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The big read. Integral Hydroponics Evolution @ 2016. 100 A4 pages on hydroponic nutrient  science. Undoubtedly the most culturally appropriate and informative material about hydroponic nutrient science for ‘hydro’ growers on the web today. Save your money – there is no reason you need to purchase a book on the subject. G. Low, the author of Integral Hydroponics was going to release this material in 2016 as a book to be sold though hydroponic stores. He instead chose to make it freely available on Manic Botanix..

Published on Manic Botanix in March 2016 by G.Low.

 

Topics

Medical Marijuana and Phosphorus

The role of nutrients in plants

The importance of understanding plant/nutrient interactions

Nutrient deficiencies and excesses – a broader understanding

Beneficial elements

A broader understanding of EC

Osmosis/osmotic pressure and EC

Salt buildup in hydroponic substrates

EC and ppm standards

pH science – a more complex understanding of a too oft dumbed down subject

Establishing ppm in solution using a hydroponic nutrient’s guaranteed analysis

Understanding the different growth phases, plant architecture/morphology and the nutrient requirements of the plant

Flushing science – a more complex understanding of the flush and plant tissue nutrient buildup

 

HYDROPONIC NUTRIENT SCIENCE

 

Because Hydroponic substrates are inert (devoid of food), the food is provided to the plant through a balanced diet of liquid nutrition (nutrient), which is added to water in the nutrient tank/reservoir. This is then fed to the plants, ensuring that they are getting adequate levels of food. This is why the pH of the water/nutrient is critical. For the plants to uptake the differing elements that make up the nutrient it is necessary to have the nutrient set at the correct pH. In addition to this, the EC is also critical. If the levels of food are too low the plant will be starved of nutrition. If the nutrient is too strong (EC is too high) other problems such as necrosis (burning/rusting) may be apparent.

 

Plant nutrition is a complex business. How each element affects the plant, whether the element is mobile or immobile within the plant and the interaction of each element with other elements along with their part in photosynthesis is always going to be a complex business to discuss.

 

Fortunately, it is something that the indoor grower doesn’t have to worry too much about today as there are many good nutrients on the market. They differ somewhat in their macro and microbalances; however, using a reputable brand should ensure that the plant is receiving the correct balance of micro and macro nutrition.

 

Often individuals will ask me to recommend a brand of nutrient over the other brands. I’m going to tell you the same thing as I tell them regarding this. “I really could not tell you. If I were to put 100 hydroponic gardeners in the same room there would be a raging debate about the best nutrient to use” (among other things).

 

What I’m saying here is that personal preference and your growing practices (recycling v. run-to-waste, nutrient tank/reservoir practices etc.) will determine the best nutrient for you. I would be extremely hesitant to recommend any one brand over another. There are many, many good nutrients on the market today.

 

Nutrients can be purchased as single part formulas, two part formulas and as triple pack formulas. Over the years I have played with all of these systems and achieved great results across the board. Again, personal preference (ease of use etc) will help you decide which is the most ideal for you.

 

My only recommendation…. the thing about nutrients that you ought to be aware of is the hype that goes with them. Be wary of paying too much for a product that probably isn’t going to perform any better than the product next to it that is 30 – 40% cheaper. You may be paying extra for an extravagant label, importation costs, and the manufacturer’s advertising campaign.

 

I’ve had some of the expensive brand names lab analyzed only to discover that they aren’t anything special compared to far cheaper brands. If you factor in the vast number of different growing methodologies practiced amongst indoor hydroponic gardeners, and if you understand what goes into a nutrient, the approximate cost of the constituents, the principles of plant nutrition and particularly the principles of the sufficiency and luxury nutrient ranges, which I discuss in detail on pages…. , you tend to be a bit sceptical about using higher priced products. For example, here is what a highly renowned PhD biochemist and plant nutrient expert had to say after analyzing perhaps the most expensive (given nutrient concentration and price) big name multinational brand on the market today.

 

(Quote)

 

“Interpretation:

 

There is nothing unusual or unexpected in the mixes. Veg is a fairly standard grow mix and Flower is a fairly standard bloom mix. Both are rather more acid­ic than optimal, due to the formulation used. Flower B contains much more ammonium than most other plant nutrients. This is because monoammonium phosphate is used to boost P to the high levels required. The following formulations will provide a solution which is close to those supplied. You can never be exact, because batch-to-batch variation between fertilizers means you will never get better than 98% reproducibility.”

 

(End Quote)

 

The store I worked in at that time was one of the top 10 Australian distributors of the line. After a falling out with its Australian distributor we had the products (nutrients and additives) lab analyzed and subsequently reverse engineered them.

 

What we immediately did was drop the price from a rather hefty $99.00 AUD for a 2 part 10ltr A&B set for the original, reducing it to $50 AUD for our product which was presented in unlabeled plain white bottles with A and B written on them in black or blue permanent marker (very high tech and truly a nutrient without the bells and whistles!). Not only this, but there was no impressive laboratory or white coats anywhere in sight – our product was knocked up by the biochemist in 200ltr plastic drums using a water pump to circulate the ingredients as he mixed.

 

We then told our customers that we were dropping their preferred line (largely preferred because we had recommended it in the first place) and that we had engaged a highly renowned PhD Biochemist to reverse engineer and “tweak” the original formulas and that the copy was as good as the original – if not better. We, however, added that if they didn’t find our nutrient to their liking we would order the original for them as they required.

 

As it turned out the product was a hit. Our PhD biochemist had weaved his magic and not only were our customers confirming the new products quality but also, in many cases, stating (insisting) that it was superior to the original, producing higher yields and a better quality end product. I myself, on trialing the product, was happy but didn’t see any differences in yields and/or quality, when compared to the original, and put my customers rave reviews down to a collective hysteria… a placebo effect based on savings. Nevertheless, the bottom line; not one single customer (not one) requested the original again and new customers began appearing at the store seeking our no thrills, cheaper product that they had heard so much about through their friends. Within weeks, the product was walking out the door and our customer base increased by approximately 20% within the year. No small feat given two new stores had opened within 10km of us in approximately the same period.

 

The moral of the story: The power of branding and marketing.

 

Were the original formulas any better than any other formula? Clearly many growers thought so. The company is multinational with a good reputation – one that it has worked hard to create – and certainly their nutrients are better than some formulas I have subsequently lab analyzed. On the other hand, as the biochemist noted, there was nothing “unusual or unexpected” about their mixes. Good formulas pretty much tend to look and perform about the same and the key difference – if any – comes down to marketing and consumer perception.

 

Why then did our store recommend the multinational product over others?

 

In truth, because it was a product I had used and liked (better than some – no better than others) and secondly, perhaps more importantly, due to its higher recommended retail price we made more profit from selling this product over cheaper brands. It is important to understand this factor and is true of all retail businesses, whether they be selling clothing or hydroponic specialty equipment. In short, hydroponic retailers by other retail standards have low profit margins on the goods that they sell; typically about 100% markup on wholesale price where consumables (e.g. nutrients and additives) are concerned and even lower margins in most cases on hardware. Think yourself lucky on this front. For instance, to generalize somewhat, clothing retailers typically markup 200- 400%. Basically though, if you purchase a $100 10L nutrient pack the retailer has made about $50.00. On the other hand, if you purchase a $50.00 10L nutrient pack the retailer has made $25.00. If you were him/her and had a business to run, with all of the associated costs (e.g. rent, insurance, wages), which product would you prefer to sell? It’s a case of simple business economics. Other than this, some nutrient manufacturers fully understand this principle as good business practice. I.e. create products that create higher profit margins for the retailer and they are likely to support it. For example, the company’s whose formulas we reverse engineered and sold to our customers was found guilty of price fixing in Australia in 2005 by an Australian Government corporate watchdog. The Australian distributor had sent an Australia wide letter to retailers demanding that they stop discounting their brand below the recommended retail price (RRP) or they wouldn’t supply them. This, as it turned out, was in breach in corporate law and as a result the company was called to task.

 

When discussing value for money, with regards to hydroponic nutrients, it is important to note that some nutrient brands are more mineral dense (concentrated) than others, meaning that some nutrients require a lower dilution rate than others to achieve the same EC in solution. Therefore, when considering the price/value of a nutrient be sure to factor in the product’s concentration.  That is, if one product is 40% more concentrated than another and costs 25% more, this product is actually 15% cheaper than its less concentrated counterpart because it will go 40% further at only 25% more recommended retail price.

 

Okay, that was pretty simple wasn’t it? Now I’m going to get somewhat technical and explain what roles the different macro and microelements play in the plant.

Before I do this, I had better point out that there are some nutrient deficiency symptoms outlined. However, it is important to note that while visual nutrient deficiency symptoms can be a very powerful diagnostic tool for evaluating the nutrient status of plants, one should keep in mind that an individual visual symptom is seldomly sufficient to make a definitive diagnosis of a plant’s nutrient status. Many of the classic deficiency symptoms such as tip burn, chlorosis and necrosis are characteristically associated with more than one mineral deficiency. Additionally, what appears to be a nutrient deficiency can be due stresses such as high ambient air temperatures, pathogens, high levels of sodium chloride in the substrate, oxygen starvation in the root zone (due to too high temperatures in the nutrient and/or substrate) and air pollution. Often, symptoms of these stresses closely resemble those of a nutrient deficiency. That is….

 

Some possible reasons for nutrient deficiency and excess symptoms beyond actual nutrient deficiencies and excesses:

 

1) The mains (municipal) water supply that you use may be high in microelements such as iron, copper and zinc. When these combine with the microelements in the nutrient solution toxicity is expressed due to too high levels of micro-nutrition in the nutrient working solution (I.e. the solution that is being fed to the plants).

2) Your mains water supply may contain high levels of both sodium and chloride. When combined this equates to high levels of common table salt (i.e. sodium chloride or NaCl) in solution. High levels of NaCl is toxic (phytotoxic) to plants and is expressed in what looks like nutrient deficiencies and/or excesses.

3) If growing in coco coir substrate, did you purchase a quality flushed and buffered coir substrate or a cheap compressed brick product? Similar to some mains water supplies, cheap coco coir products can come loaded in sodium chloride. The end result is phytotoxicty and what looks like nutrient deficiencies and/or excesses.

4) Plant Pathogens often produce an interveinal chlorosis in the leaves (yellowing or whitening of the leaf veins) that can be easily mistaken for a nutrient deficiency. Put simply, when a pathogen infects a plant, it alters the plant’s physiology, particularly with regard to mineral nutrient uptake, assimilation, translocation, and utilization. Plant pathogens/diseases can also infect the plant’s vascular system and impair nutrient or water translocation. Such infections can cause root starvation, wilting, and plant decline or death. Plant pathogen/disease symptoms can often be separated from nutritional symptoms by the rate in which they affect a population of plants. If the plants are under nutrient stress, all plants tend to develop similar symptoms at the same time. However, if the stress is the result of pathogens, the development of symptoms will have a tendency to vary between plants.

5) Too high ambient air temperatures (heat stress) severely limits utilization of absorbed light energy in photosynthesis which leads to exposure of the chloroplasts to excess energy and thus generation of reactive oxygen species (ROS) such as superoxide radical, hydrogen peroxide, hydroxyl radical and single oxygen. Therefore, oxidative cell damage is a common phenomenon in heat stressed plants.[1] The end result is what looks like nutrient deficiencies and/or excesses.

6) Excessive humidity slows transpiration in plants which results in slowing the distribution of nutrients throughout the plant. The end result is nutrient deficiencies.

7) Oxygen starvation in the root zone results in an unhealthy root system and thereby can greatly impact on nutrient uptake, resulting in deficiencies. In hydroponics, oxygen starvation in the root zone due to overly warm nutrient and/or media temperatures is probably the leading cause of root death and reduced growth rates. I.e. unhealthy roots = unhealthy nutrient uptake and, as result, nutrient deficiencies occur.

8) Salt buildup in the media and root zone can cause damage to the plants both through direct contact with the salt crystals around the plant stem, particularly in young plants, and by increasing the osmotic pressure around the roots. The end result is nutrient deficiencies.

9) pH problems are often the cause of nutrient deficiencies because pH determines the availability of mineral elements to plants. Too high or too low pH can, therefore, reduce nutrient availability resulting in deficiencies.

 

As you can perhaps see, nutrient deficiency or excess symptoms can be caused by numerous biotic and abiotic stresses. Therefore, any deficiencies and/or excesses need to be addressed holistically because often what appears to be deficiencies or excesses are caused by unfavorable environmental conditions, pathogens and/or root disease. As such, it is too simplistic to label the problem as a nutrient deficiency and/or excess before covering all of the bases. That is, if you plants show signs of nutrient deficiencies or excesses check that the roots are healthy and white (not turning brown).

 

Check that your environmental conditions are within ideal parameters (temp, airflow, humidity etc).

 

Check that your pH and EC meters are calibrated and working properly (a trip to the hydro store to have your retailer look them over may be in order).

 

Check for signs of salt buildup in the media (if in doubt flush the media with pH adjusted water). And, treat your plants for potential pathogens (speak to your retailer for more information about product options). Last, but by no means least, dump your nutrient tank and mix a fresh batch of nutrient to cover all bases. Other than this, a foliar feed will greatly help the plants recover from a deficiency. After you have implemented all of these measures, watch the plants closely to make sure that the problem starts to clear up over a few days. Old growth may not recover, but new growth should appear healthy.

 

Steps to follow where a nutrient deficiency is apparent

  • Check the roots of the plants. Are they white (healthy) or brown (unhealthy)? If the roots are brown check your nutrient tank/reservoir solution temperature – it should be below 23oC (73.4oF)
  • Dump the nutrient tank/reservoir and mix a fresh batch
  • Flush the media with pH adjusted water (salt build up and or a nutrient excess may be locking out crucial nutrients)
  • Treat the plants for potential pathogens
  • Check that all of your grow room environmental factors (air tamp, water temp, relative humidity) are within ideal parameters
  • Foliar feed the plants as a quick fix to correct a deficiency (avoid in mid to late flower due to the potential for botrytis/grey mould)
  • Ensure that your pH and EC are within ideal ranges after you have made a fresh batch of nutrients
  • If in doubt about the reliability of your pH and/or EC readings take your meters to your hydroponic retailer to have them looked at (this is recommended either way)

 

It is important to note that by the time you identify symptoms of nutrient deficiencies or excesses the problem has already impacted on yields (this is particularly true during the bloom cycle). I.e. a plant’s growth rates are reduced long before deficiencies or excesses become visually apparent.  Scientists call this phenomenon “hidden hunger” (nutrient deficiency) or “incipient toxicity” (nutrient excess).

 

We’ll be covering a great deal of material in the following chapter on hydroponic nutrient science that helps you to understand the principles of hidden hunger and incipient toxicity. However, briefly for now, let’s begin with incipient toxicity, where a nutrient excess is present.

 

As the name implies, incipient toxicity describes being in an initial stage; “beginning to happen or develop”. Therefore, incipient toxicity is where excess nutrients slowly begin to accumulate in the plant tissue to such a degree that they start to become toxic.  Signs of excess won’t necessarily become apparent for some time. However, growth will be impaired long before nutrient excess signs become apparent. Therefore, while growers are providing too much nutrient (enough nutrients to impair growth) they can be completely unaware of this because the visual symptoms that growers attribute to excess aren’t apparent. Nevertheless they are losing yield because this excess is hidden and is impacting on growth.

 

Where “hidden hunger” (nutrient deficiency) is concerned, as the name implies, the plant is hungry but we cannot see it (i.e. symptoms of hunger are “hidden”). Because the exact concentration of a nutrient below which yields decline is difficult to determine precisely, some experts define the critical level as the nutrient concentration at 90 or 95% of maximum yield. However, hidden hunger can be present well before this without visible signs of a deficiency.  For example, a grower may only be achieving 80-85% of the maximum possible growth/yield before visual symptoms of a deficiency present.

 

In fact, scientifically speaking, the expression of nutrient deficiency symptoms varies for acute or chronic deficiency conditions.

 

Acute deficiency occurs when a nutrient is suddenly no longer available to a rapidly growing plant.

 

Chronic deficiency occurs when there is a limited but continuous supply of a nutrient, at a rate that is insufficient to meet the growth demands of the plant.

 

Most of the classic deficiency symptoms described in textbooks or online are characteristic of acute deficiencies. However, the most common symptoms of low-grade, chronic deficiencies are a tendency towards darker green leaves and stunted or slow growth. So basically, where a chronic deficiency is present, the plants leaves are dark green. As such, the plant looks healthy to novice growers (if its dark green its healthy right?). The problem is that dependent on the degree of a chronic deficiency many novice growers are unlikely to be able to spot that growth rates are less than optimal. From a scientific perspective, the only way of knowing that hidden hunger or a chronic deficiency is present is through lab analyzing the plant tissue or through having another plant that is being fed with more ideal nutrition to measure growth rates to.

 

This is something that hydro nutrient manufacturers and others typically forget to mention – perhaps many of them don’t understand this themselves. That is, ‘hydro’ growers have been led to believe that if a nutrient deficiency or excess is present then the plant will tell them this through displaying symptoms of excess or deficiency.

 

Nothing could be further from the truth!

 

The fact is that yield losses can occur long before signs of excess or deficiency become apparent. The only way of knowing that these deficiencies or excesses are impacting on growth is through running tissue analysis or having a control plant which is being fed with more ideal nutrition to compare growth rates to.

 

Now that you have this important scientific fact under your belt, let’s move on.

 

[back to top of page]

The Role of Nutrients in Plants

 

Overview of Pant Nutrients 

 

Plants require 18 essential elements. Of these, 15 essential elements need to be provided to the rootzone. These are the macronutrients (needed in relatively large quantities) of nitrogen, phosphorus, potassium, sulfur, calcium and magnesium; and the micronutrients (needed in relatively small quantities) of iron, manganese, zinc, boron, copper, molybdenum, cobalt, chloride and nickel. All of these nutrients must be supplied by the hydroponic nutrient solution, although chloride, cobalt and nickel aren’t included in most recipes, as they’re typically found in sufficient quantities as impurities in fertilizers or provided through the water supply (e.g. mains water will provide chloride).

 

An element is considered to be essential if the plant cannot complete its lifecycle without it or if it forms part of an essential molecule or constituent.

 

A deficiency of one essential element causes primary metabolic defects leading to stunting or deformity of roots, stems, or leaves, chlorosis or necrosis of various organs and even plant death.

 

The essential elements that are provided to plants through fertilizers are called macro and micronutrients. The macronutrients are used in large quantities and can be categorized into two groups:

 

    1. Primary nutrients: nitrogen (N), phosphorus (P), potassium (K)
    2. Secondary nutrients: calcium (Ca), magnesium (Mg), sulphur (S)

 

The remaining essential elements are used in small quantities by the plant, but nevertheless are necessary for plant survival. These are called micronutrients and include iron (Fe), boron (B), copper (Cu), chloride (Cl), manganese (Mn), molybdenum (Mo), zinc (Zn), cobalt (Co) and nickel (Ni).

 

The following table lists the essential elements, their status as macro or micronutrients, their uptake forms, and their mobility (‘nutrient mobility’) in the plant.


 

Nutrient Mobility

 

The mobility of a nutrient relates to the ability of a nutrient to move within the plant tissue. In general, when certain nutrients are deficient in the plant tissue, that nutrient is able translocate (move) from older leaves to younger leaves where that nutrient is needed for growth. Nutrients with this ability are labelled ‘mobile nutrients’, and include nitrogen, phosphorus, potassium, magnesium, and molybdenum. Conversely, ‘immobile nutrients’ do not have the ability to translocate from old to new growth. ‘Immobile nutrients’ include calcium, sulfur, boron, copper, iron, manganese and zinc.



 

 

Macroelements

 

Element Symbol Available as Symbol Mobile in the plant
Carbon C Carbon dioxide

Carbonic acid

CO2,

H2CO3

Hydrogen H Hydron/hydrogen

Hydroxide

Water

H+

OH

H2O

Oxygen O Oxygen O2
Nitrogen N Nitrate ion

Ammonium ion

Urea

N03

NH4+

CO(NH2)2

Yes
Phosphorous P Monovalent phosphate ion

Divalent phosphate ion

H2PO4

 

HPO4-2

 

Yes
Potassium K Potassium ion K+ Yes
Calcium Ca Calcium ion Ca+2 No
Magnesium Mg Magnesium ion Mg+2 Yes
Sulphur S Divalent sulfate ion SO4-2 No

 

Microelements

 

Iron Fe Ferrous ion

Ferric ion

Fe-2

Fe-3

No
Manganese Mn Manganous ion Mn+2 No
Boron B Boric acid H3BO4 No
Copper Cu Cupric ion chelate

Cuprous ion chelate

Cu+2

Cu+

No
Zinc Zn Zinc ion Zn+2 No
Molybdenum Mo Molybdate ion MoO4 Yes
Chloride Cl Chloride ion Cl Yes
Nickel Ni Nickel ion Ni2+ Yes
Cobalt Co Divalent cobalt Co2+ Yes

 

 

The Essential Elements

 

The Major Nutrient Elements (Macroelements)

 

Nitrogen (N)

 

Of all the essential nutrients, nitrogen, along with potassium, is required by plants in the largest quantity and is most frequently the limiting factor in crop productivity. This element is part of all amino acids (hence proteins), of chlorophyll and of enzymes. It is very mobile in plants, and has a dominant effect on other nutrient use and uptake. Plants typically absorb nitrogen in the form of nitrates or ammonium, and the plant converts this ammonia to create proteins. Plants are also shown in more recent research to uptake the chemically organic, albeit synthetic, urea (another form of nitrogen fertilizer) and organic N containing amino acids (e.g. glycine).

 

Nitrogen is critical to fast, lush plant development. Photosynthesis occurs at high rates when there is sufficient nitrogen. A plant receiving sufficient nitrogen will typically exhibit vigorous plant growth. Leaves will also develop a dark green colour.

 

A deficiency in nitrogen is recognisable through yellowing of the leaves (particularly older, larger leaves), stunted leaf growth, older leaves falling off the plant, and in extreme cases a small spindly plant.

 

Phosphorus (P)

 

Phosphorus, in the form of inorganic phosphate (Pi), is one of the most important macronutrients for all organisms. It is not only used in the biosynthesis of cellular components, such as ATP, nucleic acids, phospholipids, and proteins, but it is also involved in many metabolic pathways, including energy transfer, protein activation, and carbon and amino acid metabolic processes. Large amounts of phosphate are required for cell survival. In plants, Pi is essential for growth and development.

 

A deficiency will appear as:

 

  • In the early stages the plant will appear a dark blue/green shade.
  • Plant growth and shoot development may be retarded.
  • Might result in delayed crop maturity and small fruit set
  • Purpling in older leaves
  • Leaves may curl and die

 

Generally, inadequate P slows the processes of carbohydrate utilization, while carbohydrate production through photosynthesis continues. This results in a buildup of carbohydrates and the development of a dark green leaf color. In some plants, P-deficient leaves develop a purple color. Since P is readily mobilized in the plant, when a deficiency occurs the P is translocated from older tissues to active meristematic tissues, resulting in foliar deficiency symptoms appearing on the older (lower) portion of the plant. Other effects of P deficiency on plant growth include delayed maturity and decreased disease resistance.

 

Note: phosphorus uptake is influenced by temperature and a deficiency may be induced by cool nutrient solution and/or ambient air temperatures.

 

Potassium (K)

 

Potassium is not an integral part of any major plant component, but it does play a key role in a vast array of physiological processes vital for plant growth. Potassium is used in large quantities by plants to maintain ion balances within the cells, maintain osmotic pressure throughout the plant and activate enzymes. It is also required for protein synthesis.

 

A deficiency will appear as:

  • The edges of the leaves turning rusty brown.
  • Leaf yellowing.
  • Leaf curling.
  • Plant growth being retarded
  • Flowers fail to develop
  • Older leaves develop marginal browning which can extend into the leaves, and forward curling of leaves

 

Since potassium is mobile in the plant, the symptoms appear on the older leaves first

 

Magnesium (Mg)

 

Magnesium is the essential element that forms the central atom in the chlorophyll molecule. Low magnesium levels reduce the plant’s ability to produce sugars from air and sunlight. Magnesium also helps regulate cellular pH and cation-anion balance within the plant. It is quite mobile in plants.

 

A deficiency will appear as:

 

  • Rusty spots may appear on leaves.
  • White stripes between the leaf veins may appear
  • Yellowing in veins of older leaves
  • The edges of the leaves may become yellow or bright green and may start feeling brittle to the touch
  • Older leaves are more affected than younger leaves.

 

Calcium (Ca)

 

Plants require a large amount of calcium as it is an essential part of cell walls, enzymes and chromosome structure. Calcium is also involved in the cation-anion balance and maintenance of osmotic pressure, allowing the plant to pump required nutrients around.

 

A deficiency will appear as:

  • Yellow/brown spots on leaves – the spots generally start small and slowly increase in size. The older larger leaves will show the symptoms first.
  • Plant growth is retarded.
  • Leaves fail to fully expand.

 

Sulfur (S)

Some plants require as much sulphur as phosphorus. In the plant sulphur is necessary for chlorophyll formation and is a component of methionine, cysteine and cystine, three of the 21 amino acids which are the essential building blocks of proteins.

 

A sulfur deficiency is characterised by:

  • Retarded plant growth.
  • Plants may appear very spindly
  • Purpling of leaf stems.
  • Yellowing of leaves.
  • In extreme cases the plant will have yellow leaves and purple stems.

 

Minor Elements (Micro or Trace Elements)

 

Boron (B)

 

Boron is essential to the transport of sugars in the plant, to pollen formation (fertility) and to cell wall structure (like calcium). The main functions of boron relate to cell wall strength and development, cell division, fruit and seed development, sugar transport, and hormone development. It is necessary for normal cell division, nitrogen metabolism, and protein formation. Boron is also essential for proper cell wall formation. It plays an important role in the proper function of cell membranes and the transport of potassium (K) to guard cells for the proper control of internal water balance

 

As boron is required to build plant cell walls, when not enough B is available the areas of the plant with rapidly growing new cells (i.e. the growing point and new leaves) are affected first. The growing tip often aborts (effectively “pinching/tipping” the plant). This leads to proliferation of lateral branches. The branches and new growth are distorted, thick and brittle. Also, the upper foliage can exhibit a mottled chlorosis (i.e. scattered yellowing of leaves). When the roots are examined they are often short and stubby. Boron deficiency is somewhat unique in that unlike most other nutrient deficiencies B deficiency may not appear uniform across the crop.

 

Copper (Cu)

 

Copper is an important component of proteins found in the enzymes that regulate the rate of many biochemical reactions in plants. Copper is an enzyme activator in plants and is concentrated in the chloroplasts of leaves, assisting the process of photosynthesis. Thus, copper plays an essential role in chlorophyll formation and is essential for proper enzyme activity.

 

A copper deficiency can be identified through leaves curling downwards, the tips and margins of the leaves may exhibit coppery gray or slightly blue discolorations with a metallic sort of look. In between the veins, the leaves may yellow, new growth may have a hard time opening  and leaves may appear small.

 

Iron (Fe)

 

Iron has special importance in biological redox systems involved with chlorophyll formation/synthesis and protein synthesis.

 

Because of iron’s involvement in chlorophyll synthesis a deficiency will appaear as chlorosis (yellowing) of the leaves. Often the symptoms appear near the top of the plant on newer leaves.

 

Manganese (Mn)

 

Manganese is an important enzyme activator involved in the assimilation of nitrates to form proteins and is also important for photosynthesis.

 

A deficiency will show as intervenal chlorosis. i.e. Leaves may become yellow in the veins, with mottled brown spots on the affected leaves. These brown dead patches may spread. Growth rates may slow.

 

Zinc (Zn)

 

An integral component of many enzymes. Zinc plays a major role in protein synthesis and is involved with the carbohydrate metabolic processes. Zinc is also required for maintaining integrity of biomembranes and protecting membranes from oxidative damage from toxic oxygen radicals.

 

A deficiency will show symptoms of yellowing between the veins of leaves, plant growth will slow and there will be less growth/distance between the internodes of the plant.

 

Molybdenum (Mo)

 

Molybdenum plays an important role in the enzyme system which is involved in nitrate to ammonium conversion.

 

Deficiency symptoms include failure of leaves to develop a healthy dark green colour. The leaves of affected plants show a pale green or yellowish green colour between the veins and along the edges. In advanced stages, the leaf tissue at the margins of the leaves die. The older leaves are the more severely affected.

 

Cobalt (Co) 

 

Cobalt is a transition element, and is an essential component of several enzymes and co-enzymes. It has been shown to affect growth and metabolism of plants, in different degrees, depending on the concentration and status of cobalt in the rhizosphere.

 

A deficiency in cobalt is shown in reduced Vitamin B12 production and lower nitrogen fixation which is reflected by uniformly pale green to yellow leaves. .

 

Nickel (Ni)

 

Nickel, in low concentrations, fulfills a variety of essential roles in plants, bacteria, and fungi. Nickel naturally occurs in a few plants where it functions as an essential component of some enzymes (e.g., ureases) that are involved in nitrogen assimilation.

 

Ni deficiency produces an array of effects on growth and metabolism of plants, including reduced growth, and induction of early senescence (fruit ripening) and leaf tip chlorosis (paling/yellowing).

 

Chloride (Cl-) 

 

Chloride (Cl-) is an essential element for plants. It is a component of common salt and found in seawater. It should not be confused with other forms of the element such as chlorine gas (highly toxic and unstable), chlorine in swimming pools, hypochlorite (a sterilant and bactericide), hydrochloric acid (corrosive and dangerous liquid) etc. Chloride (Cl) is the negatively charged anion of chlorine, which is the form it is found in naturally. It is non-toxic and readily adsorbed by plants.

 

Chloride regulates the function of several enzymes, it is essential (working in tandem with potassium) to the proper function of the plants stomatal openings, thus controlling internal water balance. It functions in cation balance and transport within the plant and is essential for transport of the nutrients calcium, magnesium and potassium. Studies have shown that Cl- diminishes the effects of fungal infections in an as yet to be understood way, although this might be related to Cl- reducing N accumulation in plant tissue. That is, it is speculated that Cl- competes with nitrate uptake. This may be a factor in its role in disease suppression, since high plant nitrates have been associated with disease severity.

 

Although chloride deficiency symptoms are rare, wilting is a common symptom of chloride deficiency and the leaves turn yellow to white.

 

Overall stunting of the whole plant is a sign of all nutrient disorders, and the hardest to detect unless normal plants of the same kind and age are available for comparison.

 

Beneficial Elements 

 

Other than the essential elements, there are beneficial elements for plant growth. Beneficial elements are elements that help optimize the growth and development of plants but they are not essential for growth.  When they are absent in the solution/substrate, plants can still live a normal life. Here are a few criteria that can distinguish between the essential elements and the beneficial ones. Beneficial elements can:

 

  • Compensate for the toxic effects of other elements.
  • May replace mineral nutrient in some other less specific function such as the maintenance of osmotic pressure.
  • May be essential to some but not to all plants
  • May enhance plant growth and benefit yields

 

Aluminum (Al), sodium (Na), selenium (Se), and silicon (Si) are considered beneficial elements for plants: they are not required by all plants but can promote plant growth and may be essential for particular plant species. These beneficial elements have been reported to enhance resistance to biotic stresses such as pathogens and pests, and to abiotic stresses such as drought, salinity, and nutrient toxicity or deficiency. While most of the beneficial elements can aid plant growth when present at very low levels they can also be toxic to plants at higher levels.

 

The exception here is silicon (Si) which is uptaken by plants at reasonably high levels, and is considered a ‘quasi essential’ element for plants because its deficiency can cause various problems with respect to plant growth, development and reproduction. The addition of Si to hydroponic solutions exerts a number of beneficial effects on growth and yield of several plant species, which include improvement of leaf exposure to light, resistance to lodging, decreased susceptibility to pathogens and root parasites, and amelioration of abiotic stresses. Silicon can also alleviate imbalances between zinc and phosphorus supply. In general, dicot plants (e.g. tomato, cucumber, peppers) show a tissue accumulation of Si at 0.5% or less.

 

FOUNDATIONS IN NUTRIENT SCIENCE

 

EC – Electrical Conductivity and Ions in Solution

 

Maximum crop production is primarily a function of environmental conditions and genetic potential. The extent to which this limit can be reached relies directly on the degree and effectiveness of practices which serve to optimise the plant’s environment. Fulfilling the crop’s water and nutrient requirements by providing optimal levels of plant nutrients through, among other things, closely monitoring EC and by ensuring high bioavailabity of nutrients through maintaining optimal pH levels are among the most important factors to consider when striving for maximum yields.

 

About EC, ppm and TDS

 

EC stands for electrical conductivity, which is the potential of any material to conduct electricity. Although most growers are used to measuring the amount of feed they give to their plants in ounces per gallon, millilitres per litre (ml/L), millilitres per gallon, or some other unit of measurement, EC goes a little further and gives us a reasonably precise measurement of how much plant nutrient we have in the hydroponic ‘working solution’ (i.e. the solution that is being fed to the plant).

 

When I say “reasonably precise measurement”, hydroponic nutrients consist of nutrient salts that have positive electrical charges (cations) or negative electrical charges (anions). For example, some important elements with a positive electrical charge (cations) in their plant-available form include potassium (K+), ammonium (NH4+), magnesium ( Mg++), calcium (Ca++), zinc (Zn+), manganese (Mn++), iron (Fe++), and copper (Cu+). Some of the nutrients that have a negative electrical charge in their plant-available form include nitrate (NO3), phosphate (H2PO4 and HPO4), sulfate (SO4), borate (BO3), and molybdate (MoO4).  When these elements (ions) are in solution they produce an electrical charge and conduct electricity from one to the other. Plants are able to acquire the essential mineral elements via the root system utilising the chemical properties of the ions, particularly the negatively charged anions because the plants roots have sites that are positively charged (opposites attract). The plant is also able to attract positively charged cations to negatively charged sites on the roots. The more nutritional salts (ions) added to the nutrient tank/reservoir, the more the electric conductivity (EC). EC, therefore, tells us how many nutrient salts we have in solution and, in turn, this tells us how much nutrition we are providing to the plant.

 

However, this said, no EC meter has the ability to distinguish between different types of ions. This means the use of EC measurement is only helpful in checking total salt concentrations in the solution, but the concentrations of individual nutrients may vary considerably from the desired concentration. This is because EC only tells us the relative amount of total salts (ions) and nothing about each specific nutrient concentration in the solution. So, for instance, the true concentration of N, P and K may be lacking even though the EC is ideal.

 

This is something that you need to be aware of. As we progress through this chapter you’ll come to see that some nutrients are removed from the nutrient solution very quickly by plants while other nutrients are used at far lower levels and can accumulate in the solution and substrate. Given this, incorrect maintenance practices of your nutrient tank/reservoir can lead to deficiencies of key elements even when your EC is running at optimum.

 

Plants require a wide range of nutrient ions to support growth. Each of these specific nutrient ions has an ionic sufficiency range in which growth is optimized. The uptake and utilization of nutrients depends not only on the quantities, but also on the ratios among nutrient types. If a particular nutrient is deficient, yields can be negatively affected. A similar reduction in plant growth can arise when a particular nutrient is present at a concentration that is too high. All the nutrient ion types need to be within their respective ranges if plant productivity is to be optimized. Departure from these optimal levels in any of the nutrient ion types will have an influence on all the others as well.

 

Given that EC only measures total salts (ions) and not specific ion types this measurement alone does not provide sufficient information to allow growers to realize optimum yields from a hydroponic solution fertility perspective. This is extremely important information to understand, and certainly it has implications for recycling system growers where the preferential uptake of certain nutrients by plants can quickly deplete nutrients that are in high demand (i.e. N, P, K and Mn) and leave high degrees of other ions which are in lesser demand such as Ca, Mg and S in solution. The outcome, when nutrient maintenance best practice is not implemented, is an imbalance of the nutrient ions in solution, even though the EC might be ideal.

 

Actually, this one warrants more attention….

 

Preferential Nutrient Uptake by Plants – Its Influence on Nutrient Ions in Solution and EC  

 

It is important to note that plants can quickly remove their daily ration of some nutrients while other nutrients accumulate in solution. For example, high levels of nitrogen, potassium, phosphorous and manganese are taken from solution by plants while lower levels of calcium, sulfur and magnesium are taken.  This means that the concentrations of nitrogen, phosphorous, potassium and manganese can be quickly depleted from the solution (to a few ppm) because these nutrients are in the plant where we want them. On the other hand, maintaining high concentrations of nutrients in the solution to compensate for high uptake needs can result in excessive uptake (overfeeding) that can lead to nutrient imbalances in the plant.

 

For example, according to Bugbee (1996), the water removed from a recycling hydroponic system through plant uptake and transpiration must be replaced and it is necessary to have about 15.4ppm of phosphorous in the refill solution. If the refill solution was added once each day, the phosphorous would be absorbed by the plant in a few hours and the solution phosphorous concentration would be close to zero. This does not indicate a deficiency; rather it indicates a healthy plant with high nutrient uptake. However, if phosphorous was maintained at 15.4ppm in the solution, the phosphorous concentration in the plant could increase to 1% of the dry mass, which is 3 times higher than the optimum in most plants. This high phosphorous level can induce microelement (e.g. Fe, Cu and Zn) deficiencies.

 

See following table that demonstrates the uptake elevation of essential plant nutrients (Bugbee 1996)

 

bugbee

 

 

Looking at our ‘uptake elevation of essential plant nutrients’ table, the essential nutrients can be placed into three distinct groups based on how quickly they are removed from solution.

 

Group one elements (N, P, K, Mn) are actively absorbed by roots and can be removed from solution in a few hours.

 

Group two elements (Mg, S, Fe, Zn, Cu, Mo, and Cl) have intermediate uptake rates and are removed from solution at lower rates than the group one elements, which means that the ratios/balance between the group one and two elements is altered by the preferential uptake by plants of group one (higher uptake rates) over group two elements (lower uptake rates).

 

Group three elements (Ca and B) are passively absorbed from solution and often accumulate in solution. This information brings us back to my earlier point about EC not distinguishing between different types of ions and that given N, P, K and Mn are taken up at higher rates than other elements, N, P, K and Mn may be deficient even though the EC is ideal. It’s something that you need to be very aware of, particularly in recycling hydroponic systems where the various nutrients are taken at different volumes and as a result nutrient imbalances and deficiencies can occur.

 

This is why, where growers are using mains/tap water, I recommend to dump the nutrient in recycling systems regularly (dependent on nutrient tank/reservoir volume, plant numbers and size) to ensure a well-balanced nutrient is supplied to the plants at all times. I have heard some say that dumping is unnecessary. In my mind, given the science and the demographic in the retail hydroponics sector, this is flat out bad advice that will lead to nutrient imbalances and yield losses.

 

Other

 

Later in this chapter I go into detail on adding Cal Mag to solution through the use of agricultural base nutrient fertilizers. In this material I show the theoretical ppm of nutrient salts after fertilizers have been added to solution versus a measured EC of the solution. These values being:

 

Nutrient ppm in solution

 

Cl = 60ppm

Ca = 53.36ppm

Mg =26.5ppm

NO3 N = 43.74ppm

Total ppm = 183.41

 

EC Measurement of Solution

 

EC = 570 micro Siemens (microS/cm)

 

The EC measurement is taken from a solution that consists of only RO water and fertilizers that add these elements to solution. No other nutrients have been added to solution.

 

Therefore, in this example there is a total of 183.41ppm of plant nutrients in solution (no other nutrients have been added) but when measured with scientific testing equipment the EC of the solution is 570 micro Siemens (microS/cm) or EC 0.57 (mS/cm1).

 

However, based on the EC testing standard mS/cm1 which is commonly referred to by hydroponic growers as EC, 183.41 ppm should be, at least in theory, approximately 0.287 (mS/cm1) EC – or about half of that which an EC meter is showing.

 

Other than this, 570 micro Siemens theoretically gives us a TDS (total dissolved solids) ppm of 365. So the measurement a scientific EC meter has given us is showing higher ppm of elements/ions/salts than the nutrients that are actually in solution. Quite simply, this is because EC meters not only measure electrically charged nutrient elements/ions but they also measure electrically charged non-nutrient elements/ions in solution. Basically, an EC meter measures the total electric conductivity of the electrically charged ions in solution and does not distinguish between ions.

 

Think about things this way. Hydroponic liquid nutrients and additives are produced using dry agricultural base fertilizers. In production these agricultural dry base fertilizers are added to water and this (water + fertilizers) becomes a hydroponic nutrient or additive product. However, the agricultural base fertilizers used in producing the nutrient or additive only contain a percentage of nutrient elements with the bulk of the base fertilizer often consisting of non-nutrient elements. For example, when looking at just one calcium nitrate fertilizer that might be used in producing a hydroponic nutrient or additive, the fertilizer contains16.7% Ca and 11.6% NO3 N. This means that only 28.3% of the fertilizer constituents are plant nutrients, leaving us with over 70% of other non-nutrient elements that are added to solution in the production of a hydroponic nutrient or additive. The next thing to understand is that at least some of these non-nutrient elements have electrical charges which is what an EC (Electric Conductivity) meter measures for. Therefore, not only is our EC meter telling us how many ppm/EC/TDS of calcium and nitrate is in solution but it is also reading the ppm/EC/TDS of non-plant nutrients in solution. The end result is that EC measurements are inaccurate for precisely measuring the amounts of nutrient elements/salts/ions in solution because they simply give us an overall reading of the electrically charged elements/salts/ions (nutrient salts + other salts) that are in solution.

 

EC measurements are also complicated by the fact that not all salts conduct an electric current equally. For instance, ammonium sulfate conducts about twice as much electricity as calcium nitrate and more than three times that of magnesium sulfate. Also, nitrate ions do not produce as close a relationship with conductivity as do potassium ions. Consequently, the higher the nitrogen to potassium ratio in a nutrient solution, the lower the conductivity value will be. Since every element in a multi-element solution has a different conductivity factor, EC measurements, albeit reasonably accurate, are only approximate. Following is a table that demonstrates the various ECs of several different chemicals when added to water at varying percentages.

 

Electrical Conductivity (mS/cm) of Aqueous Solutions Indicated Concentration in Mass Percent

 

All values refer to 200C (680F)

 

Name Formula 0.5% 1% 2% 5% 10% 15% 20% 25%
Ammonium sulfate (NH4)2SO4 7.4 14.2 25.7 57.4 105 147 185
Copper sulfate CuSO4 2.9 5.4 9.3 19.0 32.2 42.3
Magnesium sulfate MgSO4 4.1 7.6 13.3 27.4 42.7 54.2 51.1 44.1
Phosphoric acid H3PO4 5.5 10.1 16.2 31.5 59.4 88.4 118 146
Phosphate KH2PO4 3.0 5.9 11.0 25.0 44.6
Potassium nitrate KNO3 5.5 10.7 20.1 47.0 87.3 124 157 182
Potassium sulfate K2SO4 5.8 11.2 21 48.0 88.6
Sulfuric acid H2SO4 24.3 47.8 92 211
Zinc sulfate ZnSO4 2.8 5.4 10.0 20.5 33.7 43.3

 

References. CRC Handbook of Chemistry and Physics 70th Edition (1989): Wolf, A. V, Aqueous Solutions and Body Fluids (1966)

 

Electric conductivity (EC) requires mobile ions in solution; when the mobility rises because of increases in temperature the conductivity measured also rises. For every 1OC temperature change, the conductivity of a nutrient solution will increase by approximately 2%. This temperature coefficient varies with the type of salts in the nutrient solution, the concentration of those salts and the temperature itself. When calibrating EC meters the calibration solution temperature should be as close as possible to the nutrient solution to be tested to minimise temperature induced errors.

 

Electrical conductivity can be expressed using a number of different units, but the typical unit is siemens per meter2 per mole (S/m2/mole) or millisiemens per centimetre (mS/cm). The mS/cm unit is generally used in Europe and elsewhere as a guide to the concentration of nutrients in water. In North America, electrical conductivity is typically converted into a count of ions in the water using parts per million (ppm). Parts per million represent literally how many salts/ions you have to 1 million parts of distilled water. Parts per million can also be converted directly into milligrams per litre (mg/L). That is, 1 ppm is 1mg/l, 100ppm is 100 mg/l and so on. Basically, to dumb things down somewhat, ppm and mg/l are exactly the same thing with different units of expression.

 

The ideal EC is specific for each crop and somewhat dependent on environmental conditions; however, the EC values for hydroponic systems are typically stated to range from 1.5 to 3.0 mS/cm. Higher EC may hinder nutrient uptake by increasing osmotic pressure and reducing water uptake, whereas lower EC may severely affect plant health and yield due to there being too low nutrient levels in solution. In simple terms, if the EC is too high, specific nutrient absorption will cease/slow and the plant will be subjected to osmotic stress. If the EC is too low not enough nutrients will be available to the plant and yields will suffer as a result. Either way, where EC is too high or too low yields will be decreased, so striving for and maintaining optimum EC is a critical factor in achieving optimum yields.

 

Following is a table that outlines the recommended ECs of several specific crops.

 

Salinity Group Threshold EC, mS/cm Example of crops
Sensitive 1.4 lettuce, carrot, strawberry
Moderately sensitive 3.0 Tomato, cucumber, pepper, chili
Moderately Tolerant 6.0 Soybean, ryegrass
Tolerant 10.0 bermuda-grass, cotton

 

Author’s note #1: You can perhaps see that EC is a somewhat flawed way of measuring what actual nutrients there are in solution. EC measurements act only as a guide to tell us our nutrient strength is within an ideal/acceptable range for a given crop. However, a far more accurate way of understanding things is through analysing the ppm of individual nutrients we have in solution through dissecting a nutrient labels guaranteed analysis.

 

Optimized nutrient management programs should begin with an understanding of the nutrient solution concentrations in parts per million (ppm) for the various nutrients required by the crop of choice. By managing the concentrations of individual nutrients, growers can maintain optimal nutrition in solution. We’ll be going into detail on this later in this chapter.

 

Author’s note #2: There is scientific testing equipment that can measure specific nutrient ions in solution. However, while there are ion specific meters available that are capable of measuring the ppm (mg/L) of elements such as Nitrate N and P in solution, these meters tend to be costly (cost prohibitive), often require a high degree of technical expertise to operate correctly and to date meters don’t exist for measuring some of the nutrient ions found in hydroponic solutions.

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Osmotic Pressure

 

Another important factor to consider where nutrient strength (EC) is concerned is ‘osmotic pressure.’ Put simply, osmotic pressure reduces water potential, which is the tendency of water to move across a semi-permeable membrane from one area to another. Where nutrient levels in the solution/substrate/soil are too high osmotic pressure becomes highly negative and reduces the potential for the plant to uptake water.  Where osmotic pressure becomes highly negative and the plants can’t uptake adequate amounts of water, they are subjected to ‘osmotic stress’. The end result is a plant that can’t uptake adequate water and nutrients which results in a reduction in growth.

 

Osmosis, Osmotic Pressure and Osmotic Potential

 

In order to understand osmotic pressure and osmotic potential it is necessary to understand the principle of osmosis in plants.

 

Osmosis is a vital function to the growth and stability of plant life. Without osmosis, photosynthesis couldn’t occur and plants would wilt and die.

 

Osmosis is the transfer of water through a semi-permeable membrane driven by a difference in concentration of the solutions on either side of the membrane in the direction that tends to equalize the solute concentrations on the two sides.

 

Where plants are concerned, the cell walls of the plant roots act as the semi-permeable membrane in osmosis.

 

Therefore, osmosis uses the difference in concentrations of nutrients between the nutrient/ substrate/soil and the root to move water into the plant. More nutrient ions are in the center of the root, which is an area called the stele or vascular cylinder (higher concentration of nutrients), than are in the outside of the root (lower concentration of nutrients).

 

With normal hydroponic root zone solutions, their strength will typically always be lower than the solution within the roots. As a result, the water flow will be from the root zone solution into the plant roots.

 

With hydroponic solutions, the higher the strength (EC) of the root zone solution the smaller the difference to the internal root solution and, hence, the slower the rate of water uptake. Conversely, the weaker the root zone solution (the lower the EC) the greater the concentration difference across the root cell membrane and the faster the rate of water uptake. See following illustration which demonstrates how solution strength alters osmotic potential.

 

Osmosis-Nutrient-solution-strength

 

Based on the principles of osmosis, when a solute (e.g. nutrient salt) is dissolved in water, water molecules are less likely to diffuse away into the plant roots via osmosis than where there is no solute. The more solute that is dissolved in the water the more pronounced this situation becomes. A solution (salts + water) will have a lower and therefore more negative water (osmotic) potential than that of pure water. Additionally, the more solute molecules added to a solution the more negative the osmotic potential becomes. Therefore, the higher the EC of a nutrient solution, the more solutes in solution and the more negative the osmotic potential.

 

Osmotic potential has important implications for many living organisms including plants. If a living cell is surrounded by a more concentrated solution, the cell will typically lose water to the more negative water potential of the surrounding environment.

 

As an example of highly negative osmotic pressure, if you were to put a carrot in salty water, the salt water would draw the pure water from inside the carrot and within a few hours the carrot would be limp, its cells shriveled.

 

According to Guzman and Olave (2006) optimal yields are achieved up to a given threshold of nutrient salt concentration specific to each crop, determined by EC.[1] Beyond this threshold there is a percentage of reduction in yield for each unit increase in EC. It is well known that high EC reduces yield.[2] This is a result of reduced uptake of water into fruits/flowers caused by high osmotic pressure and as a result the fruit/flower size is smaller.[3]

 

Additionally, osmosis has extremely important implications for nutrient uptake and translocation because water movement into and throughout the plant is interrelated with the movement of nutrients into and throughout the plant. Therefore, a reduction in osmotic pressure (negative osmotic potential) will not only impair water uptake but also reduce nutrient uptake and translocation.

 

High EC and Salt Buildup in Substrates

 

A very important issue to be aware of when discussing how much nutrient is provided to the plant (measured through EC/CF/ppm) is that excess nutrient salts can buildup in hydroponic substrates.

 

A salt is simply an inorganic mineral that can be dissolved in water. When raw ingredients used to make inorganic and synthetic fertilizers are added to water they become soluble salt, often termed a fertilizer salt. Plants can readily use mineral nutrients that are in the form of soluble mineral salt ions. The roots of a plant naturally contain different levels of mineral ions called root salt that help create a stable, natural flow of water and nutrients into the plant’s vascular system. However, where nutrient concentration is higher than that required by the crop, the plants will not absorb all the nutrients, resulting in unused salts building up in the substrate. As the salt accumulates the EC in the substrate begins to rise and the salt buildup can start to disrupt the flow of water and elemental nutrients into the root. If salt levels reach a point of extreme excess they can actually begin to draw water out of the plant and back into the substrate.

 

Following is a table that demonstrates yield losses as a result of excessive salt levels in the ‘rhizosphere’ (the region of the media in contact with the roots).

 

Crop salinity sensitivity, threshold and yield decrease (Maas and Hoffman, 1993).

Crop Salinity threshold expressed in ds/m Percentage of yield decrease above the salinity threshold

% per every ds/m

Lettuce 1.3 13
Pepper 1.5 14
Cucumber 2.5 13
Tomato 2.5 9.9

 

Although some salts are absorbed by the plant, there can be a sharp increase in the concentration and a build-up of some undesirable salts. When growing plants in soil outdoors, root volume and soil space are large enough that salt accumulation does not interfere with plant growth as quickly as in hydroponics where pot size limits substrate volume, leaving little space to buffer this salt build-up.

 

When plants are supplied with mineral fertilisers, although some are consumed and some are lost by leaching, the medium solution electrical conductivity is increasing compared to the nutrient that is being fed to the plants. The accumulation is mainly of nitrate and chloride; however other salts (e.g. calcium, magnesium and sulphur) can also accumulate in the substrate.

 

Other than this, salt buildup and pH are interrelated. Substrates that have high soluble salt content will also have a high pH. As the pH of a substrate rises, the result will be a change in the overall availability of certain nutrients, and sometimes it can even cause an alteration in the form of some nutrients, changing them into plant unavailable forms. In these cases, the plant might show visual signs of a nutrient deficiency but this can be misleading. Although the apparent deficiency might be real (salt buildup is causing lockout and thus a deficiency), adding more fertilizer would create more of a problem, leading to further plant injury.

 

Therefore, providing plants excessive nutrients – a practice that is common among many indoor hydroponic growers – presents with some serious issues.

 

EC and PPM Standards  

 

You’ll note that in Integral Hydroponics I discuss nutrient and additive strengths in terms of EC (mS/cm). This is because EC is a universal measurement/standard while ppm meters do not apply a universal standard and therefore ppm varies on a country-to-country basis.

 

As such, EC is a universal standard that can then be converted into ppm based on several standards that are used worldwide. Unfortunately, the same cannot be said for ppm where different countries apply different EC to ppm conversion rates. This means that there is no single universal ppm standard, so when talking in terms of ppm things can become somewhat confusing (i.e. which standard are we talking about and how do we convert this to EC?)

 

It is important to note that all ppm meters first measure in EC (electric conductivity) and then run a conversion program to display the reading in ppm. There are three different conversion factors (standards) that various manufacturers use for converting from EC to ppm. These can be simply stated as:

 

USA 1 mS/cm (EC 1.0 or CF 10) = 500 ppm
European 1 mS/cm (EC 1.0 or CF 10) = 640 ppm
Australian 1 mS/cm (EC 1.0 or CF 10) = 700 ppm

 

For example,

 

Hanna, Milwaukee 1 mS/cm (EC 1.0 or CF 10) = 500 ppm
Eutech 1 mS/cm (EC 1.0 or CF 10) = 640 ppm
Truncheon 1 mS/cm (EC 1.0 or CF 10) = 700 ppm

 

Understanding this becomes important when interpreting data and advice given through books and other media such as magazines, blogs and forums. For instance, a grower in the U.S. may say that he/she is running their nutrient at 1000ppm (2 EC mS/cm) to get optimal results. However, if a grower in Australia uses their ppm meter to achieve the same nutrient levels in solution that the U.S. grower has recommended (1000ppm) they would need 1400ppm. Other than this, understanding how to convert ppm to EC becomes important when talking to other growers who are using different units of measurement than your own (i.e. ppm versus EC and vice versa).

 

Always read the manufacturer literature supplied with your ppm meter to establish what EC to ppm conversion factor is being used.

 

Fortunately, there is a fixed calculation for the relationship between all these units, which is given in the following table. Use this table to convert between the recommended ECs found in Integral Hydroponics and ppm.

 

 

Converting EC to ppm based on the different standards 

 

EC Hanna Eutech Truncheon CF
mS/cm
0.1 50ppm 64ppm 70ppm 1
0.2 100ppm 128ppm 140ppm 2
0.3 150ppm 192ppm 210ppm 3
0.4 200ppm 256ppm 280ppm 4
0.5 250ppm 320ppm 350ppm 5
0.6 300ppm 384ppm 420ppm 6
0.7 350ppm 448ppm 490ppm 7
0.8 400ppm 512ppm 560ppm 8
0.9 450ppm 576ppm 630ppm 9
1.0 500ppm 640ppm 700ppm 10
1.1 550ppm 704ppm 770ppm 11
1.2 600ppm 768ppm 840ppm 12
1.3 650ppm 832ppm 910ppm 13
1.4 700ppm 896ppm 990ppm 14
1.5 750ppm 960ppm 1050ppm 15
1.6 800ppm 1024ppm 1120ppm 16
1.7 850ppm 1088ppm 1190ppm 17
1.8 900ppm 1152ppm 1260ppm 18
1.9 950ppm 1216ppm 1330ppm 19
2.0 1000ppm 1280ppm 1400ppm 20
2.1 1050ppm 1334ppm 1470ppm 21
2.2 1100ppm 1408ppm 1540ppm 22
2.3. 1150ppm 1472ppm 1610ppm 23
2.4 1200ppm 1536ppm 1680ppm 24
2.5 1250ppm 1600ppm 1750ppm 25
2.6 1300ppm 1664ppm 1820ppm 26
2.7 1350ppm 1728ppm 1890ppm 27
2.8 1400ppm 1792ppm 1960ppm 28

 

 

Symptoms of Overfeeding

 

An early symptom of overfeeding is leaf and stem wilting and ‘tip burn’, where the ends of the leaves begin to look burnt and go brittle and yellow to brown. If your plants show symptoms of this you’re possibly overfeeding somewhat; although, as a word of caution, tip burn and wilting can also be caused by other factors such as overly warm ambient air temperatures or a shortage of calcium. i.e. calcium strengthens plant cell walls and tip burn results from the plants inability to supply sufficient calcium to developing leaves during periods of rapid growth. Since calcium is needed to strengthen cell walls and to maintain membrane integrity, calcium deficiencies lead to the collapse of cells, resulting in tissue enzymatic browning. This said, if you’re running your environment to optimums (temp, humidity, regular nutrient changes etc) the likelihood is that you may be overfeeding. Drop back on nutrient strength and see if this corrects the problem.

 

Symptoms due to excessive EC

 

– Wilting of leaves and stems

– Reduced growth.

– Leaves show signs of burning at tips and edges and may wither or die.

– Leaves may drop off and shoots die back

 

It is important to note that many of these symptoms are also indicative of other problems such as a lack of water, disease, nutrient deficiencies and excessive light or heat.

 

 

Optimum EC is Influenced by Genetic and Other Factors

 

It is important to note that various plant nutrition factors come into play when discussing optimum EC. For example, the nutrient itself (its NPK ratios etc) will have some bearing over optimum EC. As a simple example, in flower ammonium nitrogen, magnesium, sulfur, phosphorous and particularly potassium are in higher demand than when compared to the vegetative stage of growth. Therefore, if using a high N, low K containing fertilizer (i.e. a grow formula in flower) a higher EC would be required than compared to a high K, lower N formula to provide the required amounts of potassium for optimal flowerset (although this would also mean risking too much N in solution which would result in a high leaf to cola ratio and reduced yield). Other than this, environmental factors such as ambient air temperatures play an important role in determining optimum EC. For example, where ambient air temperatures are too high (above 300C/860F) EC needs to be reduced somewhat to compensate for a reduction in the plants rates of photosynthesis (photosynthetic potential). Other than this, genetics will play some role in determining optimum EC. That is, some varieties are heavy feeders while others are easy to over-fertilize. For this reason some cautious experimentation, to establish optimum EC for your crop/genetics, is advised.

 

pH – The Power of Hydrogen

 

pH stands for the power of hydrogen, although many refer to the meaning of pH as ‘potential hydrogen’. Either way, potential hydrogen or power of hydrogen, pH is a parameter that measures the acidity or alkalinity of a solution by measuring the hydrogen ion concentration in solution. A pH value indicates the relationship between the concentration of free ions H+ (hydrogen) and OH- (hydroxide) present in a solution. Put simply, if a solution is very acidic, there will be lots of active hydrogen ions and hardly any hydroxide ions. If a solution is very alkaline, the opposite is true. In pure water, the concentrations of hydrogen and hydroxide ions are about the same. Therefore, pure water has a pH that is neutral at pH 7.0.

 

The pH scale is a logarithmic scale that typically runs from 1 to 14. Each whole pH value below 7 (the pH of pure water) is ten times more acidic than the higher value and each whole pH value above 7 is ten times less acidic than the one below it. For example, a pH of 5 is ten times more acidic than a pH of 6 and 100 times (10 times 10) more acidic than a pH value of 7. So, a strong acid may have a pH of 1-2, while a strong base may have a pH of 13-14. See following illustration of the pH scale.

 

pH-scale-site

 

Why does pH change in nutrient solutions?

 

This comes back to understanding the power of hydrogen, otherwise known as potential hydrogen in solution and some of our earlier material on EC where we touched on positively charged cations and negatively charged anions in hydroponic solutions. Other than this, pH changes largely occur due to the principle of electroneutrality where chemical reactions take place on an equivalent basis. The law of electroneutrality states that in any single ionic solution (e.g. a hydroponic nutrient solution) a sum of negative electrical charges attracts an equal sum of positive electrical charges. Therefore, according to the principle of electroneutrality, the total charge of an aqueous solution must be zero. For this to occur, the number of positive charges contributed by cations must be equal to the number of negative charges contributed by anions.

 

Based on this, in very simple terms, when a plant removes a positively charged cation from the nutrient reservoir/tank it leaves a negatively charged anion in its place and when a plant removes an anion from the nutrient reservoir/tank it leaves a cation in its place. See following images.

 

pH-change-image-site

 

Since every macro and micro element ion in solution has an electrical charge plants can’t just take them, otherwise the electrical equilibrium would be out of balance. What plants do is swap them with equivalent amounts of H+ and OH- ions.

For example:

 

Ammonium N (NH4+) is swapped with 1xH+
Nitrate N (NO3-) is swapped with 1xOH-

Potassium (K+) is swapped with 1xH+
Calcium (Ca++) is swapped with 2xH+
Magnesium (Mg++) is swapped with 2xH+
Iron (Fe++) is swapped with 2xH+
Manganese (Mn++) is swapped with 2xH+
Zinc (Zn++) is swapped with 2xH+
phosphates (HPO4–) is swapped with 2xOH-

In this way electrical charge equilibrium remains the same.

 

The ratio in uptake of anions and cations by plants may cause substantial shifts in pH. In general, an excess of cation over anion leads to a decrease in pH, whereas an excess of anion over cation uptake leads to an increase in pH.  That is, when the anions are uptaken in higher concentrations than cations the plant excretes OH- or HCO3- anions to balance the electrical charges inside, which increases the pH value. For example, if a plant absorbs the negatively charged nitrate nitrogen (NO3-) heavily it will start to contribute more OH – than H3O + ions into the solution and the result will be an increase in pH. On the other hand, if the plant absorbs high levels of the positively charged potassium (K+) it will contribute more H3O + than OH – ions and the result will be a decrease in pH.

 

This phenomenom is frequently seen where plants are grown with a full spectrum nutrient solution that contains nitrogen either as ammonium nitrogen (NH4+) or nitrate (NO3– ) nitrogen. When plants are fed only with NH4+, cation uptake generally exceeds anion uptake and the pH of the substrate decreases. On the other hand, when the plant is fed only with NO3– the uptake of anion to cation ratio is typically higher and as a result the pH of the substrate increases. This becomes important in understanding that a well formulated hydroponic nutrient contains an ideal ratio of ammonium nitrogen to nitrate nitrogen in order to minimize this situation and better maintain pH stability in the root zone and nutrient solution.

 

As a general rule, daylight photosynthesis (when the plant is taking up high degrees of mineral nutrition) produces hydrogen ions which can cause the nutrient acidity to increase (lowering the pH). When the lights switch off photosynthesis stops and the plants increase their rate of respiration. This coupled with the respiration of microorganisms (the release of CO2 by microorganisms) uses up the hydrogen ions so the acidity of the solution tends to decrease (pH rises). Additionally, plants are known to release organic acids through their roots (root exudates), reducing pH.

 

Nutrient Availability and pH

 

This is an area that tends to be misunderstood and/or oversimplified by many hydroponic industry interests who express optimum pH as 5.5 – 5.8.

In fact, from a scientific perspective, provided that adequate nutrients are available in solution, the acceptable pH range can be expressed as somewhat wider.

 

That is, the recommended pH for hydroponic growing is specified by many hydroponic nutrient manufacturers/suppliers at 5.5 to 5.8 because overall availability of nutrients is optimized at a slightly acid pH. The availability of Mn, Cu, Zn and especially Fe are reduced at higher pH, and there is a small decrease in availability of P, K, Ca, Mg at lower pH. Reduced availability means reduced nutrient uptake, but not necessarily a nutrient deficiency.[1]

 

As such, there is some tolerance regarding pH where nutrients don’t become a limiting factor. This is because the direct effects of pH on root growth are small… the problem is reduced nutrient availability at high and low pH.

 

The pH of a solution can influence the availability of the individual ions within that solution. As pH changes one particular nutrient ion may gradually become more insoluble, leaving less of that ion available to act as a nutrient. pH is of little influence over a range, but if it goes too far, especially too high, then problems can result. Therefore, pH where nutrients are present at adequate levels is less critical than many think.

 

For example, where hydroponic techniques are used to study the growth of various species apparently preferring different pH levels, researchers usually find that they do reasonably well over a fairly wide pH range (approx. pH 5.2 to 7.5 provided a chelated form of iron is used). [2]  The real issue is in ensuring that enough of any particular ion is in solution at a given pH to cater for the plants nutritional requirements. For example, indoor plants tend to grow equally well between pH 5.2 and 6.3 if nutrients in solution do not become a limiting factor.

 

Put simply, there is some tolerance to pH where adequate nutrients are available. Therefore, while pH 5.5 – 5.8 is expressed by some as the ideal there is some tolerance with regards to pH and you will typically find that a pH of between 5.2 and 6.3 will perform equally well in indoor settings where nutrients are supplied at adequate levels.

 

 

Given the rather confusing scientific understanding surrounding pH you can perhaps understand why many hydroponic and nutrient manufacturers simplify the subject and inform their consumers that they should maintain pH between 5.5 – 5.8. However, it is also necessary to raise the point that this is a simplified version of understanding pH because you will find that some express optimum pH between 5.5 – 5.8 while others express a wider range (e.g. 5.2 – 6.3 or 5.2 – 6.5 etc). This can lead to confusion amongst hydroponic retail consumers because the information provided by one nutrient supplier may seem contradictory to information being provided by another supplier. Additionally, you will find growers give what appears to be conflicting advice on forums with some stating that their plants grow best at e.g. pH 5.5 to 5.8 while others may state that the ideal pH range is wider. However, both versions of optimal/ideal/acceptable pH ranges are, in fact, correct. It really comes down to the fact that nutrient status, nutrient availability and pH are interrelated.

 

To put things simply, for novice growers, if you strive to maintain pH between the ideals of 5.5 – 5.8 this caters more adequately in situations where nutrients may be a limiting factor. This said, pH tolerance is wider than this in situations where adequate nutrient levels are maintained in solution at all times.

 

See following image that shows each nutrient’s pH range in hydroponics.

 

pH-availability-chart

 

The Essential Nutrients and their pH Ranges

 

Nitrogen (N) 

 

Nitrogen is the plant nutrient which most influences growth and development of agricultural crops. Yield is closely related to N nutrition. Plants are surrounded by nitrogen in the atmosphere, but because atmospheric gaseous nitrogen is present as inert nitrogen (N2) molecules, this nitrogen is not directly available to the plants. Plant available forms of nitrogen in hydroponics are typically inorganic and include nitrate (NO3), and ammonium (NH4). Organic forms of nitrogen that are plant available, which are found in some nutrients and additives are N containing amino acids (e.g. glycine) and the organic chemically pure, albeit, synthetic urea.  Nitrogen is available across a wide range of pH values from 2 – 7.

 

Potassium (K)

 

There is an extremely important relationship between potassium and nitrogen in flowering/fruiting crops and although potassium is not a constituent of any plant structures or compounds it plays a part in many important regulatory roles in the plant. These include osmotic regulation, regulation of plant stomata and water use, translocation of sugars and formation of carbohydrates, energy status of the plant, the regulation of enzyme activities, protein synthesis and many other processes needed to sustain plant growth and reproduction. Additionally, potassium plays a very important role in plant tolerance of biotic and abiotic stresses.

 

Potassium is also known as the quality nutrient because of its important effects on quality factors (e.g. essential oils, flavonoids). With the exception of nitrogen, potassium is required by plants in much greater amounts than all the other nutrients. Increasing plant vegetative growth, yield as well as fruit quality and chemical composition due to increasing potassium fertilization levels has been reported by many researchers on different crops. The most prevalent nutrient found in the developed tomato plant and fruit is potassium, followed by nitrogen (N) and calcium (Ca). See graphs 1 and 2.

 

Potassium is almost completely present as a free ion (K+) in a nutrient solution and is available over a wide range of pH values from 2 to 9.

 

Graph 1: Element composition of a tomato plant (Atherton and Rudich, 1986)

 

graph-1-tomato

 

Graph 2: Element composition of a tomato fruit (Atherton and Rudich, 1986)

 

graph-2

 

Calcium and Magnesium (Ca and Mg)  

 

Like nitrogen and potassium, calcium and magnesium are available to plants across a wide range of pH; however, the presence of other ions can interfere with their availability due to the formation of compounds with different grade of solubility. For example, when the pH of the nutrient solution increases, the HPO42– (hydrogen phosphate) ion predominates, which precipitates with Ca2+when the product of the concentration of these ions is greater than 2.2, expressed in mol m-3 . Sulphate also forms relatively strong complexes with Ca2+ and Mg2+. As pH increases from 2 to 9, the amount of SO42-, forming soluble complexes with Mg2+as MgSO4 and with K+ as KSO4 increases. Practically speaking, where hydroponics is concerned, both calcium and magnesium tend to be reasonably plant available between pH 5.5 – 6.0.

 

Phosphorus (P)

 

Phosphorus (P) is an important plant macronutrient, making up about 0.2% of a plant’s dry weight. It is a component of key molecules such as nucleic acids, phospholipids, and ATP, and consequently plants cannot grow without a reliable supply of this nutrient. Phosphorus is also involved in controlling key enzyme reactions and in the regulation of metabolic pathways.

 

Phosphorus is an element which occurs in forms that are strongly dependent on pH. In the root zone phosphorus can be found as PO43-, HPO42, and H2PO4- ions; the last two ions are the main forms of P taken by plants. In inert substrates, the largest amount of P available in a nutrient solution is presented when its pH is slightly acidic (pH 5). In alkaline and highly acidic solutions the concentration of P decreases in a significant way. Namely, with pH 5, 100% of P is present as H2PO4-; this form converts into HPO4-2 at pH 7.3, reaching 100% at pH 10. The pH range that dominates the ion H2PO4-2 on HPO4- is between 5 and 6. In research surrounding P availability by Jacek Dysko et al (2008) with tomatoes grown in various hydroponic organic and inorganic substrates it was shown that regardless of the substrate type, optimum yields were gained at pH 5.5.

“The marketable yield obtained with a pH of 5.5 was significantly higher in relation to the yield obtained at pH 6.5, but it did not differ significantly from the yields obtained at pH 4.5, 5.0 and 6.0. “ Similar findings were made by Chohura et al (2004) while studying the effects of pH in tomato culture grown in rockwool. [1]

Therefore, optimum phosphorus availability in solution and substrates falls within the range of pH 5.0 – 6.0, with pH 5.5 being ideal.

 

Sulphur (S)  

 

Sulphur is used mainly in sulphur-containing proteins using the amino acids cysteine and methionine. The vitamins thiamine and biotin, as well as the cofactor Coenzyme A, all use sulphur, and so this element also plays a key role in plant metabolism. Sulphur is most available to plants grown hydroponically over a range of 6.0 to 9.5; however because of the availability of other nutrient elements and their pH ranges, sulphur in hydroponics is absorbed reasonably well between pH 5.5 – 6.0.

 

The Microelements (Fe, Cu, Zn, B, Mn and Mo)   

 

The microelements, iron (Fe), copper (Cu), zinc (Zn), boron (B), molybdenum (Mo) and manganese (Mn), become unavailable in most cases at pH higher than 6.5 and are most available in hydroponic solution at an acidic pH of 4.0-5.5, although where chelated micronutrients are used tolerance levels are higher.

 

Boron is an exception and is mainly uptaken by plants as boric acid, which is not dissociated until pH is close to 7; at greater pH values, boric acid accepts hydroxide ions to form anionic species. In simple terms, putting aside the scientific jargon, boron has a wider pH range than the other microelements.

 

Optimum pH, to accommodate for all of microelements is, therefore, typically expressed in hydroponic solutions, factoring in the availability of other nutrients, at pH 5.5.

 

However, where chelated or complexed microelements are used in hydroponic solutions the pH range of iron, copper, zinc, boron, and manganese is increased.

 

 

Chelated Microelements 

 

It would be remiss of me to talk about pH, nutrient availability and microelements without highlighting that chelation increases the acceptable pH range for microelements.

 

The word chelate is derived from the Greek word chelé, which refers to a lobster’s claw. Hence, chelate refers to the pincer-like manner in which a metal nutrient ion is encircled by the larger organic molecule (the claw), usually called a ligand or chelator.

 

Chelators, when combined with a microelement, can form a chelated fertilizer. Chelated microelements are protected from oxidation, precipitation, and immobilization in certain conditions because the organic molecule (the ligand) can combine and form a ring encircling the micronutrient. The pincer-like manner in which the micronutrient is bonded to the ligand changes the micronutrient’s surface property and favors the uptake of microelements found in hydroponic solutions. This acts to increase the acceptable/ideal pH range for microelements in solution.

 

The common forms of chelates used by many ‘hydro’ nutrient manufacturers are the synthetic chelates, EDTA (ethylenediaminetetraacetic acid) and to a lesser extent DTPA (Diethylene triamine pentaacetic acid). Chelates such as EDTA and DTPA have a high affinity for e.g. iron and generally form stable complexes with the metal across a pH range from 4 to 7.

 

Chelates have several points of attachment with which they “grasp” the trace element. EDTA has four connecting points while DTPA has five. Higher num­bers of connection points isn’t always an advantage. In some cases the four connection points may hold the element too tightly, while in a different situa­tion these may not hold it tight enough. For this reason, various chelates may prove better than others based on the ion that is chelated and the conditions in which the chelate is present.

 

The effectiveness of a chelating agent can depend on pH. In the case of iron Fe EDTA is best suited to slightly lower than neutral pH levels while Fe DTPA is most effective at higher pH values. DTPA is more costly than EDTA and less soluble and is usually found in higher quality fertilizers. DTPA is stable up to a pH of 7.5 while EDTA is stable up to a pH of approximately 6.5.

 

The most effective of the synthetic chelating agents is ethylenediaminedihy­droxy-phenylaceticacid (EDDHA). It is important to note that EDDHA can be formed only with iron and not with other essential microelements such as Cu, Zn, Mn.

 

Fe EDDHA is the most stable of all the commonly available iron chelates. This synthetic chelate is held in a bond up to 100 times tighter than DTPA because it has six molecular bonds rather than five bonds. Typically EDDHA is only found in premium fertilizers because of its higher cost. EDDHA is stable up to pH 9.0 (pH range = 4- 9).

 

In most cases combinations of chelating agents can improve stability and broaden effectiveness. That is, a mix/blend of EDTA, DTPA, EDDHA or EDTA and DTPA in formulation best ensures nutrient availability over a wide range of conditions, including those above or below optimal. For this reason, even in hydroponic growing environments where optimum pH (water temperatures etc) can be monitored and maintained, there are benefits potentially gained from using a blend of chelated elements in solution.

 

Conclusion re Optimum pH in Hydroponics

 

pH is an extremely important factor in Hydroponic gardening.  It makes your nutrients available to your plants. An unusually high pH will decrease the availability of iron, manganese, boron, copper, zinc and phosphorus. A pH that is too low will reduce the availability of potassium, sulphur, calcium, magnesium and phosphorus.

 

Based on the information that we have covered surrounding nutrient pH ranges in hydroponic settings it is possible to see that where optimum availability of all the nutrients is concerned, pH 5.5 – 5.8 offers the ideal range to work within. As previously noted, some tolerance to pH is present where adequate nutrients are in solution.

 

Optimum pH for hydroponics = 5.5 – 5.8  

 

Tolerance range where adequate nutrients are in solution = pH 5.2 – 6.3

 

 

pH Meters

 

pH meters are just one piece of equipment that is worth investing a few extras dollars in, in order to purchase a high quality scientific meter. That is, the cheaper pen meters may seem appealing based on price, but where pH meters are concerned it is safe to say that you always get what you pay for. This is a lesson I myself learned many years ago. At one point I owned several pen meters so that I could cross check pH readings. In some cases there were massive variants between the readings even though all the meters had been calibrated prior to use (I,e. one meter may have told me pH was 5.5 while another was reading pH 6.5). In the end, a year on and a hard lesson learned, I purchased a very high quality scientific meter from a laboratory supplier that in those days cost me over a $1,500.00. Cost aside, it was the best decision I ever made and I never looked back thereafter (finally accurate readings every time and confidence in the equipment I was working with). The good news is that nowadays you can get about the same quality meter for several hundred dollars through hydroponic retail stores. Speak to your retailer about product options.

 

pH-meter-combo

 

A combination pH, EC meter (top) and a quality scientific pH meter (below)

 

pH-meter-single

 

Calibrating and using a pH Meter Correctly

 

There are many factors that influence pH measurement. Compensation for or elimination of these factors is the key to accurate and precise pH measurement.

 

I’ll avoid wading into the technicalities that surrounds these factors (e.g. zero potential and theoretical values etc) but taking a pH measurement without first calibrating your pH meter is not best practice. Put simply, if you are looking for very accurate pH readings every time that you use your pH meter to monitor your solution you should calibrate first. Other than this, there are several other factors that you need to be aware of to ensure you get an accurate a pH reading.

 

Temperature plays a key role in pH measurement

 

In simple terms, pH measurement should always be performed together with temperature measurement because only pH values measured at the same temperature can be compared.

 

The pH value of the solution changes with the temperature. I.e. an increase in any solutions’ temperature will cause a decrease in its viscosity and an increase in the mobility of its ions in solution. An increase in temperature may also lead to an increase in the number of ions in solution due to the dissociation of molecules. As pH is a measure of the hydrogen ion concentration, a change in the temperature of a solution will be reflected by a subsequent change in pH. See following table.

 

Nernst-slope

 

The variation in Nernst slope with temperature for an ideal electrode

 

Because of these temperature variants, as with EC meters, when calibrating your pH meter the calibration solution temperature should be as close as possible to the nutrient solution to be tested to minimise temperature induced errors.

 

Purchase a Quality Automatic Temperature Compensated (ATC) Meter

 

Reference pH electrodes function on the basis of a chemical equilibrium between a metal and a solution of its ions which generates a potential, and which is reflected in the pH reading. This is affected by temperature because the solubility of the metal salt in the reference electrolyte solution varies with temperature.

 

This brings us back to investing a few extra dollars in quality equipment. Most good modern scientific pH meters have an auto buffer recognition facility whereby values of pH buffers at various temperatures are stored in memory. Meter standardisation and temperature coefficient of variation correction is therefore automatically done if the correct buffer is used. Meter manufacturers generally outline specific pH buffer types to be used for calibration. Be sure to read the literature that comes with the pH meter that you purchase. More importantly, be sure to follow manufacturer recommendations.

 

Speak to your hydroponic supplier for more information on ATC pH meters.

 

Practical hints for successful calibration:

 

1) Always use fresh buffer solutions – never place a used buffer back into its bottle. This will lead to a contaminated buffer solution that gives an imprecise calibration.

 

2) A two-point calibration (e.g. 4.0 and 7.0) is recommended for optimal accuracy.

 

3) Calibrating with buffers that lie within your pH measuring range increases the accuracy of the measurement. Therefore, because in hydroponics optimal pH is between 5.5 – 5.8, calibrating with pH 4.0 and pH 7.0 buffers is ideal.

 

Stirring during the calibration process also plays a role in the accuracy of the pH reading. I.e. stirring affects the pH. I have watched several growers calibrate their pH meters by using a small medicine measuring cup and then leaving the meter to sit in it without stirring for a minute or so before calibrating. I have then watched them dip the electrode of the meter into an aerated nutrient tank/reservoir and stir it around and around – often quite vigorously. This is a bad practice but one I expect many growers use. It is important to note that the stirring speed during the calibration process should be identical with the stirring speed during the pH measurement. When stirring during the pH measurement is not required, calibration should also be carried out without stirring. With hydroponic solutions it is best to calibrate first without stirring and then measure the nutrient solution pH without stirring. Further, it is best to take a sample from the nutrient tank/reservoir in e.g. a glass or medicine cup/vial and take a pH reading from this rather than placing the probe into the tank/reservoir.

 

pH-probe-med-cup

 

Best Practice for Calibration

 

  1. Thoroughly rinse the pH electrode sensing area with distilled water. Dab the electrode with a soft paper tissue to remove the distilled water (don’t rub the electrode surface – dab gently).

 

  1. Dip the electrode into buffer pH = 7, so that the diaphragm of the electrode is well immersed. If you meter has specific requirements, re temperature settings, be sure to follow the manufacturers instructions.

 

  1. Start the calibration on the meter following manufacturer recommendations.

 

  1. Rinse the electrode sensing area with distilled water. Dab the electrode with a soft paper tissue to remove the distilled water (don’t rub the electrode surface – dab gently). Dip in buffer pH = 4 and repeat the procedure.

 

  1. Take a nutrient sample from the tank/reservoir and measure the pH.

 

  1. After taking the pH reading from the nutrient solution rinse off the electrode with distilled water, put storage solution in the cap and put the cap on the end of the pH electrode.

 

 

Automatic Temperature Compensation (ATC) should always be utilized during calibration to correct for the non- Nernstian slope of electrodes. For pH meters that feature automatic buffer recognition the correct calibration buffers, as specified by the instrument manufacturer, must always be used as the meter has the temperature profile for these buffers stored in memory.

 

 

If in doubt get your pH meter checked out

 

pH meters are precise scientific equipment that rely on an electrolyte gel for reading pH. Sometimes this gel can become contaminated or dry out. What this means is that your pH readings will not be accurate. In fact, it can be miles out. Fortunately, new probes can be purchased or in many/most cases probes can be regelled. If you have any doubts about your pH meters accuracy have it looked at by your hydroponic retailer.

 

 

As a tip, I tend to keep a liquid pH test kit around in my grow room to run an occasional side-by-side test with my meter if in doubt of its accuracy. These kits are cheap (about $10.00) to purchase through hydroponic stores and are accurate enough to tell you if your pH meter reading is questionable.

 

pH meter storage (when not in use)

 

pH glass electrodes need to be kept moist at all times. Therefore, it is important that the measuring tip of the pH probe remains hydrated when not in use. pH probe storage solutions are available for this purpose. If the pH electrode dries out during storage, a regeneration procedure is required to restore the hydrated glass layer and the reference junction in order to make the electrode operable.

 

Use the solution recommended by the manufacturer. However, as a general rule, store your pH electrode in the same solution as the reference electrolyte of the electrode. In most cases this is a 3 mol/l (325g/L @ 99% purity) potassium chloride (KCl) solution.

 

pH electrode storage solutions are sold through hydroponic stores. Ask your supplier for more information.

 

 

Cleaning the pH Electrode

 

 

For optimal pH measurement response time, it may be necessary to clean the pH electrode’s glass bulb and reference junction of contaminants and precipitations. Follow the steps outlined in the pH electrode manual. Typically, it is recommended to soak the electrode for a few minutes in warm deionized water, or in a cleaning solution to maintain the probe.

 

 

PLANT NUTRIENT INTERACTIONS

 

 

mulders_chart

Mulder’s Chart (above) helps to simplify understanding the interactions between plant nutrients. Some elements work in synergy. They stimulate the uptake of other elements and increase their availability while some elements are ‘antagonistic’. They interfere with the uptake or availability of other nutrients.

 

Put simply, high levels of a particular nutrient can interfere with the availability and uptake of other nutrients. The nutrients which interfere with one another are referred to as antagonistic.

 

For example, high nitrogen levels can reduce the availability of boron, potassium and copper; high phosphorous levels can reduce the availability of iron, calcium, potassium, copper and zinc, and high potassium levels can reduce the availability of magnesium and calcium. For this reason unless care is taken to ensure an adequate and balanced supply of all nutrients too much nitrogen, phosphorus and potassium (and others) in fertilizers/nutrients can induce plant deficiencies of other essential elements.

 

Stimulation

 

Stimulation occurs when the high level of a particular nutrient increases the demand by the plant for another nutrient.

 

For example, increased nitrogen levels create a demand for more magnesium. If more potassium is used than more manganese is required etc.

 

Although the cause of stimulation is different from antagonism, the result is the same; induced deficiencies in the plant if it is not supplied with balanced nutrition.

 

Freiherr Justus von Liebig’s Law of Minumum (Liebig’s Law)

 

In 1840 German chemist, Freiherr Justus von Liebig, who made a major contribution to the science of agriculture and biological chemistry determined the ‘Law of the Minimum’, often simply called Liebig’s Law, which describes the effect of individual nutrients on crops.

 

Liebig’s Law of the Minimum is a principle developed in agriculture that states that if one of the nutritive elements is deficient or lacking, plant growth will be restricted and not in its full potential even when all the other elements are abundant. Any deficiency of a single nutrient, no matter how small the amount needed, will hold back plant development. If the deficient element is supplied, growth will be increased up to the point where the supply of that element is no longer the limiting factor. Increasing the supply beyond this point will not be helpful, as due to the laws of nutrient antagonism and stimulation some other elements would then be in minimum supply and become the limiting factor.

 

The Importance of Understanding Plant Nutrient Interactions  

 

Some hydroponic growers use staggering amounts of nutrients and additives which can potentially change the nutrient values to such a degree that the food being provided to the plant becomes highly imbalanced. As a result, antagonism and/or stimulation occur; the end result being nutrient deficiencies and lower yields. For this reason, it is important to consider just what additives you are using and what they are contributing with regards to nutrient elements in the working solution. For instance, many growers use excessive levels of PK additives for far too long. This can result in a range of deficiencies.

 

That is, excessive phosphorus will reduce the availability of iron, calcium, potassium, nitrogen, copper and zinc. This is particularly true of the microelements iron, copper and zinc. What this means is that the overuse of phosphorous in solution will potentially starve out other important nutrients/elements that are required for healthy growth/optimal yields.

 

  • Zinc (Zn) is an integral component of many enzymes. Zinc plays a major role in protein synthesis and is involved with the carbohydrate metabolic processes. Zinc is also required for maintaining integrity of biomembranes and protecting membranes from oxidative damage from toxic oxygen radicals. Severely affected plants develop small, misshapen fruits of poor quality. This is due to poor cell division early in fruit development, and fruits not getting enough sugars from photosynthesis.
  • Iron (Fe) has special importance in biological redox systems involved with chlorophyll formation and protein synthesis. Iron is essential in the enzyme system in plant metabolism (photosynthesis and respiration). The enzymes involved include catalase, peroxidase, cytochrome oxidase, and other cytochromes. Fe is part of protein ferredoxin and is required in nitrate and sulfate reductions. Fe is essential in the synthesis and maintenance of chlorophyll in plants and has been strongly associated with protein metabolism.
  • Copper (Cu) is an important component of proteins found in the enzymes that regulate the rate of many biochemical reactions in plants. Copper is an enzyme activator in plants and is concentrated in the chloroplasts of leaves, assisting the process of photosynthesis. Thus, copper plays an essential role in chlorophyll formation and is essential for proper enzyme activity.

 

 

On the other hand, too much potassium, which is absorbed rapidly from the nutrient solution, will antagonize calcium and magnesium and induces calcium and magnesium deficiencies. Keeping potassium at the appropriate levels in the root zone significantly improves calcium uptake. Large amounts of calcium are required for optimum flower development, while magnesium activates the plant enzymes needed for growth.

 

According to Hanan (1998) the essentiality of a nutrient is based on the element’s requirement for the plant to survive and reproduce – often so called “critical” level or range. In order to support optimum growth, development and yield of the crop, the fertiliser feed solution has to continuously meet the nutritional requirements of the plants.

 

Low nutrient levels will result in deficiencies, while high concentrated nutrient solutions lead to the potential for excessive nutrient uptake and, therefore, toxic effects may result.

 

This presents a very real problem given some contemporary indoor growing practices. Many people think that more is better when supplying nutrients and additives and that it is better to have excess nutrients in the solution than levels that are only adequate. This is not necessarily true and this thinking can potentially lead to serious imbalances in nutrient uptake. See following image of the nutrient response curve.

 

 

nutrient-response-curve

 

Mulder’s Chart (above) helps to simplify understanding the interactions between plant nutrients. Some elements work in synergy. They stimulate the uptake of other elements and increase their availability while some elements are ‘antagonistic’. They interfere with the uptake or availability of other nutrients.

 

Put simply, high levels of a particular nutrient can interfere with the availability and uptake of other nutrients. The nutrients which interfere with one another are referred to as antagonistic.

 

For example, high nitrogen levels can reduce the availability of boron, potassium and copper; high phosphorous levels can reduce the availability of iron, calcium, potassium, copper and zinc, and high potassium levels can reduce the availability of magnesium and calcium. For this reason unless care is taken to ensure an adequate and balanced supply of all nutrients too much nitrogen, phosphorus and potassium (and others) in fertilizers/nutrients can induce plant deficiencies of other essential elements.

 

Stimulation

 

Stimulation occurs when the high level of a particular nutrient increases the demand by the plant for another nutrient.

For example, increased nitrogen levels create a demand for more magnesium. If more potassium is used than more manganese is required etc.

Although the cause of stimulation is different from antagonism, the result is the same; induced deficiencies in the plant if it is not supplied with balanced nutrition.

 

Freiherr Justus von Liebig’s Law of Minumum (Liebig’s Law)

 

In 1840 German chemist, Freiherr Justus von Liebig, who made a major contribution to the science of agriculture and biological chemistry determined the ‘Law of the Minimum’, often simply called Liebig’s Law, which describes the effect of individual nutrients on crops.

 

Liebig’s Law of the Minimum is a principle developed in agriculture that states that if one of the nutritive elements is deficient or lacking, plant growth will be restricted and not in its full potential even when all the other elements are abundant. Any deficiency of a single nutrient, no matter how small the amount needed, will hold back plant development. If the deficient element is supplied, growth will be increased up to the point where the supply of that element is no longer the limiting factor. Increasing the supply beyond this point will not be helpful, as due to the laws of nutrient antagonism and stimulation some other elements would then be in minimum supply and become the limiting factor.

The Importance of Understanding Plant Nutrient Interactions  

 

Some hydroponic growers use staggering amounts of nutrients and additives which can potentially change the nutrient values to such a degree that the food being provided to the plant becomes highly imbalanced. As a result, antagonism and/or stimulation occur; the end result being nutrient deficiencies and lower yields. For this reason, it is important to consider just what additives you are using and what they are contributing with regards to nutrient elements in the working solution. For instance, many growers use excessive levels of PK additives for far too long. This can result in a range of deficiencies.

 

That is, excessive phosphorus will reduce the availability of iron, calcium, potassium, nitrogen, copper and zinc. This is particularly true of the microelements iron, copper and zinc. What this means is that the overuse of phosphorous in solution will potentially starve out other important nutrients/elements that are required for healthy growth/optimal yields.

 

  • Zinc (Zn) is an integral component of many enzymes. Zinc plays a major role in protein synthesis and is involved with the carbohydrate metabolic processes. Zinc is also required for maintaining integrity of biomembranes and protecting membranes from oxidative damage from toxic oxygen radicals. Severely affected plants develop small, misshapen fruits of poor quality. This is due to poor cell division early in fruit development, and fruits not getting enough sugars from photosynthesis.
  • Iron (Fe) has special importance in biological redox systems involved with chlorophyll formation and protein synthesis. Iron is essential in the enzyme system in plant metabolism (photosynthesis and respiration). The enzymes involved include catalase, peroxidase, cytochrome oxidase, and other cytochromes. Fe is part of protein ferredoxin and is required in nitrate and sulfate reductions. Fe is essential in the synthesis and maintenance of chlorophyll in plants and has been strongly associated with protein metabolism.
  • Copper (Cu) is an important component of proteins found in the enzymes that regulate the rate of many biochemical reactions in plants. Copper is an enzyme activator in plants and is concentrated in the chloroplasts of leaves, assisting the process of photosynthesis. Thus, copper plays an essential role in chlorophyll formation and is essential for proper enzyme activity.

 

 

On the other hand, too much potassium, which is absorbed rapidly from the nutrient solution, will antagonize calcium and magnesium and induces calcium and magnesium deficiencies. Keeping potassium at the appropriate levels in the root zone significantly improves calcium uptake. Large amounts of calcium are required for optimum flower development, while magnesium activates the plant enzymes needed for growth.

 

According to Hanan (1998) the essentiality of a nutrient is based on the element’s requirement for the plant to survive and reproduce – often so called “critical” level or range. In order to support optimum growth, development and yield of the crop, the fertiliser feed solution has to continuously meet the nutritional requirements of the plants.

 

Low nutrient levels will result in deficiencies, while high concentrated nutrient solutions lead to the potential for excessive nutrient uptake and, therefore, toxic effects may result.

 

This presents a very real problem given some contemporary indoor growing practices. Many people think that more is better when supplying nutrients and additives and that it is better to have excess nutrients in the solution than levels that are only adequate. This is not necessarily true and this thinking can potentially lead to serious imbalances in nutrient uptake. See following image of the nutrient response curve.

 

 

nutrient-interations-advanced-graph

 

The above image/graph provides a more complex understanding of Raviv and Leith’s nutrient response curve which shows a general crop yield-response to fertilizer application. Generally speaking, higher fertilization level gives higher yields, but only up to a certain point. Beyond this point, addition of fertilizers will not increase yields and may even reduce them as a result of too high levels of nutrient salts in the root zone which leads to toxicity.[1]

 

Looking at the nutrient response curve, for many nutrients, yield decreases before visible deficiency symptoms become apparent. This is defined as falling within the ‘hidden hunger’ and/or ‘critical range’ where the plants are being underfed but show no visible deficiency symptoms. Because the exact concentration of a nutrient below which yields decline is difficult to determine precisely, some experts define the critical level as the nutrient concentration at 90 or 95% of maximum yield. The only practical way (short of tissue analysis) to identify that a plant is being supplied with less than optimal nutrition while in the hidden hunger or critical range is to have another plant that is being fed with higher levels of nutrition to compare growth rates to. I discuss measuring growth rates against nutrient and additive regime variations on page ….. where information about running comparative side-by-side trials is covered. I highly recommend this practice (running side-by-side trials) as a means as establishing optimum nutrition for your crop.

 

Coming back to the nutrient response curve, when we increase feed levels we reach a relatively wide ‘sufficiency range’ where there is enough nutrient to ensure optimal growth. As you can see, there is some degree of tolerance within the sufficiency range where, if say we were in the middle of this range, we could marginally increase or decrease nutrient levels without impairing growth.  It is important to note, however, that the reasonable degree of nutritional tolerance only applies to some of the macroelements (e.g. N, P and K) and doesn’t apply to the microelements (e.g. Cu, Fe, Mo) where even marginally too much or too little of any one microelement can quickly result in a deficiency or excess/toxicity. To stress the point again, the microelements are far more critical in terms of their control and management than most of the macroelements, particularly in hydroponic systems. Most microelement deficiencies can usually be corrected, but when dealing with excesses (toxicity) correction can be difficult, if not impossible. Therefore, care must be taken to ensure that an excess concentration of a microelement is not introduced into the nutrient working solution.

 

Moving into the ‘luxury consumption range’ while providing no benefits to growth, this level of nutrition will not impair growth until we provide too much nutrient and, as a result, reach toxicity. Toxicity, in the more complex understanding of Raviv and Leith’s nutrient response curve, is broken into ‘incipient toxicity’ (where nutrients accumulate in the plant tissue to such a degree that they start to become toxic and growth rates begin to decline) and toxicity (where nutrients are supplied at such high levels they are immediately toxic, greatly impairing growth).

 

Last of all we have the lethal range, where nutrients are applied at such a high degree they kill the plant. This one pretty much speaks for itself, albeit toxicity symptoms would typically present well before we hit the lethal range and steps could be taken to prevent plant death.

 

For practical purposes, the point of importance is the critical level at which yield declines from an increase in nutrient concentration – the so called toxicity range at which point excessive levels of a nutrient reduce growth. If the nutrient supply is increased sufficiently, yields decline either because of an imbalance with other plant nutrients or direct toxic effects of the nutrient excess. Phosphorus, for example, at high levels can antagonize the uptake of copper, iron and zinc and be out of balance with respect to nitrogen or potassium, but it is rarely toxic per se (i.e. too much phosphorus will impair growth due to antagonizing copper, iron and zinc but phosphorus toxicity symptoms are unlikely to be apparent because P toxicity is extremely rare in most plant species). Thus, while too high phosphorus application will reduce yields through antagonizing other key nutrients such as copper, iron and zinc, visual toxicity symptoms are unlikely. How do we then know if we are over supplying phosphorus and as a result reducing yields? We don’t, unless we are able to run tissue analysis (impractical for most novice growers) or grow another clone of the plant side-by-side (i.e. an exact genetic copy being grown in the exact same environmental conditions under the same treatment with regards to feed frequency etc) using lower levels of phosphorus to compare growth rates to.  One very effective way of doing this is through running side-by-side trials.

 

Side-by-side crop trials are where identical genetics are grown under identical treatments (i.e. air temperature, RH, CO2, root zone conditions, growing system, substrate, irrigation strategy, pH, EC etc) with a single variant. This variant may be that one group of plants, the control crop, is grown with my standard nutrient and additive regime while another group of plants is grown with the same nutrient and additive regime, plus one extra additive. That is, between the two crops only a single variation in treatment exists. Therefore, if any growth and/or yield improvements occur this must have been as a result of the single variation in treatments between the two crops.  Through this system growers can then accurately ascertain/measure whether e.g. an extra additive provides anything extra to growth rates and yields. This process is called measuring through ‘scientific control’. That is, a scientific control is an experiment designed to minimize the effects of variables other than a single independent variable. This increases the reliability of the results through a comparison between the control measurement and another measurement. Scientific controls thus allow us to conclude ” the two situations were identical except for factor X. Since factor X is the only difference between the two situations, the new outcome was caused by factor X.”

 

Scientific controls (side-by-sides) are extremely handy when establishing the optimal nutrient requirements of any given crop. For example, a few plants (group X) can be grown in the same room with a larger group of plants (the control), where group X are used to compare things such as EC (i.e. does a lower or higher EC result in better yields?), additives (does a certain additive offer benefits to yields?) or nutrients (does one brand of nutrient perform better than another under side-by-side conditions?). Through this process you are then able to perfect the nutrient and additive regime that you provide to your plants.

 

Anyway, coming back to the overuse of phosphorus, in many instances where I have consulted to novice indoor growers, and where we have run tissue analysis, phosphorus excess and micronutrient deficiencies have shown in the lab results. See following…

 

cannabis-tissue-analysis

 

The tissue analysis shows excessive P and N and somewhat low (‘marginal’) K. The important thing to note, however, is the low microelement levels. After analyzing the nutrient solution that was used to grow the plant that the tissue sample was taken from we found adequate levels of Zn, Cu and B in solution (in fact, almost double the Zn and Cu that would be considered optimal) and five times more iron than would be considered optimal (explaining no signs of an Fe deficiency but also no signs of excess). As a result, after scrutinizing the tissue analysis and findings from the solution analysis we could deduce that the microelement deficiencies were caused by excess phosphorus. These deficiencies and excesses had undoubtedly impaired growth.

However, when considering our nutrient response curve and in understanding the principles of the hidden hunger, critical and nutrient sufficiency and luxury ranges – with due caution advised (I stress “due caution advised”) – we can see that it is better to have marginally too much nutrient in solution than too little. This is particularly true in recycling systems where key nutrients can become quickly depleted due to the preferential uptake of particularly N, P, K and Mn by fast growing plants with high nutrient needs. Therefore, maintaining nutrient levels somewhere in the mid to high ‘sufficiency range’ or even within the ‘luxury consumption’ range can compensate for the high uptake rates of key mineral elements, which can become quickly depleted. However, where too much nutrient is supplied, and we enter the insipient toxicity or toxic range, growth will be impaired and yield losses will result.

The key, of course, is to find the right balance between having too little or too much of any nutrient or nutrients in solution because either one of these scenarios will impact on growth.

 

Run-to-Waste v. Recycling Systems: Nutrient Requirements


Recycling System

 

In recycling systems the feed solution (water + nutrient) that is fed to the plants recycles back to the nutrient tank/reservoir and then is re-fed to the plants. During the course of each feed the plants preferentially remove nutrients at differing levels as it passes through their root systems. The nutrient that is not taken up by the plants and/or absorbed by the substrate is then returned to the nutrient tank/reservoir. Remember, at this point, the material that we have covered on Bugbee’s ‘uptake elevation of essential plant nutrients’ table (page….) where group one elements (N, P, K, Mn) are actively absorbed by roots and can be removed from solution in a few hours. Group two elements (Mg, S, Fe, Zn, Cu, Mo, and Cl) have intermediate uptake rates and are usually removed from solution slightly faster than water is removed. Group three elements (Ca and B) are passively absorbed from solution and often accumulate in solution. Keep in mind also how this effects the EC and how “EC readings do not distinguish between the different types of nutrient ions and that given N, P, K and Mn are taken up at higher rates than other elements, N, P, K and Mn may be deficient even though the EC is ideal.” 

 

Therefore, in a recycling system because some nutrients such as N, P, K and Mn are preferentially uptaken by plants at high levels while others (e.g. Ca, S, Mg) are taken at much lower levels, the nutrient that returns to the tank/reservoir is altered from the feed solution that was initially fed to them. Put simply, preferred nutrients get depleted while less needed nutrients accumulate in solution. The feed and recycling process is then repeated over and over again. On each occasion (each feed) the nutrient values are further changed as plants preferentially remove nutrients at differing levels and ratios.  As a result after several days of recycling the solution that was initially placed fresh in the nutrient tank/reservoir can be greatly altered. This can quickly lead to deficiencies in some nutrients and excesses of others. For example, as Savvas et al. (2009) put it, “an imbalance in the nutrient solution is generated by excesses of the ions least consumed by the plant (normally SO4, Ca2+ and Mg2+), which disrupts the balance of the nutrients.” [1]

 

Actually, let’s have a look (show – don’t tell) at a lab analysis of the nutrient from a recycling system after 3 weeks of not being dumped. In this case, this U.S. based ‘Med’ grower was running an interesting system which he had learnt/taken from a “respected” forum (ICMag) member where the plants were both wick fed from reservoirs/tanks below the grow pots and, additionally, top fed every 1 hour and fifteen minutes. Reverse osmosis (demineralized) water was used as the water source in the system. Two ECs/ppm were maintained throughout the entire grow. Initially, the young plants were provided with 300ppm (approx. 0.6EC) and later the nutrient was increased to 600ppm (approx. 1.2 EC) which would take the plants through until the end of bloom.  The growing media in use was ‘Turface’ – a clay aggregate substrate. As water and nutrients were removed from the system, by the plants, this grower would top up the water and bring the EC/ppm back into desired range through adding additional nutrient salts. Other than this, the grower was applying regular foliar feeds to the plants, as he put it, in a “random manner”, using multiple products made by the same agricultural manufacturer (applying plant nutrition via roots or foliar, in a “random manner”, is not recommended EVER by the way!).

 

Based on the formula being used in the nutrient tank/reservoir at 600ppm, a freshly mixed nutrient batch would have looked like this (ppm in solution of each nutrient element/ion). See following.

 

 

Elemental nutrients in solution at 600ppm total

 

N = 148.1ppm, P = 40.2ppm, K = 168.2ppm

 

S = 62.4ppm, Ca = 130.9ppm, Mg = 46.8ppm

 

Fe = 2.338ppm, B = 0.389ppm,  Mn = 0.389ppm,  Zn = 0.117ppm,  Cu = 0.117ppm, Mo = 0.078mg/L

 

The lab analysis, however, showed this.

 

nutrient-lab-anal-Tom

 

What we are looking at here is basically a nutrient disaster. N and P in solution are far too low (i.e. P is at 4.94ppm when it should be at approximately 50-60ppm while total N in solution is 1.27ppm when it should be between 100 – 150ppm); Ca is low, while and Mg is far too high; the Ca to Mg ratio is at about 1:1 when it should ideally be at 2:1; sulphur in solution is far too high; manganese is too low; copper is high; iron is far too high; zinc is at about double where it should be and the K to N ratio is miles away from where it would be at optimum. Other than this, Cl- is far too low. In very simple terms this grower is supplying his plants with extremely substandard nutrition. This situation is not too uncommon with many indoor hydroponic gardeners who are growing in recycling systems. In this instance this grower had made several key mistakes, not the least of which was taking ill-informed advice from a “respected” grow forum member (re system design and parameters). Other than the system design (which wasn’t 100% light proof, resulting in algae forming in the system) he is running the EC far too low (EC should have ideally been at about 2.4 – 2.8 (1300 – 1400ppm) in mid bloom, during the swelling/bulking phase, to compensate for the preferential removal of key nutrient ions from solution) and most importantly he wasn’t dumping the nutrient and replacing it with fresh nutrient anywhere near regularly enough. This, along with algae (which feed on N and P), led to nutrient exhaustion where excesses of some elements and depletion of others, based on the lab work, are patently apparent.
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This situation can become even more pronounced where using mains (municipal) water in the recycling system. Most mains water contains a variety of ions/elements. While this may not be immediately detrimental to plant growth, in combination with continuous nutrient solution recycling it can contribute to ion buildup over time and interfere with accurately monitoring nutrient ions in solution;[1] i.e. some ions found in mains water, at relatively high levels (e.g. sodium, chloride, calcium and magnesium), are not taken up in large amounts by plants and accumulate in solution. Keep in mind that the EC measurement is only for total ion concentration and cannot be used to determine individual nutrient ion concentrations. Therefore, as sodium (Na), chloride (Cl), calcium (Ca) and magnesium (Mg) ions accumulate in solution and we maintain the recycling nutrient tank/reservoir EC with top ups of mains water, adding further Na, Cl, Ca and Mg at each top up, Na, Cl, Ca and Mg take the place of all important nutrient ions such as N, P and K in the total EC. To give you some idea, in Australian water supplies, sodium concentrations range from 3 to 300ppm, while chloride levels range up to 350ppm depending on local source characteristics.[2] To contextualize this, 500ppm is approximately 1EC. What this means is that before even adding nutrient concentrate to the tank/reservoir, the mains water that you are using in your hydroponics system could have an EC of approximately 0.5 – 1.0 of ions that are, 1) potentially toxic to plants and, 2) unwanted by plants, or only required/uptaken at very low levels. These ions, largely then remain in solution and when we top up the nutrient tank/reservoir in between nutrient dumps/changes we potentially add a further 0.1 – 0.5 EC (dependent on the volume of water required to top up the tank/reservoir and the municipal water quality) of largely unwanted nutrient ions which replace highly desirable ions such as N, P and K in the EC.
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As a result, an imbalance can quickly occur due to buildup of ‘sodium chloride’ (common table salt) and Ca and Mg that is introduced into the nutrient solution via the mains water. The outcome is deficiencies of critical nutrient ions and excesses of largely unwanted ions, even though the EC might be ideal.

 

Other than this, many mains water supplies are treated/disinfected with chloramines. Chloramines are very persistent and take a long time to dissipate from treated water which means they can build up in hydroponic systems and cause root damage and phytotoxicity. These symptoms are very similar to many root diseases. Due to this growers aren’t necessarily aware of what is actually causing the problem.

 

This is the reason that I have always promoted the use of reverse osmosis (RO) filtered water in Integral Hydroponics. RO filtration removes the unwanted ions from the water equation and provides pure H2O which means RO water helps offset the nutrient imbalances that can rapidly occur with unfiltered mains water in recycling systems. For example, as Dr Howard Resh (2013), an author and expert in the field of hydroponics, has stressed, utilization of some type of filtration system (such as a reverse osmosis unit) is commonly advised as it removes most impurities from whatever water source is used.[3]

 

It is important to note that RO water filtration not only removes undesirable elements from mains water but also some elements that are desirable. For example, because RO filtration removes carbonates (alkalinity) from the water supply this tends to make the nutrient solution more pH unstable than where mains water is used. I go into far greater detail about this on pages …… where “Hydro for Hydro” is covered.  Be sure to read this material before using RO water in your hydroponic system.

 

 

Run-to-Waste Systems

 

In run-to-waste (RTW/DTW) hydroponic systems nutrient from the reservoir/tank is fed to the plants and the nutrient and water that isn’t taken up by the plants or absorbed by the substrate is not returned to the nutrient tank/reservoir, but instead runs off into a catchment tank/reservoir as waste. This waste is then disposed of and not run through the system again.
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What this means is that the nutrient being fed to the plants is not altered by the plants and then returned to the tank/reservoir. Therefore, nutrient deficiencies and/or excesses are unlikely to occur and any such nutrient excesses or deficiencies in a RTW/DTW system would be a result of adding an imbalanced nutrient or too little or too much nutrient to the tank/reservoir in the first instance.

 

What this means is that when compared to recycling systems RTW/DTW growing provides a far higher degree of control over what is fed to the plants.

 

 

Fertigation Frequency in RTW/DTW Systems and Nutrient Requirements   

 

Having compared RTW/DTW to recycling systems, the next important thing to understand is that optimum nutrient requirement (EC of the nutrient in the reservoir/tank) in RTW/DTW hydroponic systems is greatly influenced by ‘feed frequency’, or the amount of times fresh nutrient from the reservoir/tank is supplied to the plants over the course of a day.

 

In short…

 

Frequent application of water and nutrients in RTW/DTW systems ensures better nutrient status in the root zone. This is because frequent feeding prevents the formation of a ‘nutrient depletion zone’ (the zone around roots in which the concentration of nutrients is lower than in the rest of the substrate because of the uptake by roots) between each feed.[4] Thus, high frequency feeding can serve as an efficient means of enhancing crop yield by improving the uptake by plants of less mobile nutrients.[5] This is particularly true during periods of the most vigorous growth where plants have high water and nutrient needs.  For example, in research with bell pepper (2012) it was shown that increased feed frequency had little effect on growth rates of young vegetative plants, “possibly because the nutrient requirements are relatively low during the early stages of growth.” However, at later stages of growth when the plants were fruiting and water and nutrient demands increased higher feed frequency resulted in increased growth.[6]

 

Silber et al. (2005) have suggested that high fertigation frequency improves the uptake of nutrients through two main mechanisms: continuous replenishment of nutrients in the depletion zone at the vicinity of root interface and enhanced transport of dissolved nutrients by mass flow, due to the higher averaged water content in the growth media.[7] ‘Mass flow’ occurs when nutrients are transported to the surface of roots by the movement of water in the substrate (i.e. percolation, transpiration, or evaporation). The rate of water flow governs the amount of nutrients that are transported to the root surface. Therefore, mass flow decreases as substrate water decreases. Conversely, mass flow increases as the substrate water increases.

 

 

The Nutrient Uptake Process and ‘Mass Flow’
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For nutrient uptake to occur the individual nutrient ion must be in position adjacent to the root. This process of positioning occurs through three mechanisms:
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  • Mass flow – the most important of these mechanisms, quantity wise, is where nutrient ions that are present in the substrate solution (water + nutrient ions) flow to the root
  • Diffusion – is movement by normal dispersion of the nutrient from a higher concentration (such as near its dissolving mineral source) through nutrient solution to areas of lower concentration of that nutrient.
  • Root interception – is the extension (growth) of plant roots into new areas where there are untapped supplies of nutrients in the nutrient solution

 

All three processes are in constant operation during growth. The importance of each mechanism is in supplying nutrients to the root surface for absorption by the root varies with the chemical properties of each nutrient. However, in hydroponics because of the large amounts of water flowing to and absorbed by roots as water is transpired from the plant, mass flow is the dominant mechanism that supplies about 80% of most nutrients to root surfaces.

 

Research has shown that increased feeds significantly increases plant yield, especially at low nutrient concentration.[8] This yield improvement was primarily related to enhanced nutrient availability in the root zone. Studies have shown that a higher feed frequency maintains a higher dissolved nitrogen, phosphorus and potassium concentration in the substrate solution by shortening the period during which nitrogen, phosphorus, and potassium retention takes place in the root zone.[9]

 

Phosphorus (P) nutrient status is particularly improved under high frequency feeds.  Studies have shown that yield improvement as a result of high frequency feeds is primarily related to the enhanced nutrient uptake of P.[10]

 

Yield outcomes related to P and high frequency feeding are shown to be very similar to that of less feeds with higher levels of P in solution.[11] What this means, for practical purposes, is that if hand watering once or twice daily in a RTW/DTW situation, it would be recommended to apply higher levels of nutrient (a feed solution with a higher EC and more phosphorus) than where high frequency irrigation is used.

 

Frequent application of water and nutrients ensures that the root surface and its vicinity are well supplied with fresh nutrient solution during feeds and subsequent feeds.[12] That is, for nutrient uptake to occur, the individual nutrient ion must be in position adjacent to the root.[13] As the period between feeds becomes longer, the nutrient concentration in the root zone may be high or even excessive immediately after irrigation but may fall to below sufficiency level as time proceeds. These processes are time dependent; therefore, reducing the time interval between successive irrigations to maintain constant, optimal water/nutrient content in the root zone reduces the variation in nutrient concentrations, thereby increasing their availability to plants and reducing their leaching out of the root zone. See following image…

 


 

 

Nutrient-conc-spec-cond

Schematic representation of the time variation of nutrient concentration under conventional conditions (Silber, 2005) 

 

The time variation of nutrient concentration under conventional conditions graph by Silber (2005) demonstrates that directly following irrigation a fast surface reaction occurs where nutrients directly available to the root surface are absorbed within a short timeframe. Initially the levels of some nutrients available to the plant may be excessive; however, quite quickly nutrient levels at the root surface fall below the sufficiency range and a deficiency can occur with some nutrient ions between irrigations. Frequent application of water and nutrients ensures that the root surface and its vicinity are well supplied with fresh nutrient solution offsetting the likelihood of nutrient deficiencies between irrigations.

 


 

 

In research with bell pepper, increasing the numbers of feeds was shown to affect growth by increasing plant height, stem diameter and leaf area. It was apparent that by increasing the feed frequency from 5 to 10 and to 20 feed events per day increased bell pepper growth. The author of this study noting:

 

“Increasing fertigation frequency could serve as an effective means of enhancing crop growth and yield, by improving the nutrient uptake by plants. This study showed that fertigation with high irrigation frequency (20 irrigation events day) increased yield of bell pepper significantly over low fertigation frequency (5 irrigation events day). This accounted for 22% increase in yield.”[1]

 

Better plant growth as a result of high fertigation frequency in RTW/DTW growing has also been documented by several other authors.[2]

 

In understanding the principle of high fertigation frequency providing better nutrient status in the root zone think of things like this. Traditionally nutrient manufacturers in the hydroponics industry have recommended running high ECs (e.g. 2.8 to even 3.0 EC in bloom) in order to ensure enough nutrients are made available to the plant between feeds. For example, one coir supplier recommends that growers should irrigate their coir once a day (twice a day where growing larger plants) with a nutrient at an EC of about 2.2.

 

High fertigation frequency differs somewhat to this approach and adequate nutrients are made available by regularly supplying fresh nutrients at small intervals between feeds, at lower EC. If you like, we are increasing nutrient levels by decreasing the times between feeds, rather than supplying higher levels of nutrients (measured through the EC of the solution) with longer intervals between feeds. Through this means we are able to control the levels of nutrients provided to the plants by increasing or decreasing the numbers of feeds inline to the crops nutrient demand/requirements at different phases of the crop cycle. For example, a plant in grow has far lower water and nutrient requirements than a plant in full bloom. Therefore, while the plant is in grow we may provide nutrient at 0.8 – 1.0 EC (where using RO water as the water source) 3-4 times a day while when the same plant is in full bloom we may increase feeds (inline to its increased water and nutrient requirements) to 12 – 16 lighter feeds a day with nutrient at an EC of 1.2-1.4. What this system affords us is better control over supplying water and nutrients to cater for the different physiological stages of the crop cycle, better nutrient status in the root zone and lower salt build up in the root zone [3] when compared to feeding at higher EC with longer intervals between feeds.

 

It is important to note that when discussing high fertigation frequency in substrate based growing (e.g. coir or peat) there is an extremely important relationship between the substrate’s physical properties and optimal irrigation numbers. I go into far greater detail about this in the ‘Substrate Science’ HERE …..   Be sure to read this material before employing high fertigation frequency as a growing practice.

 

How the High Fertigation Frequency Principle Applies to Recycling Systems

 

Perhaps the most popular recycling hydroponic system on the market today is RDWC (Recirculating Deep Water Culture e.g. ‘Bubble Buckets’). There’s possibly a very good reason for this in that they perform better than many other recycling systems because the roots of the plant are in contact with the nutrient solution at all times which essentially means there is no nutrient depletion zone around the roots of plants being grown in RDWC systems. Comparatively, in say a top fed expanded clay system irrigations only occur at e.g. 15 minutes once every hour and a half. This leaves one hour and 15 minutes between irrigations in a substrate that holds very little/water nutrient. Therefore, in one system (RDWC) you have no nutrient depletion zone (re the nutrients available in solution) while in the other system (top fed expanded clay) some nutrients at the root face undoubtedly become depleted between irrigations.

 

Bubble-Buckets

 

Feed Requirements of RTW/DTW v. Recycling

 

Because critical nutrient ions can become quickly depleted in recycling hydroponic systems, studies have shown that plants grown in RTW/DTW systems require less nutrients (lower ppm/EC of nutrient in the tank/reservoir) than the same crops that are grown in recycling systems.

 

For example, in research by Gül et al (2008) it was shown that optimal growth with tomato could be achieved in a RTW/DTW system with a half strength nutrient solution when compared to a recycling system. The authors concluding that this may be the result of the depletion of some elements in the recycling system.”[4] A study by Ferrante el al (2000) came to similar conclusions. It was shown that nutrient concentrations in the substrate are lower in recycling systems than in RTW/DTW systems.[5]

 

What this boils down to is that optimal growth can be achieved in hydroponic growing situations when low nutrient concentrations are maintained (i.e. just within sufficiency range of each nutrient ion species) continuously and never allowed to deplete.[6] This is far more achievable in RTW/DTW growing where fresh, unadulterated nutrient is fed to the plants at each and every feed.

 

Therefore, the nutrient requirements of RTW/DTW systems differs from that of recycling systems and nutrients in RTW/DTW can be run at a lower EC than where recycling.

 

Additionally, because nutrients are not recycled and do not become depleted RTW/DTW growing offers far more nutrient stability and feed control. This is just one reason why I promote RTW/DTW growing in Integral Hydroponics. Quite simply, RTW/DTW growing, when handled correctly, promotes a better nutrient status in the root zone than does recycling.

 

Keep this in mind as we go through the following material surrounding the overuse of phosphorous in solution. In this discussion I will be demonstrating that 60ppm of phosphorus in solution during mid to late flowerset is about optimal in RTW/DTW growing where regular fertigation is used (e.g. I feed 12 times over the 12 hour light cycle with 10% runoff during the full bloom/swelling phase). Therefore, if I were to have 60 ppm of P in solution, each plant in the RTW/DTW system would receive 60ppm of P at each and every feed and these feeds would occur on a regular basis. This would ensure a good P status is maintained in the root zone at all times.

 

This situation does not apply to recycling systems where nutrient tank/reservoir practices differ amongst growers. That is, phosphorus (P) is uptaken by plants at high levels. If this P is not replaced daily in solution, at the same levels it is uptaken at, a P deficiency may occur over several days. So, for example, let’s say that we start with 60ppm of phosphorus in solution in a recycling system and the plants in total remove 30 ppm of P per day from this solution. Let’s also say, for this example, the nutrient tank is just topped up with water (no additional nutrients) daily to replace the volume of solution that the plants remove from the nutrient tank/reservoir every day. Therefore, on day one we would begin with 60ppm of P in solution; on day two we would begin with 30ppm of P in solution; on day 3 we would begin with 0ppm of P in solution. Therefore, within three days, under these conditions, the plants are being starved of phosphorus. Keep in mind that this is a hypothetical scenario. In reality, various factors such as plant numbers, plant size, amount of nutrient (i.e. litres/gallons of nutrient per given plant numbers) and growth stage/phase would all influence P uptake rates from solution. However, the point is that P can become quickly depleted in recycling systems if not maintained or replaced at adequate rates.

 

Given this information, for recycling system growers, due to the preferential uptake of some nutrients at high levels and the importance of maintaining adequate root zone nutrient status of phosphorus during flower, higher than 60ppm of P in solution at any one time during full bloom (the swelling phase), is more ideal. While not optimal this will not necessarily create excess where yields are reduced. To understand this, consider the nutrient response curve where there is a luxury range for a nutrient. This range won’t increase growth but neither will it create excess which impacts on growth. So, for instance, using Raviv and Leith’s nutrient response curve, 50ppm of phosphorus in solution may be at the beginning of the optimal phosphorus nutrient range (maximum efficiency) while 120ppm of phosphorus may be at the far-end of the luxury range where yields begin to decrease as a result of excess. See following image of hypothetical phosphorus range based on Raviv and Leith’s nutrient response curve.

 

Nute-Hypothetical-P-Curve

 

Based on the hypothetical nutrient response curve for phosphorus, optimal yields can be realized with phosphorus in solution at between 50-120ppm. What this means is that a reasonable tolerance can exist for some macronutrients and a little too much (within the luxury range but below the excess/toxicity level) in solution is more ideal than too little where deficiencies may occur. Under luxury nutrient conditions, nutrient concentration may be high or even excessive initially but may fall to below sufficiency levels as time proceeds.

 

It’s somewhat of a compromise situation, but one that recycling system growers need to be aware of.

 

Therefore, when disseminating the following material keep in mind that I am talking about RTW/DTW growing where fresh nutrient is delivered to the plants at each and every feed. Further, I am talking about RTW/DTW growing where high frequency irrigation is used and not in a RTW/DTW situation where say plants were watered by hand once daily. I.e. as has been previously noted:

 

 “Yield outcomes related to P and high frequency feeding are shown to be very similar to that of less feeds with higher levels of P in solution.[1] What this means, for practical purposes, is that if hand watering once or twice daily in a RTW/DTW situation, it would be recommended to apply higher levels of nutrient than where high frequency irrigation is used.” 

 

One other important factor to note is that where I discuss plant nutrition optimums these relate to non CO2 enriched environments where slightly higher levels of nutrition are optimum.

 

That is, it is important to note that depending on the crop and the growing methodology, the increased growth rate related to CO2 enrichment may require the nutrient solution to be applied at a higher electrical conductivity (EC).

 

For example, elevated CO2 is shown to increase N requirement.[2] Sucrose and starch concentration generally increase in the leaves of plants grown at above- ambient CO2 and this in part explains the decrease of leaf N concentration – a dilution effect.[3] Other than this, stomatal conductance is altered in elevated CO2 and transpiration is decreased which some authors believe reduces N mass flow. Additionally, this decrease in transpiration reduces calcium (Ca) and boron (B) uptake which may affect flower/fruit quality. Increased applications of these nutrients, within reason, will adequately compensate for decreased uptake.

 

Additionally, P demand increases due to elevated levels of CO2; i.e. several studies have reported that both the magnitude and the direction of the growth response of plants to elevated CO2 depend on P availability.[4], [5] Elevated CO2 is likely to affect the internal P requirement of plants because elevated CO2 alters P utilization within plant tissues (leaf phosphorus demand is thought to increase with increasing CO2).[6]

 

As a general rule, when running a CO2 enriched environment run your EC at 0.2 – 0.4 higher than normal. E.g. if normally running an EC of 1.8 in a non- CO2 enriched environment increase this to EC 2.0 to 2.2. This is particularly relevant when using CO2 in flower when nutrient demand is high. Keep this mind when reading the following material. More on growing in CO2 enriched environments is covered on page….

 

The following information is included in Integral Hydroponics to demonstrate how some additives can greatly affect all-important nutrient levels and ratios. This is an important topic to cover because far too many growers apply additives to their reservoir at extremely high levels, in an almost random manner, not stopping to consider just what this means in terms of plant nutrition. However, based on the theory we have covered to date you can perhaps see that this is a potentially dangerous practice that can result in excesses that ultimately impair growth.

 

Anyway, let’s take a look at the additive factor.

 

Author’s note: I go into further detail on optimal RTW/DTW growing practices and additives when discussing my preferred method of growing on pages ….  

 

 

The Additive Factor and the Overuse of Phosphorus by Indoor Hydroponic Growers

  

Let’s look at the additive factor by taking an off the shelf nutrient and then throwing in additives to demonstrate what occurs. My apologies in advance; this is where things get extremely technical and I introduce new information and terminology that we haven’t covered before. For what it’s worth don’t get caught up in the jargon (e.g. %w/v, ppm, nutrient ratios) or the equations and, instead, just skim the material looking closely at the tables which help to clarify things. I’ll go into much more detail on this subject later in the chapter when discussing nutrient labels and guaranteed analysis numbers and how to convert these numbers into establishing ppm in the nutrient working solution (i.e. the nutrient that is being fed to the plants).

 

In the following examples, I’ll be dissecting things by using the %w/v of various nutrients and additives and calculating ppm in the ‘working solution’. Additionally, in some examples I’ll list a final EC mS/cm. This EC value is based on the U.S. standard of 500ppm being approximately 1 EC. Where NPK ratios are listed these are approximate values.

 

For this example we’ll use House and Garden (H&G) Coco nutrient and then add PK 13-14 and silica. The NPK %w/v numbers that I’ll use for the H &G Coco come from lab analysis we conducted in 2010 (98% accuracy on elemental values). All following ppm in working solution and EC’s are based on using nutrients and additives in demineralised (RO) water.

 

See following table for %w/v analysis of A + B House and Garden Coco nutrient.

 

H&G Coco % w/v   A + B Nutrient Concentrate

 

Total N 6.3%
Total P 1.6%
Total K 6.8%
Total Ca 4.8%
Total S 1.4%
Total Mg 1.33%

 

 

Used at 3ml/L (i.e. 3ml nutrient to 1L of water) this would give us in solution:

 

N = 189ppm

P = 48ppm

K = 204ppm

Ca = 144ppm

S = 42ppm

Mg = 39.9ppm

 

N to P ratio = 4:1     N to K ratio = 1:1    K to P ratio = 4.25:1      NPK ratio = 4 – 1 – 4.25

 

Let’s now add PK 13-14 as P205 and K20 at 1.5ml/L and see what occurs with regards to NPK (ppm) values in solution. Because the product is listed as P as P205 and K as K2O this equates to an elemental P and K of 5.59- 11.62.

PK 13-14 @ 1.5ml/L

P = 83.85ppm

K = 174.3ppm

 

H&G Coco Nutrient @ 3ml/L + PK 13-14 @ 1.5ml/L equals: 

N = 189ppm

P = 131.85ppm

K = 378ppm

 

N to P ratio = 1.4:1     N to K ratio = 0.5:1      K to P ratio 2.8:1   NPK Ratio = 1.4 – 1- 2.8

 

You’ll note that by comparing the NPK ratios of the H&G Coco nutrient against the nutrient plus PK 13-14 that the addition of PK 13-14 has made a significant difference to NPK levels and ratios of what nutrients the plants are receiving. See following tables.

 

Total Nitrogen (N) ppm in solution Phosphorous (P)

ppm in solution

Potassium (K)

ppm in solution

Coco Nutrient @ 3ml/L 189ppm 48ppm 204ppm
Coco Nutrient + PK 13-14 189ppm 131.85ppm 378ppm

 

 

N to P Ratio N to K Ratio K to P Ratio NPK Ratio
Coco Nutrient @ 3ml/L 4:1 1:1 4.25:1  4 – 1 – 4.25
Coco Nutrient + PK 13-14 1.4:1  0.5:1      2.8:1  1.4 – 1- 2.8

 

I’ll avoid going into too much detail at this point as I discuss more about the use of PK additives on pages …. . However, you can perhaps see that by using a single PK additive the nutrient solution (elements delivered to the plant) has significantly changed from when just using nutrient alone. This potentially has implications where nutrient antagonism and/or stimulation is concerned.

 

Now, let’s also add a silica product into the mix. It is important to note that most hydroponic store sold liquid silicon (silicate) products are made using potassium silicate (K2SiO3). Potassium silicate contains 18.204% Si and 50.685% potassium (K). This means that by adding e.g. 30ppm of Si to solution we are adding 83.5 ppm of K to solution. The addition of potassium through the use of potassium silicate needs to be considered in any optimized nutrient regime.  Lets’ have a look at the effect that adding silicon at 30ppm has on our nutrient working solution.

Nutrient @ 3ml/L + PK 13-14 @ 1.5ml/L + Si at 30ppm equals:

N = 189ppm

P = 131.85ppm

K = 461ppm

 

N to P ratio = 1.4:1     N to K ratio = 0.45:1      K to P ratio 3.5:1

 

See following tables for comparison of coco nutrient alone, coco nutrient plus PK 13-14 and coco nutrient plus PK 13-14 and silica.

 

Total Nitrogen (N) Phosphorous (P) Potassium (K)
Coco Nutrient @ 3ml/L 189ppm 48ppm 204ppm
Coco Nutrient + PK 13-14 189ppm 131.85ppm 378ppm
Coco Nutrient, PK 13-14 and Silica 189ppm 131.85ppm 461ppm

 

N to P Ratio N to K Ratio K to P Ratio NPK Ratio
Coco Nutrient @ 3ml/L 4:1 1:1 4.25:1 4 – 1 – 4.25
Coco Nutrient + PK 13-14 1.4:1  0.5:1 2.8:1 1.4 – 1- 2.8
Coco Nutrient, PK 13-14 and Silica 1.4:1 0.4:1 3.5:1 1.4 – 1 – 3.5

 

 

According to Spensley et al. (1978) a typical flower/bloom nutrient solution for tomato production has the following composition: N: 150-200 ppm, P: 30- 40 ppm, K: 200-300 ppm, Mg: 40-50 ppm and Ca: 150-200 ppm and Fe: 5 ppm.[7]  This is supported by the University of Florida who recommend: N: 150ppm, P: 50ppm, K: 200ppm, Mg: 50ppm, Ca: 150ppm and S: 60ppm [8]

Therefore, if we were growing tomatoes with H&G Coco nutrient and PK 13-14 the nutrient profile would be miles out (far too much phosphorous) and this would, potentially, cause nutrient antagonism or stimulation to iron, calcium, potassium, nitrogen, copper and zinc. The end result would be nutrient deficiencies and reduced yields. It’s one of the paradoxes of additives. When used correctly at the right times and the appropriate levels they are beneficial to yields. However, when used incorrectly (too high rates etc) they can cause nutrient antagonism and/or stimulation and thereby reduce yields. That’s not to say that additives don’t increase fruit quality and yields. For example, the use of fulvic acid, silica, beneficial bacteria and/or fungi, low levels of potash and various hormone based growth or bloom enhancers all can aid in the quest for optimum yields (read more about additives that enhance on pages…..).

 

However, given tomatoes, we would have been far closer to optimum NPK nutrition by just using H&G Coco nutrient at 3ml/L alone. See following table.

Optimum Tomato Nutrition (Bloom) H and G Coco @ 3ml/L
N = 150 – 200ppm N = 189ppm
P = 30-50ppm P = 48ppm
K = 200ppm K = 204ppm
Ca = 150ppm Ca = 144ppm
Mg = 40-50ppm Mg = 39.9ppm
Total ppm = 650ppm Total ppm = 624.9ppm
EC mS/cm (approx) 1.3 EC

 

EC mS/cm (approx) 1.249 EC

 

 

N to P Ratio N to K Ratio K to P Ratio NPK Ratio
Coco Nutrient @ 3ml/L 4:1 1:1 4:1 4.-1-4.
Optimum Tomato Plant Nutrition for mid to late Bloom 3:1  0.75:1 4:1 4-1-4

 

Doctor Peter Keating, a PhD biochemist (plant physiology) and nutrient expert, makes this point about the mineral nutrition of higher plants.

(Quote)
;br]

“Plants grown by hydroponics experts have very high quality and very high yields – near to their genetic potential, and tend to have mineral nutrient levels about 20 – 30% higher than “Ideal” levels published in various guides for the elements potassium, calcium and phosphorus. All the other elements tend to be at, or perhaps no more than 10% higher that the “ideal” levels.

 

These “ideal” levels do not vary much between all monocots (grasses) between all dicots (tomatoes etc) and between all gymnosperms (pines), but are different between the groups. Give any plant excess above what it needs, and it will not get any bigger or better. Give it high excess above its needs and you will subject it to toxicity and/or osmotic stress. As a result, growth rates will be affected and yields will be reduced.”  (Dr Peter Keating, PhD biochemistry/plant physiology)

 

(End Quote)

 

Dr Keating raises an extremely good point where he states that the ideal levels do not vary much amongst plants of the same species. For example, tomato is a dicot and when looking at the nutrient requirements of other dicot species we are able to see that the nutritional requirements of each dicot is similar to others. This is particularly true of phosphorus where the P requirements of various dicot species range between 30 – 50ppm. See following table.

 

Recommended Major Element Concentrations in Nutrient Solutions by Crop in mg/L/ppm

 

Crop Nitrogen Phosphorus Potassium Calcium Magnesium
Cucumber 230 40 315 175 42
Eggplant 175 30 235 150 28
Tomato 200 50 360 185 45
Lettuce 200 50 300 200 65
Melon 186 39 235 180 25
Pepper 175 39 235 150 28

 

Source: Schon, M, 1992, in Proceedings of the 13th Annual Conference on Hydroponics, Hydroponic Society of America, ed. D. Schact, 1992, Hydroponic Society of America, ed. San Ramon, CA

 

Author’s note: Upon dissecting H&G Coco’s ppm in solution I was impressed with their coco formula and would recommend it as a coco bloom nutrient over a few other coir formulas I have looked at through lab analysis and ppm in solution equation. However, if used with RO (demineralized) water be sure to add ‘Cal/Mag’ (calcium and magnesium) to the solution. I’d advise you to contact H&G directly for their recommendation as to how much Cal Mag to add when using RO water.

 

Other than this, if growing for an extended period of time at 18/6 (vegetative) I’d recommend to use a ‘Veg’ formula over the H&G Coco which has a relatively high K to N ratio (i.e. it is a coco bloom formula). Speak to your hydroponic retailer for further information about coco grow/vegetative formulas.

 

Medical Marijuana and Phosphorus

 

Actually, for novelty’s sake let’s also have a look at cannabis which is yet another dicot, or technically an annual dioecious (unisexual) flowering plant. As medical and recreational marijuana is now being legalized in many U.S. States and Canada, ‘Med’ is also important to discuss given the phosphorus factor in hydroponic solutions and its implications to legal cannabis.

 

Please note that if you’re growing cannabis outside of this legal framework, or if you are under the age of 18, by-pass this material. That is,,,, WARNING: the use and/or cultivation of cannabis is a criminal offence and failing to comply with the law may result in persecution, prosecution, harassment, entrapment, the presumption of guilt before innocence, incarceration, job loss, a criminal record, asset seizure, bankruptcy, possible extortion, threats against loved ones, intimidation, stand over, relationship breakdown, police surveillance, tedium, paranoia, sweats, vomiting, and eventual death. However, oddly (paradoxically) in some parts of the world cannabis is seen as an amazing medicine with many therapeutic properties while in other parts of the world it is considered a dangerous hard drug. While this situation is highly confusing, “Marihuana is a short cut to the insane asylum” (Harry J Anslinger) “unless it is medicine” (Glow). For those of you growing legal medicine or legal ‘rec’ read on. For those of you who are growing a “deadly new drug called skunk” (Channel 9, 60 Minutes, Australia) “just say no” (Nancy Reagan) and go to page….

 

Phosphorus and Medical Marijuana

 

Comprehensive tissue tests/analysis conducted in 2003 by Advanced Nutrients through BC Research Inc on several cannabis strains demonstrated that marijuana plants require far less P than is present in many hydroponic nutrient and additives formulations. The tests also demonstrated that N, Ca and K are required at far higher levels than P and P, while required at somewhat higher levels in bloom than grow (as is shown in numerous studies surrounding dicots), is required at far lower levels than would be expected. Another surprising outcome was N requirements in flower were higher than previously believed.

 

Based on the Advanced Nutrients data, cannabis tissue presents, when looking at just one set of genetics that were tested (White Rhino/Medicine Man), an optimal NPK ratio of approximately 6.4 – 1 – 4.5 during vegetative stage and 4 – 1 – 5 in bloom. You’ll note in the case of the latter that this NPK ratio isn’t so dissimilar to tomato plants which, dependent on author, has an optimal bloom nutrient requirement of approximately 4- 1- 4.25 to 4- 1- 5.5 NPK.

 

Other than this, Szarvas shows that hemp tissue tests present an NPK nutrient ratio of: N at 5-6%, P at 0.5 – 0.6% and K at 2.7-3.0 %, or an NPK ratio of approximately 10 – 1 – 5.4. See following table.

 

Plant nutrient element status for hemp (Szarvas. 1999, 2003)

 

Nutrients Low Satisfactory High
N% < 5 5 – 6 >6
P% < 0.5 0.5 – 0.6 >0.6
K% < 2.7 2.7 – 3.0 >3.0
Ca% < 2.4 2.4 – 3.0 >3.0
Mg% < 0.6 0.6 – 0.8 >0.8
Fe (mg kg-1) < 65 65 – 105 >105
Mn (mg kg-1) < 85 85 – 130 >130
Zn (mg kg-1) < 25 25 – 40 >40
Cu (mg kg-1) < 2 2 – 5 >5

 

And from Starcevic, 1979, see following.

 

phosphourous-med-1

Source: STARCEVIC, Lj.: A study of relations between some anions and cations (N, S, P, K, Ca and Mg) andtheir effect on yield, fiber quality and content in different parts of hemp plant (in Serbo-Croat, with summary in English). Proceedings of Natural Sciences of Matica Srpska 57, 109-172, Matica Srpska, Novi Sad, Yugoslavia (1979)

 

Again, as with the tissue analysis conducted by Advanced Nutrients these findings show that the phosphorus requirements of cannabis are inline to other dicot species and that N and K are in far higher demand than P. Additionally, as with the Advanced Nutrients research, this data indicates that phosphorus is being overused by many medical marijuana growers.

 

Therefore, to reassert the point and bring you back to Mulders chart and nutrient antagonism, excessive phosphorus will reduce the availability of iron, calcium, potassium, nitrogen, copper and zinc. This is particularly true of the microelements iron, copper and zinc. What this means is that the overuse of phosphorous in solution will lock out other important nutrients that are required for healthy growth and optimum yields.

 

Of course, some might rightfully say at this point, “but yes, theory aside, PK 13-14 works great” (and it does). However, what this really comes down to is; 1) the principle of luxury consumption, where more nutrient than is required isn’t benefitting growth, but nor is it at the level where it is toxic and; 2) phosphorous toxicity symptoms are rarely seen in plants so where oversupplying P growth rates/yields may be reduced while no excess P symptoms are present; 3) flowering/fruiting plants have high K requirements in mid to late bloom and PK 13-14 provides high levels of K. I.e. used at 1.5ml/L PK 13-14 contributes 174.3ppm of K to the nutrient working solution.

 

Having looked at H&G Coco nutrient which has an elemental NPK ratio of 4-1-4 and having established that optimal medical marijuana nutrition in bloom is approximately NPK 4-1-5, there is simply no need to add further phosphorus to solution through the use of phosphorus containing additives. Given that coco substrate naturally contains some levels of potassium that will contribute more K to the H&G 4-1-4 NPK ratio in the substrate/roots (I.e. potassium in the nutrient + potassium in the coir substrate), what is perhaps needed to achieve optimal nutrition for medical marijuana, when using H&G Coco nutrient, is just a fraction more potassium that could easily be added to the nutrient working solution through the use of a K only additive or fertilizer (e.g. potassium citrate or potassium hydroxide).

 

Actually, let me make a very important point here. NPK ratios mean very little without considering the ppm in solution of each nutrient species. For example, based on the cannabis tissue analysis conducted by Advanced Nutrients™ a 4- 1 – 5 NPK ratio is about ideal for indica or indica dominant varieties of cannabis. However, a 4 – 1 – 5 NPK ratio could mean we have 4ppm of N, 1ppm of P and 5ppm of K in solution. Or it could mean that we have 20ppm of N, 5ppm of P and 25ppm of K. So while the NPK ratio is ideal with these ppm values the plants would quickly starve. However, a 4 – 1 – 5 NPK ratio could also represent this.

 

Cannabis Nutrition Figures   

 

NO3 N =160ppm

P = 40ppm

K = 200ppmm

 

Now let’s compare these figures to other dicot species.

 

Recommended Major Element Concentrations in Nutrient Solutions by Crop in mg/L/ppm

 

Crop Nitrogen Phosphorus Potassium
Cucumber 230 40 315
Eggplant 175 30 235
Tomato 200 50 360
Lettuce 200 50 300
Melon 186 39 235
Pepper 175 39 235

 

 

Now let’s increase our 4-1-5 cannabis nutrition figures to match somewhere around the ppm in solution NPK nutritional requirements of tomato. To do this I will add 25% to all of ‘Cannabis Nutrition’ figures meaning that we haven’t changed the NPK ratio but instead simply applied equivalent amounts by percentage per nutrient species of N, P and K to solution.

 

NO3 N =200ppm

P = 50ppm

K = 250ppmm

 

Now compare these NPK figures to the NPK requirements of tomato. Are we getting anything from this yet? What we can see is that both dicots have very similar nutrient requirements, albeit that at least based on one hydroponic nutrient manufacturer’s tissue testing tomato theoretically has higher K requirements than cannabis (i.e. the K to N ratio ideal re plant nutrition is higher for tomato than cannabis… at least on paper).

 

Before moving on and discussing, among other things, optimum plant nutrition for various dicot crops there is a couple of things that should be said first with regards to optimized medical marijuana nutrition.

 

Firstly; it has been my experience that each different set of genetics perform best under marginally different nutrition to achieve optimal yields (genetic potential). This has been supported by the Advanced Nutrients tissue test data where it was shown that some variance of nutritional requirements existed between the different genetics that were tested.

 

As such, there is no one size fits all nutrient regime for the numerous genetic variations of a single genus.

 

Other than this, most experienced growers who have worked with numerous strains would understand and attest to the fact that some genetics perform extremely well under conditions of high levels of nutrition while at the same time displaying high levels of tolerance to nutritional variations while other genetics tend to be extremely finicky (fussy) regarding nutrient requirements and difficult to grow to optimum. Due to this, where it comes to optimizing a nutritional regime for a given strain some careful (conservative/cautious) experimentation with nutrients and additives is advised.

 

Secondly; largely due to the marketing of a just a few hydroponic companies there is a belief amongst some sections of the indoor hydroponic growing community that the nutritional needs of medical marijuana differs significantly from other dicots. Other than this, according to some, seemingly this dicot acts differently from all others and needs innumerous additives (which of course these same companies supply at a premium to growers), beyond a good standard plant nutrient and the careful use of just a few additives (e.g. a root stimulant, low levels of potash and silica), to realize optimum yields.

 

For example, one North American based multinational tells hydroponic consumers that their competitors’ nutrients are developed for tomato and cucumber crops and as a result, when using their competitors’ products many hydroponic consumers are getting inferior yields (re weight and quality). What this does is create a mindset amongst some sections of the largely chemistry and plant physiology non-formally educated hydroponic retail consumer demographic that the nutritional requirements of each dicot plant is vastly different from the other. However, for anyone who understands nutrient chemistry and plant physiology and who has dissected this company’s formulas along with many of their competitors formulas through laboratory work these claims are extremely suspect (read more about this in “About Hydroponic Nutrients” on pages….).

 

Having looked at House and Garden Coco nutrient and established that its NPK is about optimum for tomato production, and having looked at what is optimum for medical marijuana cultivation, it is easy to conclude that the nutrient requirements of tomato and cannabis aren’t so dissimilar.

 

Given, as Dr Keating, points out – “These “ideal” levels do not vary much between all monocots (grasses) between all dicots (e.g. tomatoes) and between all gymnosperms (pines), but are different between the groups.” – this is perhaps not too surprising.

 

Why Then the Overuse of Phosphorus in Hydroponics?

 

Indoor hydroponic growers are applying too much phosphorous for three key reasons; 1) based on hydroponic nutrient and additive manufacturer recommendations they are purchasing and using products that contain too much phosphorus; 2) many hydroponic PK additive formulations are based on outdoor agricultural practices, which simply don’t apply in hydroponic growing and; 3) due to the fact that hydroponic nutrients and additive regimes -feed chart recommendations – were developed to cater for a vast array of growing methodologies (e.g. recycling versus run-to-waste growing) these regimes/recommendations are, at best, a compromise to provide reasonable plant nutrition for a multitude of growing situations.

 

On point two (i.e. many hydroponic PK additive formulations are based on outdoor agricultural practices), while chemically phosphorus is a very stable element, fertiliser phosphorus does not move far from where it is applied in soils. Because the mobility of phosphorus in soil is very limited plant roots can take up phosphorus only from their immediate surroundings.

 

Additionally, phosphorus is easily bound in soils, making it unavailable for plant uptake. In acidic soils (below pH 5.0) phosphorus tends to react with aluminum, iron and manganese, forming insoluble compounds (e.g. aluminum phosphate) while in alkaline soils the dominant fixation is with calcium – forming insoluble calcium phosphate.

 

Because phosphorus is so easily bound in soil, crops and pasture take up only 5–20% of phosphorus applied to the soil.

 

However, this situation does not apply in hydroponics where phosphorus remains highly available in solution and where about 80 – 90% of the P in solution is available for plant uptake (once irrigated into the root zone). From this, we can deduce that the excessive P values in our N-P-K ratios are not necessary for indoor hydroponic gardens where readily bioavailable phosphorus maintains a high level of contact within the root zone of plants grown in substrates and in containers commonly found in indoor hydroponic settings.

 

On point three (hydroponic nutrients and additive regimes – feed chart recommendations – were developed to cater for a vast array of growing methodologies) having looked at the fact that plants have preferential nutrient uptake needs and some nutrients are taken up by plants at far higher rates than others, off-the-shelf hydroponic feed regimes need to accommodate for this reality.

 

For example, where recycling systems are used, certain nutrients stand to become depleted over a given number of days with elements such as N, P, K and Mn being removed from solution at very high levels, while other elements such as Ca and S are removed from solution at lower levels. This means that for adequate phosphorus to remain available in solution, where recycling is concerned, higher levels of P than would otherwise be desirable/optimal in a RTW/DTW system, where regular fertigation is applied, are required in solution from the outset (to offset the potential for P depletion/exhaustion in the nutrient tank/reservoir over any given number of days).

 

Having looked at Raviv and Leith’s (2008) nutrient response curve we can see that at low concentration, small increases in nutrient availability results in large changes in growth (A). Further increase in nutrient has smaller effect on growth as the nutrient level approaches optimal level (B). At some point additional amounts do not increase growth. This is the range of luxury consumption (C). At high levels toxicity is reached and growth diminishes (D).[1]

 

nutrient-response-curve

 

The question is, of course, at which point does growth diminish? How many ppm of phosphorus can we safely have in solution before yields decrease?

 

The bottom line here is that, while various factors will influence optimum P in solution, growers may be wiser to use PK additives that contain comparatively lower levels of phosphorus to potassium (i.e. PK 9-18 versus PK 13-14).

 

To the hydroponic industry’s credit they have done an extremely good job of formulating nutrients and additives, and creating feed charts that cater for a vast number of growing situations; however, at best, this approach is a compromise, and in understanding plant nutrition you, yourself, can do much better and tailor (optimize) your nutrient regime for the growing methodology of your choice.

 

As a tip, your plants should be dark green and healthy until the flush. Some of the older, larger shade leaves may be pale to yellow and this is natural. However, all of the lateral leaves should be dark green with no signs of deficiencies and/or excesses (i.e. no yellowing, burning, patching, interveinal chlorosis or other signs that indicate nutrient deficiencies or excesses). Put very simply, a healthy plant tells you it is a happy plant and a happy plant will reward you with optimal yields. Therefore, the best way to finish your product is to maintain healthy leaves until the flush.

 

The Problem with the Overuse of Phosphorus in Solution in Medical Marijuana Crops

 

There are several problems associated to having too much phosphorus in solution. Firstly, high P levels are associated to plant stretch. That is, as University Professor Paul V. Nelson (Department of Horticultural Science, North Carolina State University) points out: “low phosphate levels result in compact plants, while high phosphate levels result in tall plants.” [1] It is my belief that this factor, along with the prevalent use of HPS lamps alone (versus blended spectrum HPS + MH lighting), and the all too common practice of inadequate plant spacing is, at least, in part, responsible for the high use of likely harmful to human health synthetic PGRs (subclass growth retardants) amongst medical marijuana growers. I’ll avoid discussing synthetic PGRs here as I go into more detail about them on page… when covering material on synthetic PGR free ways of reducing stem elongation (stretch), just one of these being to reduce phosphorus ppm in solution during the stretch phase of the crop cycle.

The second key issue that presents with the overuse of phosphorus is that high levels of P will antagonize other key nutrients such as nitrogen, potassium, iron, copper and zinc. This can lead to nutrient deficiencies and reduced yields.

 

Thirdly, another issue that potentially presents with the overuse of phosphorus in medical marijuana crops is that P may remain residual at high levels in the plant tissue if overused.

This has implications where combustible crops are concerned; i.e. burning phosphorus with sufficient oxygen results in the formation of phosphorus pentoxide (P4O10). Phosphorous pentoxide is an irritant to the skin, mucous membranes, and respiratory tract/system even at concentrations as low as 1 mg/m3. What this means is that if high levels of residual phosphorus is present in a combustible crop (after drying and curing) the produce when ingested via inhalation will be harsh and chemically tasting. This may have health implications for the end user if they are ingesting phosphorous pentoxide on a regular basis.

 

Last, but by no means least, another key problem regarding the use of phosphate fertilizers are the heavy metals’, arsenic, cadmium, selenium, cobalt, mercury and lead, with cadmium being the most notable of these where plant uptake and health implications are concerned.

 

The problem here is that the phosphate rock derived fertilizers (e.g. monopotassium phosphate, monoammonium phosphate) that are used in hydroponic nutrients for providing P are a key source of cadmium (Cd) contamination in fertilizers. That is, phosphate fertilizers are shown to possess more Cd contaminants than other fertilizers which are used in producing hydroponic nutrients and additives. As an absolute rule, the higher the heavy metal levels in media and fertilizers, the higher the heavy metal uptake by plants. I.e. heavy metal concentration in soils, substrates and hydroponic solutions is the dominant factor in heavy metal plant tissue contamination. Therefore, reducing the use of phosphorus in solution will reduce the levels of heavy metal contaminants that are available for plant uptake.

 

Jarup, L (1998) notes that the population group at the highest risk of cadmium exposure is tobacco smokers. The absorption of cadmium in the lungs is 10-50%, while the absorption in the gastrointestinal tract is only a few percent. Smokers have about 4-5 times higher blood cadmium concentrations (about 1.5 micrograms/l), and twice as high kidney cadmium concentrations as nonsmokers.[2] The national geometric mean blood cadmium level for adults is 0.47 μg/L. A geometric mean blood cadmium level of 1.58 μg/L for New York City smokers has been reported. The amount of cadmium absorbed from smoking one pack of cigarettes per day is about 1–3 μg/day. Direct measurement of cadmium levels in body tissues confirms that smoking roughly doubles cadmium body burden in comparison to not smoking.[3] This information has telling implications for consumers of combustible crops.

 

Anyway, let’s rewind this a bit. All this talk of parts per million (ppm) in solution and nutrient ratios, no doubt, sounds something like a foreign language to some (many) readers. Don’t worry – there is a method to my madness. My aim in discussing the overuse of phosphorus and demonstrating this from a ppm in solution perspective was to tweak your interest and demonstrate the importance of understanding how to dissect nutrients and additives to establish what nutrients (ppm) are being delivered to the plant.

Once we have all our environmental parameters (e.g. air temp, relative humidity, CO2, light) in check getting the nutrients right makes the difference between achieving genetic potential and lack luster yields – or, in a worst case scenario, catastrophic failure.

 

In order for any plant to achieve genetic potential (whether that be medical marijuana or tomatoes or peppers or cucumbers etc) optimum nutrition to cater for the specific needs of the crop throughout its entire lifecycle is required. The uptake and utilization of nutrients depends not only on quantities, but also on the ratios of the nutrient types. If a particular nutrient is deficient, yields can be negatively affected. A similar reduction in plant growth can arise when a nutrient is present at a concentration that is too high. All the nutrient ion types need to be within their ideal ranges if yields are to be optimized. Departure from the ideal levels of any one nutrient will have an influence on all others as well.  In the case of phosphorus, we have seen that high levels of P in solution at the wrong times can result in plant stretch. Other than this, while phosphorus toxicity is rarely seen, too high phosphorus application can result in deficiencies of nitrogen, potassium, iron, copper and zinc. This is particularly true of iron, copper and zinc.

The key to successful management of a plant nutrient program is to ensure adequate concentrations of all nutrients throughout the life cycle of the crop. Inadequate or excessive amounts of any nutrient results in impaired growth and reduced yields.

 

Optimized nutrient management programs should begin with an understanding of the nutrient solution concentrations in parts per million (ppm) for the various nutrients required by the crop of choice. By managing the concentrations of individual nutrients, growers can maintain optimal nutrition in solution.

 

Therefore, understanding how to establish the ppm of each nutrient in solution better enables us to provide optimal nutrition to our crop. Further, in understanding how to establish the ppm of nutrients in solution we are able to dissect a hydroponic nutrient (an off-the-shelf product) and analyse its suitability for use in growing a given crop. For example, prior to this we looked at the ppm in solution of H&G Coco nutrient and were able to conclude, 1) that it was fairly ideal as a bloom nutrient for use with several dicot crops (e.g. tomato, cucumber, medical marijuana); 2) that when used with RO water it would require the addition of some Cal Mag and; 3) it ideally would require a boost with some potassium and low levels of phosphorus during the swelling/bulking phase to provide ideal levels of P and K.

 

Other than dissecting nutrients, in understanding how to equate the ppm of nutrients in solution we are able to establish what additives contribute to the nutrient solution re specific nutrients when applied at a given ml per litre (or ml per gallon) usage rate.  This better enables us to monitor exactly what nutrition is being provided to the plants.

 

Reliable information can be found on the Internet (academic sites etc.) and in literature that outlines optimal nutrition for various crops such as tomato, cucumber and peppers. This information is typically expressed in ppm of each nutrient in solution. So, for example, when looking at various dicot crops the recommended ppm in solution may look something like this.

 

Recommended Major Element Concentrations in Nutrient Solutions by Crop in mg/L/ppm

 

Crop Nitrogen Phosphorus Potassium Calcium Magnesium
Cucumber 230 40 315 175 42
Eggplant 175 30 235 150 28
Tomato 200 50 360 185 45
Lettuce 200 50 300 200 65
Melon 186 39 235 180 25
Pepper 175 39 235 150 28

 

Source: Schon, M, 1992, in Proceedings of the 13th Annual Conference on Hydroponics, Hydroponic Society of America, ed. D. Schact, 1992, Hydroponic Society of America, ed. San Ramon, CA


The micronutrients should remain at the same concentration throughout the life of the crop. Optimum concentrations for the micronutrients are: Boron 0.44ppm, Copper 0.05ppm, Manganese 0.62ppm, Molybdenum 0.06ppm, Zinc 0.09ppm, and Iron 2.5 ppm.

Given this information, through being able to analyze ppm in solution we are then able to compare these numbers to our own nutrient and additive regime to check that the nutrient we are providing to our plants is within ideal parameters.

As such, let’s look at how to establish ppm in solution now. 

 


 

 

Phosphorus Deficiency Symptoms

Purpling and reddening: accumulation of anthocyanin pigments causes an overall dark green color with a purple, red (particularly in the stems of the plant), or blue tint, and is the common sign of phosphorus deficiency.

Be aware that the reddish-purple color does not always indicate phosphorus deficiency, but may be a normal plant (genetic) characteristic.

Other than this, towards the end of the crop cycle you may notice purpling in the older leaf material. This is does not necessarily indicate a phosphorus deficiency. P is a mobile element and the P in the larger shade leaves is being translocated to the flowers/fruit where it is most needed for creating biomass.

Overall stunting (reduced growth rates) of the whole plant is a sign of all nutrient disorders.

Phosphorus Excess Symptoms


Phosphorus excess can be expressed as potential deficiencies in nitrogen, potassium, iron, zinc, and copper – with particular emphasis on copper, iron and zinc. Where a nitrogen deficiency occurs the foliage becomes yellow (chlorotic) starting in older leaves. Some crops (e.g. tomatoes) may show a reddish colour instead of yellow. Potassium deficiency is expressed as localized necrosis (“firing,” “leaf burn,” or death of patches or spots or margins on leaves). Iron deficiency can be identified through interveinal chlorosis beginning on younger leaves. Zinc deficiency can be identified through shortened internodes; young leaves are small, and they may show interveinal chlorosis. Copper deficiency can be identified through leaves curling downwards, the tips and margins of the leaves may exhibit coppery gray or slightly blue discolorations with a metallic sort of look. In between the veins, the leaves may yellow, new growth may have a hard time opening and leaves may appear small.

 


 

 

Establishing ppm in Solution from Hydroponic Nutrient Labels

 

 

nutrient-label-Ca-Compliant

 

 

Above is an example of a nutrient label (“guaranteed analysis”) that would be compliant to California ‘specialty fertilizer’ regulations. I’ve used the California compliant label as an example because many multinational companies (those supplying their products worldwide) tend to adhere to U.S. (particularly Californian) standards where they aren’t in conflict with local fertilizer guaranteed analysis regulations in other regions. Therefore, in many locales, this is what a nutrient label will look like. 

Firstly, the guaranteed analysis tells us how much of each element (NPK etc) is in the hydroponic nutrient concentrate at percentage weight by volume (%w/v). So, for example, when looking at a single part, full spectrum hydroponic nutrient the guaranteed analysis may look something like this.

Guaranteed Analysis:

 

  • Total Nitrogen (N): 2.0%, 0.09% Ammoniacal Nitrogen, 1.91% Nitrate Nitrogen
  • Available Phosphate (P2O5): 2.0%
  • Soluble Potash (K2O): 3.0%
  • Calcium (Ca): 2.5%
  • Magnesium (Mg): 0.5%, 0.5% Water Soluble Magnesium (Mg)
  • Sulfur (S): 1.14%, 1.14% Combined Sulfur (S)
  • Manganese (Mn): 0.05%, 0.05% Water Soluble Manganese (Mn)
  • Molybdenum (Mo): 0.0005%

 

 

This provides us with enough information to dissect that nutrient and establish a reasonably (I stress “reasonably”) accurate ppm in solution.

 

By saying “reasonably accurate”, one important thing to highlight here is that analyzing the ppm in solution from fertilizer labels typically won’t provide 100% accurate ppm in solution numbers because what is listed on a label’s guaranteed analysis doesn’t necessarily always represent what is actually in the bottle. In other cases it may not even be close. That is, due to labeling ‘compliance’ regulations and issues such as batch irregularities – largely due to sloppy production standards or simply more a case of non compliant labelling – the nutrients that you are actually adding to solution can be somewhat different from what is listed on the label. For example, where labeling compliance regulations are concerned, fertilizers sold across the US and elsewhere are often only required to be listed accurately to within 0.4% at best to their listed NPK ratios. That is, they can only be below the labeled guarantee percent by this percentage. So if a nutrient has 1% in solution, manufacturers can often get away with listing 0.6% on the label.

 

When looking at Colorado fertilizer listing regulations, listing requirements are a sliding scale based on the guaranteed percent that is listed. For example, if the guaranteed percentage is 4% or less, a product needs to be listed within 0.49% for N, 0.67% for P, and 0.41% for K. As the guaranteed percentage rises, so does the range of error. This caps at that guaranteed percentage of 32% or more which allows for an error of 0.88% for N, 0.76% for P, and 1.44% for K. This also means manufacturers can get away with not listing the NPK ratio if the product is below 0.49% for N, 0.67% for P, and 0.41% for K. For calcium and sulfur, manufacturers don’t even have to list them on the guaranteed analysis if they are below 1% for calcium and below 0.5% for magnesium. For micro nutrients, the cutoff is different with each ion species having their own value assigned to them. All of this makes it difficult to know exactly what an off the shelf hydroponic nutrient or additive really contains.

 

To meet CDFA (Californian Department of Food and Agriculture) compliance, every nutrient is simply required to be labeled above a guaranteed minimum. What this means is that is a nutrient were to say contain 4% N the manufacturer could list N on the label as 3.5% and because the actual N in the product was 0.5% more than listed this product would meet labelling compliance.

 

Basically, many nutrients and additives are labeled in an ambiguous way where manufacturers aren’t required to even specify that micronutrients are in the product. Essentially the legislation surrounding nutrient labeling allows the manufacturers to be deliberately vague about what a formula actually contains.

 

Given this, if the data on the labels isn’t 100 percent correct what you equate from this data isn’t strictly accurate where ppm in solution is concerned. However, in most cases with the macronutrients the labels guaranteed analysis listings are reasonably accurate and will provide close to actual ppm in the nutrient solution figures.

 

Besides this, from a practical perspective, without lab analyzing a nutrient (about $40 per solution in the U.S) for its macro and micronutrient guaranteed analysis, listings on labels is all we have to work with in establishing the ppm of each nutrient species in solution the product provides when used at a given ml/L usage rate.

 

In other words, analysing the ppm in solution from the guaranteed analysis is not perfect and in some cases will be far from perfect, but when compared to EC measurements it provides us with a far more accurate picture of what is actually in solution with regards to nutrients (species and levels).

 

This said, for those of you that really want to understand precisely what ppm of nutrients are in solution there are two ways of doing this; 1) lab analyse the nutrient and work from this lab analysis or 2) produce nutrients and additives yourself at home. You’ll find a lot of information on www.manicbotanix.com about nutrient and additive formulation. We even supply formulation software and have online calculators in order to help growers formulate at home. Check out the ‘DIY Formulation’ link on the menu for formulation know-how and the ‘Manix Calculators’ link to access the four calculators we have programmed to date. Additionally, to check out the software (HydroWare) visit the Manix HydroWare link.

 

Of course, this approach (formulating yourself) isn’t for everyone so let’s look at dissecting nutrient and additive labels to establish what they provide re ppm in solution.

 

 

Interpreting the Qualities of a Nutrient from the ‘Guaranteed Analysis’

 

Fertilizer formulations are defined and listed by manufacturers in percentages, either as percent weight by volume (%w/v) or percent weight by weight (%w/w). For now, %w/v is the important measurement, so let’s focus on this.

Various regulatory bodies around the world require that NPK etc values to be presented in a somewhat ambiguous fashion. Therefore, listings for the same nutrient or additive may appear to vary on a country-by-country or, even, state-by-state basis. For example, when looking at our California compliant label ‘guaranteed analysis’ you will note that it states; 1) “Available Phosphoric Acid (P2O5) and 2) “Soluble Potash (K2O)”. This information becomes important when interpreting the guaranteed analysis.

 

That is, it is important to note that the P and K numbers found on the guaranteed analysis do not always reflect the actual amounts of elemental phosphorous and potassium by percentage. With our California compliant label example this is the case and P (phosphorus) is listed as P2O5 (phosphorous pentoxide) and K (potassium) is listed as K2O (potassium oxide) % w/v. When looking at nutrient’s label/s check them closely to see whether they list P as P2O5 and K as K2O because this becomes important in interpreting the data and using this data to establish any nutrients qualities.

 

When phosphorus is listed as P2O5 it is only 43% elemental P and when potassium is listed as K2O it is only 83% elemental K.

 

Therefore, when this system is in use, a 20-20-20 NPK ratio fertilizer truly reflects elemental NPK 20- 8.6- 16.6.

 

However, in some other countries (e.g. Australia) you may find that some labels list P and K as actual elemental P and K. This means that in some countries NPK ratios will reflect actual (elemental) NPK ratios while in other countries the P and K numbers will be much higher.

 

This situation becomes even more confusing in places such as Europe where P and K can be listed as elemental P and K or as P205 and K2O or both. Additionally, other nutrients such as calcium (Ca), magnesium (Mg), sodium (Na) and sulfur (S) can be listed in their oxide form (CaO, MgO, Na2O, SO3) or in elemental form, or both.  It’s a crazy situation (no single universal standard) but one that you need to be aware of.

If a product is listed as 20% “P as P2O5” and 20% “K as K2O”, convert K2O to elemental K by multiplying by 0.83 and convert P2O5 to elemental P by multiplying by 0.43.

 

E.g. NPK (P as P2O5 and K as K2O) = 20-20-20

 

N = 20 (elemental value)

P as P2O5 = 20 x 0.43 = 8.6

K as K2O = 20 x 0.83 = 16.6.

 

= NPK (elemental) 20- 8.6- 16.6

 

To convert other nutrient listings that may appear on some labels use these equations.

 

Ca0 to Ca multiply by 0.714

MgO to Mg multiply by 0.6031

SO3 to S multiply by 0.4

 

Understanding Percentage Weight by Volume (%w/v) Listings

 

In chemistry the concentration of a chemical solution (e.g. a hydroponic nutrient concentrate) refers to the measure of the amount of solute that is dissolved in a solvent. We normally think of a solute as a solid that is added to a solvent (e.g. adding table salt to water), but the solute could just as easily exist in another phase. For example, if we add a small amount of ethanol to water, then the ethanol is the solute and the water is the solvent. If we add a smaller amount of water to a larger amount of ethanol, then the water could be the solute.

 

Rule 1. In chemistry, a solution is the mixture of two or more substances that are dissolved and mixed until homogenous. The mixture is made up of a solute dissolved in a solvent. E.g. nutrient salt (solute) is dissolved in water (solvent).

 

Rule 2. In chemistry, concentration is used to express the amount of a given substance mixed with another substance. This applies to any sort of chemical mixture of solids, liquids or gases, but more frequently is used to measure active ingredients in homogenous solutions (the amount of solute in the solvent).

 

The way we measure how much solute is in a given solution is typically via %w/v (weight by volume) or %w/w (weight by weight). The concentration can also be measured in lots of other units like moles, but for now you need to know about %w/v.

 

Percentage Weight by Volume (%w/v)

 

A simple way of understanding how to convert a %w/v listing found on the guaranteed analysis into grams per litre is by understanding that 1ml of demineralized water weighs 1gram. Therefore 1000ml (1L) of demineralized water weighs 1000 grams.

Percentage weight by volume (%w/v) refers to the total weight of elements contained within a finished concentrate of a given total volume. For example, 10%w/v equals 10% of an element incorporated/integrated within a total volume of 1000ml (i.e. water plus element equals 1000ml).

 

So, in the case of a 1 litre nutrient concentrate that lists total N as 10%w/v this would mean that there is 100grams of N in 1Ltr of nutrient concentrate.

 

A handy sum for equating %w/v is:

 

1000 (ml nutrient concentrate) times 10% (%w/v listing) = 100 (grams N)

 

By the way, when using this sum on a calculator be sure to use the % button to arrive at the final calculation.

 

More Examples of %w/v

 

0.5%w/v equals 5 grams per litre

1.0%w/v equals 10 grams per litre

2.5%w/v equals 25 grams per Litre

5.0%w/v equals 50 grams per litre

 

Establishing how many parts per million of a nutrient is added to the nutrient working solution by using the guaranteed analysis of a given nutrient concentrate at a given ml/L usage rate

 

This is perhaps more chemistry than the average novice grower needs right now (enter at your own risk). However, I will cover it in depth for the more advanced grower. Knowing these equations will enable the reader (you) to dissect hydroponic nutrient formulas and establish their suitability for use with various crops. This said, for the hydroponic newbie and advanced grower alike, you’ll find a calculator on the Integral Hydroponics website at www.integralhydroponics.com that does these calculations for you. Simply use this calculator by entering how many ml/L of nutrient you are using; then enter the data from the guaranteed analysis (%w/v or %w/w with SG) and push “calculate”. The calculator will then tell you how many ppm of each element is in the nutrient working solution.  It’s a handy little calculator we devised and programmed for hydroponic growers. Other than telling you the ppm in solution it converts ml/L to ml per U.S. gallon. Additionally, there are calculators on the same page that convert K2O to elemental K and P205 to elemental P, and ppm to %w/v and vice versa. This said, for those of you who want to understand the chemistry let’s run the sums. You may find it helpful to grab a pen and paper and calculator and work through things as we go.

 

Converting %w/v to ppm and ppm to %w/v

 

To establish ppm from %w/v you simply need to multiply by 10,000. I.e. 3 (%w/v) x 10,000 = 30,000 (ppm)

 

To establish %w/v from ppm you simply need to divide by 10,000. I.e. 30,000 (ppm) ÷ 10,000 = 3 (%w/v).
Nutrient element concentration in the working nutrient solution yielded by a specific dose working from guaranteed analysis at %w/v

To establish the concentration of individual elements in the ‘working nutrient solution’ (the solution that is being fed to the plants), the guaranteed analysis %w/v specification should first be converted into ppm, then multiplied by the usage rate (per litre), then divided by 1000. For example, if a nutrient lists 2.5% nitrogen (N), when it is used at 4ml per litre it will yield 100ppm N in the nutrient working solution.

 

That is:

 

Step 1. N = 2.5 (%w/v) x 10,000 = 25,000 (ppm)

 

Step 2. 4ml per litre yields 100ppm. I.e: 25,000 (ppm) x 4 (ml/L) ÷ 1,000 (ml) = 100 (ppm).

 

Keep in mind that where elements such as phosphorus (P) are listed as P2O5 and potassium (K) is listed as K2O you first need to convert P205 to elemental P and K2O to elemental K before using these equations.

 

So if our guaranteed analysis listed 2%w/v available phosphoric acid as P205 we first convert P2O5 to elemental P with the sum 2 (% P205) x 0.43 = 0.86 (% elemental P).

 

Then we would establish the ppm using the sum 0.86 x 10,000 = 8,600 (ppm of elemental P).

 

Then at 4ml/L we use the sum 4 x 8,600 ÷ 1000 = 34.4 (ppm elemental P in the working solution).

 

i.e.

 

Step 1.  P as P2O5 = 2%w/v  =  2 x 0.43 = 0.86 (elemental P)

 

Step 2.  0.86 (%w/v) x 10,000 = 8,600 ppm (elemental P in concentrate)

 

Step 3. 4 (ml/L) x 8,600 ÷ 1000 = 34.4 (ppm elemental P in working solution)

 

Comparison of Two Single Part Nutrients – Flairform GreenDream Bloom versus Botanicare CNS17 2-2-3 Bloom

Okay, so let’s now put the ppm in solution theory into practice to demonstrate how we can establish what we are feeding to the plant/s from the guaranteed analysis. We’ll begin by comparing two single part bloom formulas against one another and then lining them both up side-by-side against the optimum nutrient ppm in solution required by the tomato plant (mid to late bloom). 

A single part nutrient is a full spectrum hydroponic nutrient in a single bottle and this is reflected on the labels guaranteed analysis. So, for instance, when looking at Botanicare CNS Bloom Formula NPK 2-2-3 the guaranteed analysis reads:

 

Guaranteed  Analysis:

 

  • Total Nitrogen (N): 2.0%, 0.09% Ammoniacal Nitrogen, 1.91% Nitrate Nitrogen
  • Available Phosphate (P2O5): 2.0%
  • Soluble Potash (K2O): 3.0%
  • Calcium (Ca): 2.5%
  • Magnesium (Mg): 0.5%, 0.5% Water Soluble Magnesium (Mg)
  • Sulfur (S): 1.14%, 1.14% Combined Sulfur (S)
  • Manganese (Mn): 0.05%, 0.05% Water Soluble Manganese (Mn)
  • Molybdenum (Mo): 0.0005%

 

 

While the guaranteed analysis of Flairform GreenDream single part bloom reads:

 

Total Nitrogen (N)          2%

0.1% Ammoniacal Nitrogen

1.9% Nitrate Nitrogen

Available Phosphate (P2O5)      2%

Soluble Potash (K2O)                6%

Calcium (Ca)                             1.4%

Magnesium (Mg)                       0.6%

0.6% Water Soluble Magnesium (Mg)

Sulfur (S)                                  1.2%

 

 

Flairform Green Dream has an NPK ratio of 2- 2- 6 and Botanicare CNS17 Bloom has an NPK of 2- 2- 3.

 

Keep in mind that by North American standards P is listed as P2O5 and K is listed as K2O. This means that if the products were listed in elemental P and K the would be 2- 0.86- 4.98 (Green Dream) and 2- 0.86- 2.49 (CNS17).

 

When comparing these two products macronutrients side-by-side it looks like this in elemental form (i.e. P2O5 as elemental P and K2O as elemental K). See the following table.

 

Nutrient Elemental Values %w/v Botanicare CNS17 2-2-3 Flairform GreenDream Bloom
Total N 2.0% 2%
Ammoniacal Nitrogen 0.09% 0.1%
Nitrate Nitrogen 1.91% 1.9%
Elemental Phosphorous (P) 0.86% 0.86%
Elemental Potassium (K) 2.49% 4.98%
Calcium 2.5% 1.4%
Magnesium 0.5% 0.6%
Sulfur 1.14% 1.2%
Total Nutrients %w/v 9.49% 11.04%

 

Let’s now break this down into ppm in solution when each product is used at 5.5ml/L. I’ll also compare the ppm in solution from the CNS17 and Flairform Green Dream to optimum ppm in solution for tomato plants in mid to late bloom. See the following table.

 

Element CNS17 @ 5.5ml/L in working solution (ppm) Flairform @ 5.5ml/L ppm in working solution Optimum Tomato Nutrition mid/late bloom
Total N 110ppm 110ppm 150-200ppm
Ammoniacal Nitrogen 4.95ppm 5.5ppm ———-
Nitrate Nitrogen 105.05ppm 104.5ppm ———-
Elemental Phosphorous (P) 47.3ppm 47.3ppm 30-50ppm
Elemental Potassium (K) 136.95ppm 273.9ppm 200ppm
Calcium 137.5ppm 77ppm 150ppm
Magnesium 27.5ppm 33ppm 40-50ppm
Sulfur 62.7ppm 66ppm 60ppm
Total Nutrients ppm 521.95ppm 607.2ppm 710ppm
EC mS/cm @ 500ppm = 1EC 1.04 1.21 1.42

 

 

Interpreting these Results

 

What we are looking at here is two single part bloom nutrients made by different manufacturers. In this example, the Flairform product (made in Australia and sold internationally) is 16% more concentrated than the Botanicare product (made in North America) so if you were paying the equivalent price for 1L of each product you’d be getting 16% more bang for your buck if purchasing the Flairform product. However, there are more things to consider than price. For instance, if one product yields a few percent more produce than another product which is marginally cheaper, saving a few dollars (price of nutrient) to lose even more dollars (based on yield) is simply bad mathematics. This said, it is a math that far too many indoor growers apply and thus lose yield and profit as a result.

 

Besides the concentration differences, we are looking at quite different formulations. For instance, the calcium (Ca) to magnesium (Mg) ratio in the Botanicare CNS17 is 5:1 while the Ca to Mg ratio in the Flairform Bloom is 2.3:1 (rounded down = 2:1). CNS17 is quite unusual on this front. To touch on the Ca to Mg ratio of hydroponic nutrients, many plants have a Ca to Mg ratio of about 2:1 in their tissues. Because of this, a nutrient formulated for these plants should reflect this and contain about a 2:1 Ca to Mg ratio. Most single part nutrients typically have a 1.5:1 to 2.5:1 Ca to Mg ratio, so the 5:1 Ca to Mg ratio in CNS17 presents as higher than is normally found in many other single part hydroponic store sold nutrients.

 

Additionally, the N to K ratios are significantly different between the two products. In the case of the Flairform Bloom, the N to K ratio is 0.4:1 while the CNS17 has an N to K ratio of 0.8:1 (double the N to K ratio of the Flairform bloom).

 

Because the concentration of the two products is 16% different let’s compensate for this by using 16% more of the CNS17 (6.4ml/L or 24.22ml per U.S. gallon) and have a look at what this equates to as ppm in the working solution when compared to Flairform Green Dream used at 5.5ml/L . See the following table.

 

Element CNS17 @ 6.4ml/L in working solution (ppm) Flairform @ 5.5ml/L ppm in working solution Optimum Tomato Nutrition mid/late bloom
Total N 128.16 110ppm 150-200ppm
Ammoniacal Nitrogen 5.76 5.5ppm ———-
Nitrate Nitrogen 122.24 104.5ppm ———-
Elemental Phosphorous (P) 55.04 47.3ppm 30-50ppm
Elemental Potassium (K) 159.36 273.9ppm 200ppm
Calcium 160 77ppm 150ppm
Magnesium 32 33ppm 40-50ppm
Sulfur 72.96 66ppm 60ppm
Total Nutrients ppm 607.52 607.2ppm 710ppm
EC mS/cm @ 500ppm = 1EC 1.21 1.21 1.42

 

 

Keep in mind that we are establishing ppm in solution using demineralised water (e.g. RO water) and when working with mains water, nutrients from the water supply would also need to be factored in. i.e. using an example of just one mains water supply, see the following tables.

 

Mains Water Analysis 

 

Phosphate 1.69
Nitrate 0.15
Ammonium 0.30
Potassium 7.4
Calcium 43
Magnesium 14.4
Sulfate 12.3
Iron 0.07
Sodium 55
Chloride 97
Total ppm 231.31

 

Element CNS17 @ 6.4ml/L in working solution (ppm) Flairform @ 5.5ml/L ppm in working solution Optimum Tomato Nutrition mid/late bloom
Total N 128.45 110.15ppm 150-200ppm
Ammoniacal Nitrogen 6.06 5.5ppm ———-
Nitrate Nitrogen 122.39 104.65ppm ———-
Elemental Phosphorous (P) 56.73 48.99 30-50ppm
Elemental Potassium (K) 166.76 281.6ppm 200ppm
Calcium 203 118.9ppm 150ppm
Magnesium 46.4 47.4ppm 40-50ppm
Sulfur 85.26 78.3ppm 60ppm
Total Nutrients ppm 686.6 685.34ppm 710ppm
EC mS/cm @ 500ppm = 1EC 1.37 1.37 1.42

 

It’s worth noting that I haven’t equated the ppm of the iron, chloride and sodium found in the mains water because I’m demonstrating the ppm of particular nutrients and not the total values of all known elements combined. If I did factor in the sodium, chloride and iron this would give us a total of 838.83ppm (EC 1.67) in the CNS17 working solution when used with this particular mains water supply. Keep this in mind when calculating ppm in solution; ideally you want to factor in all known elements to establish the exact ppm of each element that is being provided to the plant via the nutrient working solution.

 

Comparing Two Part Formulas 

The guaranteed analysis for two part nutrients are usually based upon the %w/v or %w/w of nutrients/elements in the individual bottles. As such, to establish a total weight by volume of each nutrient’s value that will ultimately be in the working solution (as ppm) when part A and B are used at equal proportions it necessary to add up all the guaranteed analysis numbers to establish e.g. A + B = N total, A+B = K total, A+B = Mg total etc. Let’s take an example of a U.S. based multinationals bloom product to demonstrate. I’ll only list the macronutrients in this example to keep it simple. However, when adding part A and B nutrients be sure to equate all of the nutrient elements in the bottles (i.e. calculate all of the macronutrients and micronutrients from each bottle).  See the following table.

 

Part A %w/v Part B %w/v Part A + B totals
Total N 7.55 2.3 9.85
P as elemental P Nil 2.3 2.3
K as elemental K 3.9 5.5 9.4
Ca 4.9 Nil 4.9
Mg Nil 0.97 0.97
S Nil 0.56 0.56

 

Using the totals in the right column we can then begin calculating %w/v to ppm in the working solution.

 

Three Part Nutrients

Three part nutrients, unlike two part formulas, are typically not used at equal ratios in solution. E.g. the manufacturer may recommend 4ml/L part A, 3 ml/L part B and 5 ml/L part C etc. Therefore, to calculate the ppm in solution from a three-part formula you will need to equate the recommended dilution rates of each bottle separately and then use these to establish totals.

Percentage Weight by Weight (% w/w) v. Percentage Weight by Volume (% w/v)

In some instances nutrient labels may list the nutrients in the product as percentage weight by weight (% w/w). Therefore, we had better cover the difference between %w/v and %w/w and discuss how to convert %w/w into ppm in solution.

At this point you already understand the concept of %w/v. That is, if we have a solution that contains 10g of a solute (e.g. calcium nitrate) in a 1L product you have 1%w/v. If we have 100g of solute in a 1L solution we have 10%w/v and so on.

 

So when discussing %w/v to mix a 1L solution which has 100g of calcium nitrate in it we would start with, for this example, 700mL of demineralized water. We would then add 100g of calcium nitrate, mix/circulate until the calcium nitrate dissolves completely into solution and then top up with demineralized water to 1L. Final volume of solution equals 1L; therefore the calcium nitrate in weight is expressed as a percentage of the total volume (1000mL). 10%w/v of calcium nitrate (100grams per 1000mL).

 

On the other hand, when comparing %w/v to %w/w let’s say that we make the 1L solution above with 100g of calcium nitrate. It takes 965mL of water to dissolve the calcium nitrate and make up the final volume to 1L. That would mean that we have a 1L solution with a total weight of 100g + 965g = 1065g. We know we have a solution that is 10%w/v but what if we want to convert it to %w/w? To do this we would simply express the100g of calcium nitrate as a percentage of 1065g. This gives a concentration of 9.389%w/w. I.e. 100 (grams calcium nitrate) ÷1065 (grams) = 0.09389 x 100 = 9.389 (%w/w). So, the concentration of the calcium nitrate solution is 10%w/v or 9.389%w/w.

 

Now, let’s say that we take 1L of water and dissolve into it 200g of calcium nitrate, what will we have now? Well, firstly, we’d have more than 1L (1000mL) of final solution. I.e. 1000mL (solvent) + 200g (solute) = ? (ml). Secondly, we would have a final solution that weighed 1,200g (i.e. 1000mL demineralized water = 1000g + 200g calcium nitrate = 1,200grams). We know the weight of the final solution and the weight of the calcium nitrate. This gives us a figure of 16.66%w/w. I.e. 200 ÷ 1,200 = 0.16666 x 100 = 16.66 %w/w.

 

To convert this to %w/v we will need to know one of two things, the final volume or the specific gravity of the solution. For now let’s say that we measure the final volume and use this in our calculations. We will talk about specific gravity a bit later. Therefore, we measure the volume and we have 1.07L or 1070mL. We can then calculate the %w/v and we will have 18.69%w/v. I.e. 200 (g calcium nitrate/solute) ÷ 1070 (ml) = 0.1869158 x 100 (%) = 18.69 (%w/v).

 

Therefore, 16.66%w/w equals 18.69%w/v in the equivalent volume of solution.

 

Based on this information %w/v gives the impression of a more concentrated solution than %w/w as the same concentration is expressed as a higher percentage (i.e. 16.66%w/w versus 18.69%w/v in 1L). However, this is deceptive because the equivalent %w/w and %w/v listing would tell us that the %w/w product is more concentrated than the %w/v product.  In other words a 1L product that lists NPK percentage as 5%w/v N, 0.8%w/v P, 4%w/v K, is less concentrated (i.e. less solutes in solution) than a 1L product listed with 5%w/w N, 0.8%w/w P and 4%w/w K.

 

This is generally true except for some cases where the solute actually weighs less than water, usually a gas that has been dissolved in solution. In this situation the weight of the solution decreases as the concentration rises. Of course, this information isn’t relevant to hydroponic solutions as the solutes used in formulation are always heavier than water.

 

Nutrient element concentration in the working nutrient solution yielded by a specific dose working from guaranteed analysis at %w/w

 

In order to calculate how many ppm of any nutrient will be contributed to the working solution from a nutrient concentrate that has a guaranteed analysis listed in %w/w it is necessary to know the specific gravity (SG) of the product. Where a product has a guaranteed analysis listed in %w/w the SG should also be listed. However, if the SG is not listed it is easy to establish the SG by taking a precise liquid volume of the solution and weighing it. So, for example, if you take 500mL of a solution and weigh it and it weighs 650g you divide the volume by the weight and get an SG of 1.3. I.e. 500ml of liquid weighs 650grams. Therefore 650 ÷ 500 = 1.3 (SG)

 

 

About Specific Gravity: SG will be listed as either s.g or SG. SG stands for Specific Gravity. Some use the term density but density can also refer to solids. Specific Gravity refers to the weight/volume of liquids in relation to water, usually measured in g/mL or grams per millilitre. So one ml of pure water will weigh 1.0g and has an SG of 1.0. 1ml of Phosphoric Acid 75% weighs 1.58g so phosphoric acid at 75% is 1.58 times the weight of water and has an SG of 1.58. Therefore 1,000L will weigh 1,580kg (1000 x 1.58 = 1580). SG is usually measured at a standard tem­perature of 25 degrees Celsius (77F).

 

You are able to measure the SG yourself by taking a precise liquid volume of the solution and weighing it. So, for example, if you take 500mL of a solution and weigh it and it weighs 650g you divide the volume by the weight and get an SG of 1.3.

 

I.e. 500ml of liquid weighs 650grams. Therefore 650 ÷ 500 = 1.3 (SG)

 

You can also use an instrument like a hydrometer to measure the SG of a liquid. A hydrometer is a sealed glass vial with a bulb on the bottom. It is placed in the liquid and floats. The tall thin part at the top has measurements inside that you read against the meniscus of the liquid i.e. it will have gradations like 1.11, 1.12, 1.13.

 

Okay, so let’s now hypothetically say that we have N listed at 2.5%w/w and an SG of 1.2.

 

To equate how many ppm of N this would contribute to the nutrient working solution when used at 4ml per litre we would run the same sums as when we were working with %w/v but then multiply the final figure by 1.2 (the SG).

 

I.e.

 

  • 5 (%w/w N) x 10,000 = 25,000 (ppm)
  • 25,000 (ppm) x 4 (ml/L) = 100,000 ÷ 1000 = 100 (ppm)
  • 100 (ppm) x 1.2 (SG) = 120ppm (N in working solution)

 

 

Anyway, let’s for a moment take a break from the chem quagmire of %w/v and %w/w to ppm conversion and ppm in solution material and mix things up a bit with some less intense theory on how and why this cluster of E equals MC squared (E equals what?) type material is important to understand. Take a breather, relax and let’s talk about off-the-shelf hydroponic nutrients and mains water supplies.

We’ll be tossing around a load more numbers in this material. The good news is that the numbers aren’t so important as the message so you can give your brain some much needed rest. 

Keep in mind that we have a calculator on www.manicbotanix.com that does all these equations for you.

 

UNDERSTANDING THE DIFFERENT GROWTH PHASES, PLANT MORPHOLOGY (ARCHITECTURE) AND THE NUTRIENT REQUIREMENTS OF THE PLANT   



Photosynthesis, Biomass Production and Water and Nutrient Demand  

 

Biomass is the amount of organic matter, such as plant tissue, found at a particular time and place. The rate of accumulation of biomass is termed productivity. Primary production is the rate at which plants produce new organic matter through photosynthesis. In order for photosynthesis to occur the plant needs light energy, chlorophyll, CO2, water and nutrients.

 

CO2 plays the most important role in biomass production because more than 90% of dry matter of living plants is derived from photosynthetic CO2 assimilation.[1] Plants use the carbon from CO2 and convert it into carbon compounds such as glucose, carbohydrates, lignin, and cellulose which is what becomes the biomass of the plant. This is why by increasing the levels of CO2 in the grow room we are able to increase the rate of photosynthesis, which increases the rate of biomass production. In simple terms more CO2 means more available carbon for the plant to use in biomass production.

 

For now, however, we are discussing the water and nutrient demand of the crop so let’s focus on these elements re the plant and biomass production.

The first thing to understand is that nutrients play a crucial role in photosynthesis. For example, chlorophyll is necessary for photosynthesis and the nitrogen and magnesium ions provided by nutrients are central for chlorophyll production; potassium and magnesium affect the production of ATP (adenosine triphosphate which is essential for life functions in plants) and the productivity of enzymes which are required for the fixation of CO2 in the chloroplasts of the plant; similarly, phosphorus plays a central role in ATP and virtually all major metabolic processes in plants. Basically, without nutrients photosynthesis would not be able to occur and without photosynthesis the plant could not fix CO2 to create biomass.   

Plant species with high biomass usually have increased nutrient demand which results in high biomass yield and nutrient assimilation in plant tissues. Thus, one of the most important factors influencing biomass production (growth) is nutrient availability. As a rule, the faster the plant creates biomass the higher its nutrient demand for building this biomass. Therefore, plants that are forming biomass at a rapid rate need large amounts of nutrients supplied to the roots. These nutrients are then taken up by the roots and translocated throughout the plant where they are then used in the processes of photosynthesis. 

This is where water becomes important. Water is crucial for moving nutrients into and throughout the plant. Firstly, water and nutrients enter the roots and then move to the xylem (the vascular tissue in plants that conducts water and dissolved nutrients upward from the root).  Once water enters the xylem, it can move upward through the plant carrying with it nutrient ions to areas of the plant where nutrients are required. Most of the force which enables water and nutrient to move upwards is “pulling” caused by water evaporating through the stomata on the leaves and other plant surfaces (a process in transpiration). Therefore, transpiration from the leaves creates a pull and indirectly plays a role in helping water, with its dissolved nutrient ions, enter the root cells. The process of transpiration results in plants releasing large amounts of water, as water vapour, through their leaves. The release of water vapour through the leaves draws more water and nutrient into the plant resulting in more nutrients being delivered to the plant for photosynthesis. Keep in mind that the speed at which photosynthesis is occurring (‘photosynthetic rate’) is pivotal in this process. Put simply, the higher the rate of photosynthesis the higher the rate of carbon fixation and biomass production; the higher the rate of carbon fixation and biomass production the more water and nutrients needed for processes that are pivotal to photosynthesis.

I’ve dumbed things down somewhat (so much in fact a plant physiologist may have a laugh)  because the actual processes taking place in photosynthesis/biomass production are extremely complex; for example, the plant needs to allocate nutrient and carbon resources and tradeoffs are made between nutrient and carbon acquisition. However, this dumbed down explanation rounds things up for the layman where understanding the relationship between photosynthesis, biomass production and the water and nutrient requirements of the plant/crop. This, in turn, helps the reader (you) to understand why the nutrient and water demand of the plant changes during the crop cycle.

Crop Phases, Photosynthetic Rates, Biomass Production, Water and Nutrient Demand 

Author’s note: where I discuss specific timeframes, re days or weeks, in the following material I am referring to crop cycle that incorporates the use of clones, has a short vegetative period and where short finishing time, heavy flowering dominant genetics are grown/used.

There are several phases to the growth and bloom cycles in indoor crops. These can be defined as:

Settling Phase

The first phase where using ‘clones’ (i.e. cuttings that have been rooted – a genetically identical copy from a another plant) and an 18/6 grow light cycle (18 hours of light and 6 hours of darkness) is what I call the settling phase. This is where clones are placed into the growing system and where they must adapt to high intensity lighting, place roots down into the media (initiate adventurous root growth) and begin growing. This phase can be stressful to young plants as high intensity lighting and transplanting can shock the plant/s if things aren’t handled correctly.

That is, if handled incorrectly the settling phase can take some time and plant health can be impacted. However, if handled correctly – e.g. clones are hardened before being placed in the growing system, cool day temperatures of about 24-25oC are initially maintained, humidity is kept high (approx. 75%) in the grow room to reduce transpiration initially, slowing the plant’s metabolic processes to help it adapt, MH lighting is implemented at the appropriate height above the young plants and an auxin based root stimulant (e.g. Canna Rhizotonic, House and Garden Roots Excelurator)  is used to encourage adventurous root growth – the settling phase should only take a few (3-5) days, at which point the plants have adapted to their new environment and begin producing healthy leaves and growing, at first slowly and then far more vigorously. During the settling phase the plants have quite low photosynthetic rates (PR) and low biomass (i.e. the mass is small and the plants aren’t producing more mass quickly) and, as a result, low nutrient requirements. When the young plants have settled into the system they exit the settling phase and begin growing more quickly, creating new leaves and shoots. I call this phase the rapid vegetative stage.

 

Rapid Vegetative Phase

 

During the rapid vegetative stage the plant’s PR increases; root growth extends considerably to cope with the increase in nutrient demand, large shade leaves begin to grow to produce adequate surface area for photosynthesis; plant volume increases and transpiration is dramatically increased so water and nutrient demand increases.

 

One important thing to stress about the rapid vegetative stage is related to plant height. That is, the height in which we allow vegetative plants to grow is critical in indoor crops where artifical lighting is used to replace the sun. Basically, in under light situations we don’t want plants getting too tall due to lamps only being able to penetrate so much plant canopy (re height and density) effectively. For this reason, turn down times (switching to the 12/12 light cycle to induce flowering) become critical because the plant produces high amounts of biomass after the lights are switched to 12/12. For example, to generalize somewhat because genetics influence outcomes greatly, if we were to switch down an 8-10 inch plant this plant is likely to finish flowering at about two to two and a half feet tall (this relates to predominantly early finishing heavy flowering genetics). Actually, to give the subject of plant height in rapid vegetative stage, switch down times and artificial lighting the attention it deserves let’s briefly touch on why plants should ideally be grown smaller indoors (artificial lighting) than outdoors (the sun).

Why Shorter Plants are the Ideal in Indoor Growing Situations

Plant Canopies, Light and Photosynthesis

The term “plant canopy” refers to the spatial arrangement of the above-ground organs of plants in a community. Leaves and other photosynthetic organs on the plant act as light energy collectors and exchangers for gases such as oxygen and CO2. Stems and branches support these exchange surfaces in such a way that radiative and convective exchange can occur in an efficient manner.[2]

 

The structure of the plant canopy affects the difference in light the top of the plant receives compared to what the lower regions receive. That is, the leaves and stems in the upper canopy act to shade the leaves and stems below them. As such, there are light gradients from the top to the bottom of the plant. For example, on a sunny day, inside the canopy some leaves are fully sunlit, others are exposed to frequent sunflecks and the remainder exist in deep, but occasionally punctuated, shade.

 

These gradients in light have implications to the levels of photosynthesis that occur in different regions of the plant. For example, in the case of C 3 plants, less rubisco is allocated to shaded leaves than to sunlit ones.[3] This effect causes their photosynthetic rates to saturate at lower light levels.[4] It also causes their photosynthetic capacity to be lower than that of leaves at the top of the canopy. This is just one reason why the fruit/flowers high up on the plant (those fruit/flowers that receive more light) are larger and denser than the fruit/flowers found in the lower regions of the plant.

 

Canopies normally are not solid sheets where light is blocked by the outer canopy from reaching areas of the inner canopy. Instead they are a loosely stacked formation of leaves which help plants to effectively absorb most of the incident light with the leaves near the top of the canopy absorbing near maximum light and the lower leaves receiving light of a reduced intensity and also an altered spectral composition.

Recent studies have shown that plants which are grown under artificial light sources receive far less light in the lower regions of the canopy than plants that are grown in natural light.[5] This comes down to the inverse square law. 

The Inverse Square Law

In studies with artificial lighting, light gradients in vegetation are not only dependent on canopy characteristics, but are also strongly driven by the distance between light source(s) and vegetation.[6]

Compared with natural sunlight, plant lighting conditions are dramatically different when grown under artificial light. In the case of artificial light, light intensity strongly decreases with increasing distance between the lamp and plant leaves.

This is because of the inverse square law which states the intensity (of light) is inversely proportional to the distance from the source. 

What this means in practical terms for indoor growers is:

1) Light sources emit energy in a 360 degree, circular pattern. In other words, light spreads out evenly and in all directions from its source. As the light spreads from the source its intensity diminishes exponentially. Technically, the inverse square law tells us that light that travels twice as far from the source is spread over four times the area and hence has one fourth the light intensity.  For example, if the light intensity from a lamp was measured 0.5 metres away from the lamp and it was 1000 lumens M2, then 0.5 metres away from this point it would be 250 lumens M2.

2) Light intensity is directly relative to the distance from the light source. Plants grown outdoors under the sun are unaffected by this phenomenon because the sun is a huge and very powerful light source and the distance from the sun to the earth is so great that a matter of a few feet from the top of the plant to the bottom is insignificant. Plants grown indoors under artificial lighting, however, are extremely susceptible to the inverse square law because the light source is relatively small and the light energy is only traveling a few feet so the distance from the top of the plant to the bottom is very relevant to the total distance traveled by the light. Thus, is you were growing a 1 metre tall plant it would be receiving significantly higher levels of light at the top of the plant canopy than it would be receiving at its base. You can perhaps then imagine that if you were growing a plant that were two metres tall, while it may be receiving high levels of light at the top of the plant it would be receiving hardly any light (if any) three quarters of the way down. Any grower who has experienced large fruit/flowers on the top portion of a plant and small, undeveloped fruit/flowers on the lower section of a plant has directly observed the inverse square law.

See following image that demonstrates the Inverse Square Law.

Inverse-Square-Law

 

In understanding the inverse square law we can see that if we were growing a six foot plant indoors it would take a very powerful lamp to provide enough light to all regions of the plant for optimal photosynthesis to occur.

For this reason, where growing with overhead artificial HID lighting, indoor growers produce shorter plants than would be produced outdoors under natural sunlight. This helps to ensure the plant is receiving adequate levels of light throughout the plant’s canopy to promote high photosynthetic rates and optimal growth.    

While some growers do produce larger plants under HID lighting these plants are typically grown by tying/training the plants down and spreading a single large plant under multiple lights (e.g. one six foot plant may be tied down to three to three and a half foot in height and spread under two 600 watt lamps) and/or ‘interlighting’ might be employed.

Interlighting refers to where lights are not only placed over the top of plants but also at the sides of the plants or dropped down in between plants that are being grown in the crop. This provides areas of the plant that would otherwise be recieving very little or no light with light for photosynthesis. 

However interlighting presents with a couple of issues in that; 1) lamps emit heat which means additional heat produced from additional lamps (lamps used for intercooling) equates to, particularly in warmer climates, additional cooling is required to offset lamp heat and; 2) the intercooling lamps, when placed down beside, between or inside of plants, can/will burn and damage leaves unless some form of lamp cooling device/s (e.g. cool tubes or water jackets) are used.  

I’ve dumbed down the subject of lighting and plant height somewhat because I’ll be covering more on light in other areas of the book. However, based on the information covered here we can understand why we ideally want to produce shorter plants in indoor growing situations.

There is also one other benefit associated to growing shorter plants in indoor, under light growing situations. 

CROP CYCLE TIMES, PLANT FINISHING HEIGHT AND YIELDS: THE GROWER’S LAW OF PER PAR WATT PER SPACE PER TIME YIELD POTENTIAL

 

An important factor to discuss in relation to yields is crop cycle times, or the time it takes from day one (planting) to finish (harvest). This one really comes down to a few simple rules.

 

  • The rapid veg time determines the time from start to finish of a crop
  • The longer the rapid veg cycle the longer the time from the start to finish
  • The shorter the rapid veg cycle the less the time from start to finish
  • Shorter rapid veg times equal smaller plants and lower yield per plant
  • Longer rapid veg times equal larger plants and higher yield per plant
  • Only so much weight can be produced in a given space, per PAR watt, per year over multiple crop cycles
  • Multiples of small plants produced in an meter square (M2) area will produce about the same yield as one large plant grown in that same M2 area (given all factors such as light, CO2, temps, RH, nutrition etc are equal)
  • Because multiples of smaller plants take less time to finish the crop cycle more yield can be produced in a M2 area over multiple crop cycles than where growing larger plants which have undergone a longer veg cycle

 

Okay, that may sound somewhat confusing to the newbie albeit more advanced growers undoubtedly get what I am saying. Basically SOG (Sea of Green), where multiples of small plants (e.g. 16- 24 plants) and grown in a M2 area versus say one large plant grown in that same M2 area, ultimately produce more yield when averaged over multiple crop cycles due to the time it takes for the SOG plants to finish flowering and become ready for harvest.

 

For example, one large plant in a M2 area may be grown in veg for 2 ½ – 3 weeks and the crop cycle time from start to finish may take 15 – 16 weeks. This plant may yield a pound and half to two pounds. Conversely, let’s say 16-24 SOG grown plants, are placed into the system and grown in veg for two days at which point flowering is induced by switching to the 12/12 light cycle. These plants (dependent on genetics) may be ready to harvest in as little as 9 weeks.

 

The SOG plants all finish at about 1 – 1 ½ foot tall but there are 16 to 24 plants in an M2 area which produce about an ounce and a half per plant. The end result is the same 1 ½ to 2 pounds per M2 area but instead of the crop cycle taking 15-16 weeks (1 large plant) it has taken 9 weeks to produce the same yield as the one larger plant. This represents massive savings in time. What this means is that you may harvest 6 crops per year SOG growing while, where growing larger plants for longer, you may only harvest 3-4 crops in that same year. When this is equated in terms of yield per m2, per PAR watt, per year this equates to SOG producing a far higher yield per m2, per PAR Watt than growing larger plants in the same area with the same levels of lighting over the same period of time.

 

However, growing SOG means having large numbers of plants. Conversely, larger plants will produce about the same yield in the same m2, albeit taking longer to harvest. This means that fewer plants are required per crop cycle.

 

Rapid veg times ultimately determine the finishing heights and the biomass of the plant at harvest point. Through understanding this, you can experiment with various genetics to refine your grows to any number of plants within an M2 area.

 

For example, I tend to grow four plants per m2 which finish at about 2 ½ feet tall. To achieve this I grow the plants in veg to about 8 – 9 inches and then switch them into the 12/12 light cycle. Using the types of genetics I like to work with, the entire crop cycle takes about 12 weeks. I find this provides a nice balance between plant numbers and yield. Other growers might, however, only grow one large plant (e.g. a 5-6 foot plant) per m2 and yet others may grow huge tomato plants that take up 1.5 m2 or more under two lights and take many weeks to finish.

 

This one obviously comes down to grower choice. More small plants equals higher harvest per m2, per year, while few larger plants that take longer to harvest equals lower yields per m2, per year.

 

Thus, rapid vegetative times become a very important point to consider in indoor growing for two reasons. One; shorter plants are more suited for under light growing due to the inverse square law and plant canopy light gradients and two, higher numbers of shorter plants will provide more yield over multiple crop cycles than fewer larger plants produced under the same PAR watt in the same M2 area. 

With that aside, let’s now move to the next growth phase of the crop cycle.

 

Pre-flower (the stretch phase)

 

The next important phase of the plant’s lifecycle is when the plant has reached the desired height in the ‘rapid vegetative stage’ and we switch into the 12/12 light-cycle (12 hours of light and 12 hours of darkness) to induce flowering. This phase is what I refer to as pre-flower because due to being a photoreceptor plant, a reduction in light hours triggers hormonal changes and there is a transitional phase between vegetative and flower production. Therefore, pre-flower is the transitional phase from vegetative growth leading to the beginning of flower (hence pre-flower).

 

This phase is commonly referred to by many ‘hydro’ growers as the ‘stretch phase’ because rapid upward growth occurs as the plant grows as much as possible before its hormonal changes signal reproduction.

 

The pre-flower (stretch) phase becomes extremely important to understand because the level of stretch (stem elongation) during this phase will have a significant impact on the growth (flowering) phases which follow pre-flower. Therefore, I will go into some detail on pre-flower beyond the nutrient requirements of the plant and later we will follow this up with a lot more information on plant stretch/elongation and ways of reducing stretch.

 

For now I want to drum in the importance of pre-flower/stretch and lay down a few basic principles surrounding plant physiology. Consider this the dumbed down version of plant physiology because we only need to know a few things with regards to stretch and pre-flower.

 

Firstly, let’s take a look at an illustration of a plant.

plant_pic_site_ready



The illustration shows us that a plant has a main (apical) stem and lateral (secondary) branches. The plant has leaves which are joined to the stem via the petiole. The base of the petiole is attached to the stem at the node.  At the top of the plant on the right of the image we can see an axillary bud which grows at the axil where the leaf petiole connects to the stem. To the left of this we can see nodes. The distance between the nodes is the internode. Understanding that the plant has an axil points, axillary buds, nodes and internodes becomes important for understanding plant stretch/elongation.

 

During the pre-flower phase the plant’s apical/main stem and secondary lateral branches elongate (or stretch as it is typically referred to by many ‘hydro’ growers). The level of stretch that occurs becomes extremely important because if the stems and branches elongate (stretch) too much, the plant becomes tall and leggy and large gaps form between the nodes of the plant. Young axillary buds grow from the axil point or where the leaf petiole meets the node. The distance between the axillary buds that grow above and below one another is measured by the internode. This becomes important to understand because an axillary bud is a bud that grows from the stem (at the axil) and dependent on the phase of growth – i.e. vegetative or flower – develops into either a lateral branch (vegetative phase) or a flower cluster (flower phase). Therefore, the axillary bud is a young flower that grows further and develops into a larger flower (through a process of growing out and setting more buds which all join to form a singular large flower). The distance between these young flowers (axillary buds) is measured by the internode.

 

Our aim as indoor, under light growers is to produce shorter compact plants with small internode distance (I.e. each node of the plant is close to nodes above and below it). The small internode distance means that axillary buds, the beginning of floral clusters, set close to one another. When axillary buds set close to one another this helps to form heavy dense fruit/flowers/colas as the plant develops throughout flower. In turn, this helps to produce optimal yields.

 

Therefore, our aim as growers is to produce short squat plants with small internodes (i.e. the space between nodes is small).

 

During the pre-flower phase the plants can rapidly elongate/stretch. This results in large spaces between the nodes (large internodes). As a result, during the flowering phases that follow pre-flower the flowers/fruit/buds/colas may fail to form well. As a result, growers get less than optimal yields.

 

To round things up somewhat, pre-flower precedes flower and it is vitally important to how the plants will flower because this is the point of the plant’s lifecycle where it begins setting young buds which turn into flowers. These young buds are later joined by new bud growth that occurs later in plant’s lifecycle during what I call the ‘bud set bloom stage’. If the plant isn’t set up correctly in pre-flower this will have a major impact on how the buds set in the bud set bloom stage.

 

Conversely, if we handle things correctly in pre-flower we encourage the plant to grow nodes closely together (small internode distance) which facilitates ‘bud stacking’ (numerous buds forming closely to one another) and prolific flowering. This, in turn, helps greatly to optimize yields.

 

During pre-flower, plants should be grown under metal halide (MH) lighting because blue spectrum light promotes a more compact plant with closer nodes while high pressure sodium (HPS) lighting, which provides high levels of yellow and red spectrum lighting, promotes stretch/stem elongation.[1] Other practices to encourage small internodes and bud stacking can also be implemented. I will go into far greater detail on this subject somewhat later. For now, let’s look briefly at the nutritional requirements of the plant in pre-flower.     

The pre-flower phase typically lasts between 2 – 4 weeks, dependent on plant genetics. For example, equatorial genetics will typically have a longer pre-flower phase than early finishing, heavy flowering varieties. 

During the pre-flower phase, rapid upward growth and secondary lateral branching occurs.

The plant grows leaves along the apical/main stem and lateral secondary branches to produce more surface area for photosynthesis; plant volume increases dramatically, roots grow to facilitate high levels of nutrient uptake and transpiration is increased. Numerous axillary buds (AB) grow from the axils of the apical/main stem and the secondary lateral branches.  If the plant were left in the 18/6 light cycle it would remain hormonally in vegetative mode and the AB would develop into another set of vegetative lateral branches (i.e. the plant would produce a third set of vegetative branching from the AB that are growing on the secondary lateral branches). However, in the 12/12 flowering light cycle the plant is hormonally transitioning between vegetative and flowering and these AB, towards the latter part of pre-flower, begin turning into young buds – the first sign of flowers. Therefore, in the latter stages of pre-flower fructification begins as the plant switches hormonally from vegetative to flower mode.    

During pre-flower the biomass of the plant can double, so water and nutrient demand increases from the rapid vegetative stage. This nutrient demand increase is relative to the growth rates during pre-flower and therefore is incremental (i.e. as the plant grows/stretches more and more over the 2 – 4 weeks of pre-flower nutrient demand increases relative to biomass and biomass production).

Bud Set Bloom


After the initial buds have set in the latter stages of pre-flower and the first sign of fructification is apparent, the plant enters its next phase of growth. I refer to this phase of the plant’s lifecycle as the ‘bud set bloom stage’.

Hormonally, the plant has now switched from vegetative to flowering mode and it is directing all of its energy towards producing flowers. During the bud set bloom phase the plant sets more buds above the buds that set in pre-flower. The plant is still growing upwards (growth in height is still occurring), albeit at a lower rate than in pre-flower. Therefore, the buds that set during this stage of the plant’s lifecycle form closer to one another than the buds that set during pre-flower. If things are handled correctly in pre-flower and bud set bloom, numerous buds will form (‘stack’) closely to one another, setting the plant up to produce large healthy flowers. This said, if things aren’t handled correctly, the distance between the buds can be extended (large internode distance) resulting in less than optimal yields.

The bud set bloom phase takes about 2 – 3 weeks (genetic dependent). Throughout the 2 – 3 weeks the plant sets numerous buds, the buds grow, join and begin putting on bulk. Because the plant has high PR, high biomass to sustain and is creating additional biomass, nutrient demand is higher than it is in pre-flower.

During the bud set bloom phase you should ideally implement a combination of metal halide (MH) and high pressure sodium (HPS) lighting (as opposed to HPS alone which is the norm for many indoor growers). The combination of MH and HPS should be introduced at the very beginning of the bud set bloom phase or towards the end of the pre-flower stage.  The red light from HPS is critical to stimulate prolific flowering; the blue light from MH also stimulates physiological responses related to flowering while at the same time helping the plant to maintain a more ideal architecture/morphology (i.e. blue spectrum light helps to reduce stem elongation, creating a more compact plant with smaller internode distance). Additionally, a combination of MH and HPS lighting throughout the entire flowering stage of the crop cycle better promotes resin and essential oil production. The end result is that the combination of MH and HPS lighting better promotes optimum yields and high quality produce.

Swelling/Bulking 

The plant then moves into its next phase of growth where the growing buds rapidly put on breadth/volume over the course of about a week (roughly 7 -10 days genetic dependent). I refer to this phase as the swelling phase. Others refer to it as the bulking phase. Either way, names aside, during this phase upward growth ceases as the plant directs all of its energy towards producing the flowers/fruit/colas. This phase of the plant’s lifecycle is where nutrient demand peaks because the plants kick into overdrive as the flowers/fruit rapidly swell (it is a sight to see indeed!).

Hardening

The next phase of the plant’s lifecycle is what I call the hardening phase. At this point of the plant’s lifecycle photosynthesis slows[2] and while the colas become denser and heavier (the flowers put on mass and weight) biomass production (gains in size/matter) begins to decrease. Therefore, nutrient demand during the hardening phase is lower than that of the bud set bloom and the swelling phases.

Ripening      

The last phase of the plants lifecycle is what I call the ripening phase. This phase takes about 2-3 weeks. At this point of the plant’s lifecycle photosynthesis and biomass production dramatically slows. The plant is nearing the end of its lifecycle and is beginning to die. For part of this phase some hardening occurs and the flowers continue to put on weight. As a result, one of the biggest mistakes growers make is harvesting too early and losing weight. This said, nutrient demand is extremely low during the ripening phase. At approximately 60% brown-off (60 – 75% of the white hairs on the colas turn brown) the flowers/colas have ripened and are ready to harvest. I’d specify here that the level of brown-off and the ideal harvest point is very much genetic specific. For example, some genetics might be best harvested at 80 or even 90 percent brown-off etc. This one (harvest point) is a bit of an art in itself and typically it is reliant on some trial and error… knowing the genetics that you are working with very much helps refine this one.   

Because photosynthesis and biomass production dramatically slows during the ripening phase, I begin tapering off the numbers of feeds per day and reducing EC incrementally over the course of this phase, essentially supplying very low amounts of nutrient from the beginning and then almost no nutrient towards the end of the ripening phase.

When the plants are about ready for harvest I then flush with pH adjusted water for 3-5 days. 

To summarize the lifecycle of the plant and its nutrient demand we can dumb things down a bit and say:

  • The plant begins its life as a clone or seedling with low water and nutrient demand because it has low biomass and low PR initially.
  • As the plant begins putting out shade leaf growth it moves into rapid veg; it creates more biomass (puts on more volume); its PR increases; root growth extends considerably to cope with the increase in nutrient demand, large shade leaves begin to grow to produce adequate surface area for photosynthesis; secondary/lateral branching from the apical/main stem begins; transpiration dramatically increases so water and nutrient demand increases.
  • We switch to the 12/12 light cycle to induce flower. At this point the plant moves into the pre-flower phase where the plant switches hormonally from vegetative to flower mode. During this period nutrient and water demand increases because biomass can sometimes double in a short space of time (typically 2 – 4 weeks dependent of genetics) as rapid upward stem growth occurs and the plant puts out more leaf matter and lateral/secondary growth. Towards the end of pre-flower young buds set and fructification begins.
  • The bud set bloom phase begins. Upward growth occurs, albeit at a lower rate than in pre-flower; the plant places out a little more leaf matter and more buds set. Buds begin joining and putting on bulk/girth. This phase lasts about 2 -3 weeks.
  • The swelling phase begins. The growing fruit/flowers/colas rapidly put on breadth/volume over the course of about a week. This phase of the plant’s lifecycle is where nutrient demand peaks.
  • The plant then enters the hardening phase. While, visually, the buds/flowers/fruit don’t appear to be putting on much size, the buds are hardening and putting on weight. During this period nutrient demand drops to lower levels than are required in the bud set bloom and swelling phases. The hardening phase typically lasts 2-3 weeks.
  • Finally, the plant enters the ripening phase. Cellular activity related to growth dramatically slows. The plant has reached the last phase of its lifecycle and is beginning to die. Nutrient demand is extremely low. This phase lasts for 1 – 2 weeks.

 

Based on the lifecycle of the plant we can see that nutrient demand starts off low and rises to a peak for about a week during the swelling phase and then begins to reduce until harvest. If you like the nutrient demand of the crop can be considered as being something like a pyramid. See following image.

 

nute-pyramid



While this nutrient demand pyramid isn’t strictly accurate, it helps to visualize things where the growth phases and the nutrient demand of the crop is concerned. In reality (coming back to not strictly accurate) each phase tends to blend into another and there is overlap between the phases. For example, hardening is still occurring during some of the ripening phase. This said, the pyramid gives us insight in how nutrient demand slowly and steadily increases then peaks and then begins to decrease during the course of the crop cycle.

 

That is, in the settling phase the plant has very low nutrient requirements; this increases incrementally in rapid veg; it increases further in pre-flower/stretch and bud set bloom. The swelling phase then occurs and nutrient demand peaks for about a week; then nutrient demand begins to reduce as swelling (biomass production) slows; nutrient demand reduces further as PR slows and the flowers/fruit/buds begin to harden. The ripening phase then occurs and nutrient demand drops significantly over this period. Finally, it reaches a point where cellular activity related to biomass production essentially ceases and nutrients aren’t required for further growth. At this point we flush. I’ve left flushing out the nutrient requirements of the crop discussion for now because we are going to cover a lot of ground on this subject in a second (just following).Therefore, for now, when looking at the nutrient demand pyramid think of the ripening and flushing areas of the pyramid as somewhat of a grey area re the precise time to begin flushing.

 

One thing I hope the reader (you) take from this material is that all the growth phases will play a role in determining what occurs in the following growth phase/s. For example, if the pre-flower phase is handled incorrectly this impacts on the following flowering phases (bud set bloom, swelling and hardening). If rapid vegetative is handled incorrectly, this will impact on the pre-flower phase and so on. Therefore, getting things right at every point of the crop cycle is the ideal and efforts should be focused on this.

 

One other thing I hope to have imparted is that nutrient demand is more like a pyramid than a linear, segmented line where someone can say use this EC through this entire phase; use this EC throughout this entire phase; use this EC throughout this entire phase and so on.  Quite simply, the nutrient demands of the plant are incrementally, subtly changing at every phase and within each phase of the crop cycle. On the latter, I mean that even in a single phase of the crop cycle, due to increases in biomass and biomass production from point A (start) to point B (finish), subtle nutrient tweaks are the ideal to cater for subtle changes in the nutrient demand of the plant during any given phase. So, for example, nutrient manufacturers take a linear, segmented approach to EC recommendations where a single EC is recommended for an entire phase. Then another single EC is recommended for another entire phase. This approach makes a good deal of sense because it keeps things simple. However, when we look at the nutrient requirements of the plant in the pyramid model we can see that nutrient demand is slightly increasing or decreasing at all times. For advanced growers this becomes important to understand because advanced/expert level growers are able to read the plant (often knowing what the plant wants before she does) and make slight tweaks throughout the crop cycle (phase to phase and within phases) to truly meet the demands of the plant. This becomes far more viable under high fertigation frequency where not only can we make slight tweaks to ECs but we are also able to increase or decrease irrigation frequency to provide more nutrients more regularly or vice versa inline to the plant’s needs. I’ll go into far more detail about this when discussing ‘my preferred method of growing’ on pages ….

 

Anyway, let’s talk flushing….    

Overfeeding during the latter part of the hardening phase has some implications to the accumulation of salts in the leaf tissue. Traditionally, many growers have flushed at the end of the crop cycle with water (no nutrients) for 7 – 14 days or more to reduce nutrient buildup in the plant tissue. The logic behind flushing is that by starving the plant of nutrient, by only supplying water during the flush, the plant uses up the nutrients stored in its tissues for photosynthesis. Essentially, to dumb things down somewhat, the principle being that by starving the plant of nutrients growers are forcing it to feed off itself (i.e. the plant is forced to access the existing mobile nutrients in the plant tissue as a source of nutrients because no nutrients are supplied to the roots). This theoretically reduces the levels of the inorganic macroelements such as N, P and K and microelements such as Fe, Mn, Cu, Zn etc in the plant tissue. This is argued to create a better tasting, cleaner end product.   

For example, organically bound nitrogen and free nitrate as well as free and bound phosphates decompose when burnt, resulting in undesirable products. Nitrogen-containing proteins and amino acids not only glow badly but also stink like burnt hair. Phosphate when combusted becomes P Pentoxide which is an irritant to the lungs and mucous membranes.

Therefore, the aim of flushing (in theory) is to reduce the levels of these undesirable elements in the harvested product. 

I should note that the subject of flushing while long promoted by many in the ‘hydro’ industry as best practice is an area of controversy amongst many growers. Some will tell you flushing is imperative while others will tell you it achieves little to nothing and that by starving the plant of nutrients during the flush you are slowing the metabolic processes of growth and hence compromising yields and quality. There is no doubt some basis to this (“slowing the metabolic processes of growth and hence compromising yields and quality”) where flushing is introduced too early. For example, nitrogen is needed to build chlorophyll, amino acids, and proteins. Phosphorus is necessary for photosynthesis and other growth processes. Potassium is utilized to form sugar and starch and to activate enzymes. Magnesium also plays a role in activating enzymes and is part of chlorophyll. Calcium is used during cell growth and division and is part of the cell wall. Sulfur is part of amino acids and proteins. Prolonged N and P starvation will reduce many organic constituents such as amino acids, proteins, soluble sugars and starch. These constituents and their levels and ratios play an important role in determining flavour/taste. They also play a significant role in resin production.  As such, starving a plant of vital nutrients while important growth processes that rely on these nutrients are still taking place can only lead to negative outcomes re growth and quality. Therefore, where the practice of flushing is used, care is required to get things right; i.e. on the one hand flushing may indeed improve taste and quality where handled correctly while on the other hand it may also compromise yields, taste and quality due to nutrient starvation where flushing is introduced too early. Sounds complex doesn’t it? Well, not really.

If you begin to flush at a point where your plants are absolutely ready to harvest, and then flush for 3-5 days there is very little chance that flushing will result in lower weight or quality.

Actually, this one (flushing) warrants some attention because in some ways, as a result of the unique cultural practices of many indoor ‘hydro’ growers, the process of flushing needs to be considered as a phase of the crop cycle in relation to nutrients (albeit that it has nothing to do with plant growth/biomass production).

My own view on flushing is that it is a subject that has been oversimplified by many and due to luxury to excessively feeding hydroponically grown plants for too long, based on many hydro industry feed chart recommendations (too high EC recommendations during the swelling and ripening phase, resulting in high nutrient accumulation in the plant tissue), flushing serves a good purpose (i.e. because high levels of salts have accumulated in the plant tissue it is possible that starving the plant of nutrients over an extended period of time may help to reduce the levels of some nutrients in the plant tissue?). This said, a similar thing can be achieved through not excessively feeding the plants during the ripening phase. This reduces the time that flushing is required (3-5 days versus 7-14 days) while not compromising yields and quality. One obvious advantage to this system is that growers are able to harvest sooner because they are saving several days on flushing.

To understand the concept of reducing nutrient provided to the plant to reduce flushing time, think of things this way.

Organically grown indoor crops tend to taste great with no chemical taste – an attribute (“chemical taste”) associated to poorly grown hydroponic crops. However, many organic growers never flush and yet, scientifically, organically produced plants can only uptake the exact same inorganic elements that hydroponically produced plants receive.

That is…

Soils whether organic or inorganic naturally contain levels of the essential macro and microelements that are required for plant growth. Plants cannot absorb most organic elements because the molecules are too large, so these substances are broken down into simpler soluble inorganic elements by microflora (bacteria and fungi) and plants are then able to use them. Mycorrhizal fungi, for instance, increase the uptake of poorly soluble organic and inorganic sources of phosphorous (P) such as iron and aluminium phosphate and rock phosphates by converting non-bioavailable organic phosphates into bioavailable inorganic H2PO4 (Pi) and HPO4-2 phosphorus. Hence, organic phosphates must become inorganic soluble phosphorus before it can be uptaken by plants (once inorganic P is uptaken it is then synthesized within the plant into organic materials/molecules such as DNA).  Similarly, the nitrogen in organic matter is largely found in organic forms that plants, for the most part, cannot uptake. Bacteria found in soils convert organic forms of nitrogen to inorganic forms that the plant can then use. Nitrogen (N) is only available to plants as either inorganic ammonium (NH4+) or inorganic nitrate (NO3 ) or, to a lesser extent, as organic amino acids (e.g. Glycine, Methionine, Lysine, Argenine).

To a plant inorganic nutrition is life. There is, in fact, technically speaking, no such thing as organically grown produce because all plants require and uptake inorganic nutrients. Therefore, in organics as in hydroponics plants receive their food in the same form – inorganic ions dissolved in water. From a scientific/biochemical perspective an atom of nitrogen, whether it is derived from chicken manure or synthetically derived from air (as is the case in the manufacture of most nitrogen fertilisers), is exactly the same thing to the plant. That is, potassium is potassium, nitrogen is nitrogen, phosphorus is phosphorus etc, etc, etc. As far as a plant is concerned there can be no distinction made between two atoms of the same chemical element. As such, synthetic, man-made fertilisers and organic fertilisers to a plant result in the exact same thing and they are converted within the plant to the exact same thing (i.e. sugars and carbohydrates that are used in photosynthesis, and for protein synthesis, nucleaic acids for RNA and DNA synthesis to name a few). However, one fundamental difference exists between organics and hydroponics. That is, the nutrients in hydroponics are provided to the plant in at high levels in highly bioavailable inorganic form while in organics the nutrients in many cases are supplied at lower levels in less available form which means these nutrients, as a result of bacteria and fungi interaction, become more slowly available to the plant. The end result is that organic produce isn’t, for lack of a better term, being force fed high levels of nutrients. As such, organic produce is being fed nutrients within the lower to mid sufficiency range while hydroponic produce is often being fed nutrients within the mid to high luxury range. One other thing of interest here is that several studies comparing organically grown crops to crops produced using synthetic fertilisers have shown that phosphorus is significantly higher while nitrogen is lower in organic versus inorganically grown produce.

So how do we achieve a more organic approach, without compromising yields, when growing hydroponically?

Well one obvious thing is to reduce nutrients by tapering them off during the ripening phase and feed the plant within the low to mid sufficiency range during this period. Additionally, another sound practice is to not feed the plants too excessively during the swelling phase which leads to excess nutrient buildup in the plant tissue. Further, drop N to the lower or even marginally below sufficiency range for the last 10 or so days of the crop cycle. Lastly, when the plants are ready to harvest and cellular activity related to growth has dramatically slowed or ceased, a flush for several days can’t hurt. The key though to achieving optimum yields while also producing the highest quality produce possible is not to starve the plants of nutrients when cellular division and expansion is still taking place.  Other than this, a good cure (aging/storing/curing the produce in airtight glass jars that are stored in a dark place) improves aesthetics, flavour and taste.

Actually (show, don’t tell), here are two lab analyses that were conducted on a crop being grown hydroponically. This crop was grown using low EC nutrients (EC 1.2 throughout bloom along with intermittent foliar feeds) in a recycling system where RO water was used as the water source. In this case, no adaptations to N in the nutrient were made during the hardening/ripening phase; i.e. this grower maintained the same nutrient NPK ratio throughout the entire bloom stage.  One analysis (1/30/2015) is of the crop just before the flush and the other analysis (2/16/15) is of the same crop after the flush.  The analysis measures the nutrients found in tissue samples.

 

flush-tissue-analysis



Keep in mind that this crop was grown in a recycling system with an EC of 1.2 throughout the swelling and hardening/ripening phases. Comparatively, some hydroponic nutrient suppliers recommend running an EC of about 2.8 (or even higher) during the swelling phase and tapering off to perhaps 1.8 to 2.2 during the hardening/ripening phase. That’s about double the EC (twice the amount of nutrient) that has been used here.

What’s notable in the lab analysis is that contrary to the flush diminishing the levels of nutrients found in the harvested plant tissue, while P is marginally lower and K, Ca and Mg levels somewhat lower, N levels have actually increased.  What’s also apparent is the microelements have remained about the same or in some cases increased. Where the increase of N is concerned, a possible explanation for this might be that because N is highly mobile in the plant, as a response to nutrient stress N which accumulates in high amounts in the leaves and stem of the plant has been translocated to the flowers to compensate for nutrient starvation (i.e. the plant scavenges any nutrients it can access to offset nutrient starvation in general). In fact, flushing in this instance may have even resulted in a negative outcome due to N increasing in the harvested flower matter.  Basically, however, the flush has done little to reduce certain nutrients while seemingly other nutrients have increased.

This is perhaps not too surprising. From a plant physiology perspective the “buds” are the most valuable reproductive organs of the plant and hence the last plant part to be affected by nutrient starvation. Therefore, flushing would have limitations on reducing nutrient levels in the “buds”. I.e. the plant would translocate mobile nutrient (e.g. N, P and K) reserves from the roots, leaves and stem towards the buds to ensure they were not being starved.    

Typically, a broad brush has been applied to the issue of flushing – I myself being guilty of this in the past; however, the key is to grow clean produce, while also ensuring plant health and metabolic activity aren’t compromised as a result of overly aggressive flushing. This is a fine line to walk and often it needs some genetic specific experimentation to get things absolutely right.  By taking care in how we go about feeding our plants and by tapering off nutrients inline to the plants nutrient requirements and not overfeeding, with a flush in the last 3-5 days before harvest (i.e. let your plants get to the point where they’re absolutely ready to harvest and then flush for 3-5 days) we are able to mimic a more organic approach to plant nutrition while also ensuring that the plant receives optimal nutrition throughout all phases of its lifecycle where important cellular growth is taking place.

As a tip, a plant that remains visually healthy and is lacking for nothing throughout its entire lifecycle will produce the highest yield and quality. One way of achieving this is to keep feeding the plants right up to harvest. However, this often results in harsh chemically tasting produce due to excessive nutrient levels being sustained for too long. A better way of approaching things is by only supplying enough nutrients to meet demand (not exceed demand) throughout the hardening/ripening phase to maintain plant health while ensuring the plant is not being starved of any essential nutrient elements. Under these circumstances a flush for 3-5 days prior to harvest should prove very effective at creating an extremely high quality, clean end product. 

Some growers flush for weeks. The end result is a plant that pales and yellows; leaves even fall off the plant (coined by some growers as the “flush till they drop” approach). This practice, while producing a clean end product has also, no doubt, cost many growers in yield and has also, in all probability, impacted on essential oil production (i.e. reduced the quality of the final product). Anyway, let’s leave flushing there for now. The point really is that for eons certain sectors of the hydroponics industry have promoted aggressive flushing to produce a clean end product. There is some sense to this because certain sectors of the hydroponics industry have also long promoted overfeeding hydroponically produced plants. However, in understanding that overfeeding will result in the high accumulation of nutrients salts in plant tissue growers can then modify their growing practices to reduce the need for extended flushing periods.   

Coming back to plant development, where left to grow naturally (i.e. the plant is not tied/netted down) the largest single flower/cola on the plant will form at the top of the apical/main stem, with prolific secondary cola formation also occurring on the lateral branches, with the heaviest of these secondary colas forming at the top of these branches.

I’ve oversimplified things somewhat because various factors will come into play regarding node and bud formation and branching; factors such as plant size and plant genetics. For example, if a plant is left in vegetative growth for a long period (i.e. where growing large plants) the key/primary lateral buds may set on a third set of lateral branches etc, and where genetics are concerned, e.g. long day equatorial genetics may act somewhat differently than described and the buds may not join resulting in loose, stringy or fluffy flower clusters that are spaced apart (as opposed to larger, dense flowers as is typically seen in the case of short height, early finishing, heavy flowering genetics).

I’ve covered the entire life cycle of the plant because each phase is interrelated to overall yield and quality and one cannot consider one phase without considering all others. That is, if the rapid vegetative phase is not handled correctly this may impact on the preflower phase and if the preflower phase is not handled correctly this will impact on the bud set bloom phase. Quite simply, achieving optimum yields and quality is a holistic process where optimums (plant health and development) throughout all phases of the plant’s lifecycle will determine outcomes.

For now, let’s rein this in a bit and look at the importance of the preflower phase with regards to plant architecture and ultimately flower formation and yields. The preflower cycle is a very important subject to cover because stretch is a serious issue in indoor, under light crops.

We’ll add more soon


Further recommended reads to this material are:

About hydroponic nutrients – A MUST READ that dissects hydroponics nutrients through lab analysis to demonstrate significant variations between formulations that claim to be crop specific. Additionally, this article looks at the important role that the water source plays in determining what nutrient should be added to solution

Hydroponic substrate science – the importance of the root zone air water relationship to the uptake of nutrients

 

 

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