Excerpt from Integral Hydroponics Evolution by G.Low. Coming soon.




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 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.


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 occurs. 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.




Raviv and Leith’s (2008) nutrient response curve. 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]


Actually, this one warrants more discussion.



See following image.




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…




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.




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.


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.


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.


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’

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:

  • 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.[1] 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.[2]


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.[3]


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.[4] 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.[5] That is, for nutrient uptake to occur, the individual nutrient ion must be in position adjacent to the root.[6] 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…





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’ section of IH on pages …..   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.


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.




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 ….









[1] Mohamad Zamri Sabli, PhD thesis (2012) Fertigation of Bell Pepper (Capsicum annuum L.) in a Soil-less Greenhouse System: Effects of Fertiliser Formulation and Irrigation Frequency

[1] Raviv, M. & Lietch, J. 2008. Soiless Culture Theory and Practice, London, Elseiver.

[1] Savvas, D., Sigrimis, N., Chatzieustratiou, E., & Paschalidis, C. (2009). Impact of a progressive Na and Cl accumulation in the root zone on pepper grown in a closed-cycle hydroponic system. Acta Horticulturae, 807, 451-456.

[1] Resh, H.M. 2013. Hydroponic Food Production: A Definitive Guidebook for the Advanced Home Gardener and the Commercial Hydroponic Grower. 7th ed. Boca Raton, FL: CRC Press.

[2] Water Corporation Australia, 2012/13, ISSN 2202-879X

[3] Resh, H.M. 2013. Hydroponic Food Production: A Definitive Guidebook for the Advanced Home Gardener and the Commercial Hydroponic Grower. 7th ed. Boca Raton, FL: CRC Press.

[4] SILBER, A., XU, G., LEVKOVITCH, I., SORIANO, S., BILU, A. & WALLACH, R. 2003. High fertigation frequency: the effects on uptake of nutrients, water and plant growth. Plant and Soil, 253, 467-477.

[5] Mohamad Zamri Sabli, PhD thesis (2012) Fertigation of Bell Pepper (Capsicum annuum L.) in a Soil-less Greenhouse System: Effects of Fertiliser Formulation and Irrigation Frequency

[6] Mohamad Zamri Sabli, PhD thesis (2012) Fertigation of Bell Pepper (Capsicum annuum L.) in a Soil-less Greenhouse System: Effects of Fertiliser Formulation and Irrigation Frequency, pp.152

[7] SILBER, A., BRUNER, M. & KENIG, E. 2005. High fertigation frequency and phosphorus level: Effects on summer-grown bell-pepper grown and blossomend rot incidence. Plant and Soil, 270, 135-146.

[1] SILBER, A., XU, G., LEVKOVITCH, I., SORIANO, S., BILU, A. & WALLACH, R. 2003. High fertigation frequency: the effects on uptake of nutrients, water and plant growth. Plant and Soil, 253, 467-477.

[2] SILBER, A. 2008. High frequency irrigations as means for enhancement of nutrient use efficiency: Soilless grown bell pepper as a model plant Acta Horticulturae 779.

[3] Mohamad Zamri Sabli, PhD thesis (2012) Fertigation of Bell Pepper (Capsicum annuum L.) in a Soil-less Greenhouse System: Effects of Fertiliser Formulation and Irrigation Frequency

[4] SILBER, A., BRUNER, M. & KENIG, E. 2005. High fertigation frequency and phosphorus level: Effects on summer-grown bell-pepper grown and blossomend rot incidence. Plant and Soil, 270, 135-146.

[5] SILBER, A., BRUNER, M. & KENIG, E. 2005. High fertigation frequency and phosphorus level: Effects on summer-grown bell-pepper grown and blossomend rot incidence. Plant and Soil, 270, 135-146.

[6] Mengel, D. (1995) Roots, Growth and Nutrient Uptake, Purdue University Agronomy Department

[1] Mohamad Zamri Sabli, PhD thesis (2012) Fertigation of Bell Pepper (Capsicum annuum L.) in a Soil-less Greenhouse System: Effects of Fertiliser Formulation and Irrigation Frequency

[2] AL-JALOUD, A. & ONGKINGCO, C. T. 1999. Effect of fertigation on growth and yield of greenhouse grown cucumber. Saudi Journal Biological Science 6, 156- 167. (AND) SILBER, A., BRUNER, M. & KENIG, E. 2005. High fertigation frequency and phosphorus level: Effects on summer-grown bell-pepper grown and blossomend rot incidence. Plant and Soil, 270, 135-146. (And) SILBER, A. 2008. High frequency irrigations as means for enhancement of nutrient useefficiency: Soilless grown bell pepper as a model plant Acta Horticulturae 779.

[3] Kang, J-G. and van Iersell, M. W. (2009) Managing Fertilization of Bedding Plants: A Comparison of Constant Fertilizer Concentrations versus Constant Leachate Electrical Conductivity

[4] A. Gül, F. Kıdog˘lu, Y. Tüzel1 and I. H. Tüzel (2008) Effects of nutrition and Bacillus amyloliquefaciens on tomato (Solanum lycopersicum L.) growing in perlite

[5] FERRANTE A., MALORGIO F., PARDOSSI A., SERRA G., TOGNONI F., 2000. Growth, flower production and mineral nutrition in gerbera (Gerbera jamesonii H. Bolus) plants grown in substrate culture with and without nutrient recycling. Adv Hortic Sci 14(3), 99-106.

[6] ADAMS P., 1993. Crop nutrition in hydroponics. Acta Hort 323, 289-306.

[1] SILBER, A., BRUNER, M. & KENIG, E. 2005. High fertigation frequency and phosphorus level: Effects on summer-grown bell-pepper grown and blossomend rot incidence. Plant and Soil, 270, 135-146.

[2] STITT, M & KRAPP, A. (1999)The interaction between elevated carbon dioxide and nitrogen nutrition: the physiological and molecular background, Plant, Cell and Environment (1999) 22, 583–621

[3] Wolfe, D. W. et al. (1998) Integration of photosynthetic acclimation to CO2 at the whole-plant level

[4] BassiriRad H, Gutschick VP, Lussenhop J. 2001. Root system adjustments:regulation of plant nutrient uptake and growth responses to elevated CO2.Oecologia 126: 305_320.

[5] Jin J, Tang C, Armstrong R, Butterly, C, Sale P. 2013. Elevated CO2 temporally enhances phosphorus immobilization in the rhizosphere of wheat and chickpea. Plant and Soil 368: 315–328.

[6] Niu YF, Chai RS, Dong HF, Wang H, Tang CX, Zhang YS. 2013a. Effect of elevated CO2 on phosphorus nutrition of phosphate-deficient Arabidopsis thaliana (L.) Heynh under different nitrogen forms. Journal of Experimental Botany 64: 355–367.



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