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HYDROPONIC SUBSTRATE SCIENCE: OXYGEN MOISTURE RATIO AND IRRIGATION FREQUENCY

 

Excerpt from Integral Hydroponics Evolution (coming soon!)

 

Having looked at nutrient science, there is now one other extremely important subject that needs to be addressed, and that is substrate science. There is, after all, very little point in providing optimum nutrition to an oxygen or water starved plant. Such a scenario would result in, among other things, reduced nutrient uptake.

 

As with other key environmental factors such as ambient air temperature, thermoperiod (DIF), light intensity and colour spectrum, relative humidity, CO2 levels and plant nutrition, an optimised root zone environment is a crucial environmental element. In order to ensure optimum growth this means providing an ideal balance of oxygen and water to the roots.

 

As with all other key environmental factors, should the root zone environment be less than ideal, growth will be reduced and yields will suffer as a result. For this reason, understanding how to best capitalize on the physical properties of a hydroponic substrate is pivotal in achieving optimum yields.

 

 

The Importance of Oxygen and Water in Hydroponic Substrates

 

 

Oxygen

 

Nutrient salts don’t leak into the roots of the plant. Nutrient uptake is an active process that relies of several factors, one of which is that satisfactory levels of oxygen are available to the roots of the plant.

 

Roots pump nutrients from the outside to the inside of the root where they are transported to the leaves and fruit/flowers. This pumping process requires energy. The roots get their energy from respiration. In turn, respiration requires energy which is achieved by burning sugar. Part of the sugar made in leaves by photosynthesis is transported to the roots to power the nutrient pumps.

 

Photosynthesis converts sugar and oxygen from carbon dioxide, nutrition and water using energy from light.

 

Respiration is the opposite. Respiration makes energy by burning sugar, supplied by the leaves of the plant, and oxygen to make carbon dioxide. It is this energy that powers, among other things, the root nutrient pumps. In turn these pumps deliver the nutrition that is critical to sugar production within the plant.

 

Unlike sugar, oxygen is not transported from the leaves to the roots. This means that the roots must get their own oxygen.

 

If the roots can’t get sufficient amounts of oxygen their pumping capacity is greatly reduced. This will lead to a stressed plant and reduced yields.

 

Further, it is important to note that by far the most common cause of root disease (root browning, root death) in hydroponic systems is rootzone oxygen starvation. I.e. roots growing in poorly aerated media are weaker and more susceptible to micronutrient deficiencies and root rot pathogens such as Pythium and Phytophthora than roots growing in well-aerated media.[1] A decrease below 3 or 4 mg L-1 of dissolved oxygen, inhibits root growth and roots begin browning, which can be considered as the first symptom of the oxygen starvation.[2]

 

Plants are just like people. If they can’t get enough oxygen they suffocate!

 

 

Water  

 

Water fills a number of important roles in the physiology of the plant; roles which only water can play as a result of its unique physical and chemical properties.

 

All living organisms need water to stay alive, and plants are living organisms. Plants, however, need much more water than many living things because plants use much more water than most animals. Plants also contain more water than animals – plants are about 90% water, with animals as little as 75 percent water by weight. The amount of water a plant needs depends on the type of plant, how much light the plant receives, the size of the plant and other environmental factors such as ambient air temperatures and relative humidity. When plants do not receive enough water they wilt. This is because of turgor pressure, which is water pressure inside the cells that make up the plant’s skeleton. Water enters a plant through its roots and travels up the stem to its leaves. When a plant is properly hydrated, there is enough water pressure to make the leaves strong and sturdy. Conversely, when a plant doesn’t receive enough water, the pressure inside the stems and leaves drops and they wilt.

 

Plants also need water for photosynthesis. Photosynthesis is the process whereby plants produce their food, and water is pivotal in this process. Water, which also carries the nutrients for growth, enters a plant’s stem through the roots and travels up to its leaves, which is where photosynthesis takes place. Once in the leaves, water evaporates as the plant exchanges water for carbon dioxide. This process is called transpiration and it happens through tiny openings in the plant’s leaves, called stomata. Nearly all water taken up is lost by transpiration and only a tiny fraction is used within the plant. The water from the leaves evaporates through the stomata, and carbon dioxide enters the stomata, taking the water’s place. When the pores close, little carbon dioxide or water enters or leaves the plant.  Plants need this carbon dioxide to make food. If the flow of water from the roots cannot match that lost through the stomata, then the plant will close the stomata and shut down in response to water stress. This means photosynthesis and growth will slow or stop.

 

Ultimately, a lack of water results in an unhealthy, stressed plant and a reduction in growth.

 

 

 stomata-transpiration

 

 

You can perhaps see the importance of ensuring that both adequate levels of oxygen and water are available to the roots of a plant. This comes down to maintaining an ideal oxygen water ratio in the root zone. However, this is more easily said than done because a yin and yang relationship exists between water and oxygen in hydroponic substrates. Put simply, too much water will reduce available oxygen while too much oxygen would mean too little water. Therefore, getting the balance right between plant available oxygen and water is crucial to realizing optimum yields.

 

 

OVERVIEW OF OXYGEN AND WATER IN HYDROPONIC SYSTEMS

 

As Integral Hydroponics is about “hydroponics”, a term technically applied to non-substrate, water based growing systems – e.g. NFT (Nutrient Film Technique), DWC (Deep Water Culture) and Aeroponics, it would be remiss of me not to discuss how plants get their root zone oxygen in water based systems where a growing media isn’t present. Additionally, this subject becomes important when talking about substrate based hydroponic, technically called “soilless”, systems because some oxygen is made directly available to the roots via the irrigation water.

 

Oxygen concentrations are much higher in air, which is about 21 percent oxygen, than in water which is a tiny fraction of one percent oxygen. Where the air and water meet, this large difference in concentration causes oxygen molecules in the air to dissolve into the water. Water absorbs about 4% of its bulk of air from the atmosphere. If the roots extract this oxygen from the water the water will take in a fresh portion of oxygen from the atmospheric air that floats above it.

 

Water (H20) itself is composed of two hydrogen atoms and one oxygen atom, but the oxygen in the water molecule is tightly bound making it largely unavailable to plants. Instead, the oxygen that plant roots receive is essentially the same atmospheric oxygen that we breathe; it’s just that when it comes into contact with water, it dissolves into it in much the same way that, say, sugar does. This oxygen is referred to as ‘dissolved oxygen’ (DO) and it readily available to the roots of plants.

 

If we were to place a plant’s roots in a bottle only partially filled with water, with the remainder occupied by atmospheric air, the oxygen in this air would slowly diminish. It will be absorbed by the roots through the medium of the water. This is because the roots absorb gaseous substances from the air which surrounds their roots through the medium of water.

 

Other than this, water can be oxygenated through the use of either oxygen injection, through agitating the water via stirring, or through using an aquarium air pump and air stone to pump air into the water.

 

As a result, if a plant’s roots are submerged in highly oxygenated water this water can supply adequate levels of oxygen to the root zone to facilitate growth. This is why plants can be grown in water based systems such as NFT and DWC.

 

Now a rule of physics comes into play. Temperature effects how much oxygen water can hold… the colder the water, the more oxygen – the warmer the water, the less oxygen.

 

See the following table that demonstrates oxygen ppm in pure water at given temperatures.

 

Solubility of oxygen in pure water at various temperatures

 

Temperature oC Oxygen solubility ppm of pure water
10 11.29
15 10.08
20 9.09
25 8.26
30 7.56
35 6.95

 

Oxygen is considered to be moderately soluble in water and this solubility is governed by a complex set of physical conditions that include atmospheric and hydrostatic pressure, turbulence, salinity and, as noted, temperature. Temperature, more than any other physical condition, affects the solubility potential of dissolved oxygen in water.

 

Water that is rich in oxygen has been shown to help irrigated crops achieve greater growth rates. Plants absorb more nutrients when irrigation water is saturated with oxygen.

 

However, the amount of oxygen in solution can never exceed the maximum saturation level possible for the temperature of the water. The warmer the water, the less oxygen – the colder the water, the more oxygen!

 

So why not just keep the nutrient in the tank/reservoir freezing cold to ensure high oxygen content?

 

This is where another rule comes into play. If the feed solution is too cold it reduces water and/or nutrient uptake by plants. For example, Kafkafi (2000) reported that the rate of water flow through the stem in tomato was increased by 250% when the root temperature changed from 12 to 20 °C (53.6 to 68 °F). Additionally, the uptake and translocation of nitrate, phosphate, and potassium is very much reduced with low root temperatures.[3]

 

Therefore, a yin and yang relationship exists between maintaining enough oxygen in solution and at the same time ensuring water is at a temperature that doesn’t negatively impact on water and/or nutrient uptake.

 

To generalize somewhat, most plants require about 8ppm of oxygen in the root zone to maintain optimal growth. The maximum saturation level of dissolved oxygen in 21 ˚C (70˚F) water is approximately 9 ppm. Therefore, while the ideal nutrient temperature appears crop specific, the sweet spot for promoting both adequate oxygen in solution and healthy nutrient uptake is 20- 22oC (68 – 71.6 oF). For example, Graves (1983) observed that at temperatures below 22 °C (71.6 oF) the dissolved oxygen in the nutrient solution is sufficient to cover the oxygen demand of tomato plants grown in NFT[4] while Zheng et al. (2007) reported that very little (“minor”) growth benefit was achieved in tomato when DO (dissolved oxygen) was elevated from 8.5ppm to 30ppm.[5]

 

One other important factor that also needs to be taken into consideration is the EC of the nutrient solution. The higher the EC/ppm of a nutrient solution, the more dissolved solids in solution, the lower its ability to hold oxygen. For example, saltwater holds about 20% less dissolved oxygen than freshwater,[6] so be aware that over fertilizing (high EC) often occurs in conjunction with low oxygen levels in solution.

 

In water based ‘hydroponic’ systems such as NFT and DWC the plants roots are surrounded by oxygen saturated water/nutrient. Therefore, plants are provided with enough oxygen from this water/nutrient to grow. As such, a plant’s roots can be constantly immersed in water as long as this water holds enough oxygen to provide for plant growth. In this sense, water becomes a hydroponic growing medium where the roots are provided with enough oxygen to facilitate respiration and growth. See following image of a DWC (‘Bubbleponics’) system…

 

 

Bubble-Buckets

 

In substrate based ‘soilless’ systems things differ. Rather than the plant’s roots being surrounded by oxygenated water, they are surrounded by a hydroponic growing medium/substrate. This substrate then has to provide enough oxygen for plant growth.

 

While the irrigation water fed to the plants can provide some short term oxygen this is quickly used by plants. For example, researchers measured the consumption of oxygen from the irrigation water of tomato and cucumber grown in rockwool. The initial decrease in oxygen levels close to the roots was very fast; i.e. within 15 minutes concentration dropped from 21% to less than 5%. From these profiles the researchers could determine that this would mean that the plants would need to be supplied with 4Ls of fresh water every hour to supply the plants with adequate oxygen. Based on this, they were able to conclude that the supply of oxygen to roots can only partly (10 – 20%) be supplied by the irrigation water.[1]

 

Given this information we can then understand that that plants grown in hydroponic substrates can only get limited oxygen from the irrigation water and, therefore, they must get oxygen from the growing substrate between irrigation events.

 

Substrates

 

The substrate, root zone environment tends to be one of the more misunderstood, oversimplified and/or overlooked environmental factors where growers and, indeed, some hydroponic retail and wholesale industry interests are concerned. This is perhaps not too surprising given the complexities of substrate science, which pretty much requires both a chemistry and physics degree to truly come to terms with. This said, in understanding some basic principles of substrate science we can get a much better idea of how any given substrate should be treated to create an ideal oxygen to water ratio in the root zone environment. This largely comes down to the physical properties of a substrate and the irrigation practices employed by growers. That is, regardless of the substrate being used as a growing medium, an extremely important factor in determining the oxygen to water ratio in the root zone environment is the irrigation strategy. [2]

 

In the following material we look closely at how to best optimize the root zone environment which will be addressed in terms of oxygen, water and nutrient availability to the roots.

 

Further, as a continuum to information that we covered in the nutrient section (page ….) surrounding high frequency fertigation (high numbers of feeds) creating a better nutrient status in the root zone we will look at what types of substrates are best suited to this practice under run-to-waste/drain-to-waste (RTW/DTW) growing conditions.

 

Physical and Chemical Properties of Substrates and Their Influence on Irrigation and Nutrition

 

“Oxygen deficiency in the root zone has immediate effects on the water uptake while nutrient uptake is also affected within a short space of time.” [3]

 

Let’s for a moment compare soil to hydroponic substrates with oxygen, water and nutrient availability in mind.

 

We have established that oxygen and water in the root zone is crucial for promoting healthy growth. It has also been noted that the oxygen to water ratio in a substrate is yin and yang; i.e. too much water will reduce available oxygen while too much oxygen could easily result in too little water. Therefore, getting the balance between oxygen and water right is crucial to realizing optimum yields.

 

Given this information, improving the root zone environment is an essential part of providing the plant with optimum environmental conditions; however, soil is not an easy medium to provide the crop with the ideal combination of moisture and aeration. When the moisture content is ideal, the aeration tends to be inadequate, and when the aeration is ideal then moisture tends to be problematic. This is because the ‘physical properties’ of many soils are less than ideal for facilitating both adequate oxygen and water at the same time.

 

This has implications to plant nutrition. For example, where oxygen is inadequate phosphorus (P) uptake can be reduced by as much as 50%. Conversely, plants can take up only those nutrients that are dissolved in the soil solution. The term ‘soil solution’ refers to the film moisture in soil together with its dissolved substances (e.g. nutrient ions). Therefore, if water levels are inadequate nutrient availability is inadequate. However, as soil moisture begins to become excessive, while nutrients are available in adequate quantities, excess water excludes the needed oxygen from the soil and nutrient uptake begins to suffer due to the lack of oxygen.

 

You can perhaps see the delicate balancing act that occurs in growing media, whether that be soils or hydroponic substrates, where oxygen, water and nutrients are concerned. Thus, even the most ideal plant nutrition under less than ideal root zone conditions will be greatly compromised.

 

Fortunately, hydroponic substrates are superior to soils where providing ideal levels of oxygen and water is concerned. Unlike soils, hydroponic substrates can accommodate for both adequate moisture and aeration at the same time. The yield increases associated to hydroponic/soilless growing are, in fact, attributed to several things, one of which is an improved root zone environment which is directly related to the physical properties of hydroponic substrates.

 

Physical properties relate to the physics of the media; things such as particle size, particle size distribution and pore space. These physical properties determine, among other things, how quickly water flows from the top (point of irrigation) to bottom (point of drainage) of a media and how much water and air the media can hold. In turn, the oxygen to water balance of a media, determined by the physical properties, plays an all important role in nutrient availability.

 

The significant difference between the oxygen and water capacity of soils and hydroponic substrates largely comes down to the particle size and pore space differences of the two media. We’ll be looking in depth at these physical properties shortly. For now, pore space allows water to flow through a media and pores hold air. The larger the pores, the more quickly water flows through a media and the more air the substrate can hold. Pore space is determined by particle size. A media with large particles has high pore space while a media with small particles has low pore space. To dumb things down somewhat, soils have small particles and low pore space; therefore they have low air capacity and a tendency to hold too much water. The problem is that because of the low air space when enough air is available to accommodate for optimal growth the water levels in the soil are too low. Conversely, because many soils have high water and low air capacity, when water is adequate air is less than optimal. Therefore, maintaining both adequate air and moisture in soil at the same time is compromised by the physical properties of that media.

 

Hydroponic substrates have comparatively larger particles than soils and therefore have greater air space. This means that they become less water logged than soils and provide more air. This enables growers to create an optimal balance of both air and water in the substrate at the same time.

 

Another very important factor also needs to be raised with regards to the provision of adequate levels of oxygen and water. This is, the irrigation strategy must be tailored to match the oxygen and water holding capacity of any substrate. I.e. different hydroponic substrates possess very different physical properties which reflects on their water and air properties. For example, when looking at inorganic substrates just one study shows that perlite has high total pore space, higher than 93%, but low water holding capacity. Vermiculite has high total pore space (92-94%) and good water holding capacity. Rockwool granulate contains 90-96% total pore space and has moderate water holding capacity. Expanded clay aggregate has relatively low total pore space (60-70%) and water holding capacity (18-26%).[4]

 

In general, when looking at various substrates we can see that different substrate types have very different water and air properties. See following table…

 

Growing Media Water Holding Capacity Air Porosity
Coconut Coir 88.35 23.5
Perlite 19.63 41.1
Peat-Lite 84.78 20.0
Coir/Perlite 57.40 35.0
Rockwool 86.85 10.3

Table information extracted from Comparing Five Growing Media for Physical Characteristics and Tomato Yield Potential. Jensen, M., Rorabaugh, P. and Garcia, M.. 2007[5]

 

Further, when looking at the same substrate type, such as coir, things can be just as variable. That is, coir substrate products are produced in several countries, and they have been shown to vary in physical properties according to the quantity of fibrous particles contained. Increased fibre is generally associated to increased air filled porosity and decreased water holding capacity. [6] For example, in one study, the physical properties of 13 coconut coir dusts from Asia, America, and Africa were analyzed. All properties studied differed between and within the sources of coir. Coir dusts from India, Sri Lanka, and Thailand were composed mainly of pithy tissue (small particles <1mm in diameter), whereas most of those from Costa Rica, Ivory Coast, and Mexico contained abundant fiber which was reflected by larger particle size distribution (percentage by weight of particles larger than 1 mm in diameter). Coir dust was evaluated as a lightweight material, and its total porosity was above 94% (by volume). It also exhibited a high air content (from 24% to 89% by volume) but a low easily available and total water-holding capacity which ranged from <1% to 36% by volume and from 137 to 786 mL·L–1, respectively. Physical properties of coir dust were strongly dependent on particle size distribution. Both easily available and total water-holding capacity declined proportionally with increasing particle size while air filled porosity increased.[7] To summarize this information, in simple terms, smaller particle coir substrates do not hold much oxygen while larger particle coir substrates do not hold much water.[8]

 

What this means, is that the optimal irrigation requirements of various coir substrate products will differ. That is, to achieve comparative air and water status between a coir that consists of a high degree of fibre, or large particles, and a coir with far less fibre (consisting of a high degree of small particles) more irrigation events would be required in the fibrous/ large particle substrate (low water holding capacity and high air filled porosity) than the low fibre/small particle substrate (high water holding capacity and low air filled porosity). This needs to be stressed. Too often industry interests and growers apply a broad brush and discuss coir growing optimums in one size fits all terms. Nothing could be further from the truth. Organic substrates (e.g. peat and coir) by their very nature are not uniform and particle size distribution can vary greatly between one product and the next. This has significant implications where ideal irrigation practices are concerned.

 

This information is perhaps too much too early. However, for now, suffice to say, that the physical properties of the various substrates available through hydroponic stores can differ greatly. Additionally, the physical properties of the same type of substrate (e.g. coir) can be significantly different. This has implications to their oxygen to water ratios and, in turn, to what would be considered an ideal irrigation strategy.

 

For now and very simply, the message I want to impart is over irrigate even the most air filled substrate and you will starve the plants of oxygen. Under irrigate and you will starve the plants of water. It doesn’t matter what that substrate’s physical properties are, these properties govern a substrate’s ability to retain or release moisture while feed frequencies can take advantage of a substrate’s moisture and release properties to realize better growth and yields. Therefore, once we have a substrate that is capable of accommodating for adequate levels of oxygen and water by far the most important factor in determining the oxygen and water ratio in the root zone is the irrigation strategy. [9]

 

It is important to note that research has shown oxygen deficiency in the root zone has immediate effects on the water uptake while nutrient uptake is also affected within a short space of time.[10] Therefore, it is imperative that adequate oxygen levels are maintained within the root zone for as much of the time as possible. What I mean by this (“as much as the time as possible”) is that directly following an irrigation, even in the most ideal substrate, oxygen levels may fall below desired levels due to water flooding the substrate and forcing out air. However, as the substrate drains, air space becomes available and oxygen levels begin to rise. If a substrate releases water quickly and has high pore space, oxygen will quickly rise and meet plant growth requirements in a short space of time. Therefore, the time in which oxygen falls below the sufficiency range in the root zone is short, meaning water and nutrient uptake remains optimal the vast majority of the time.

 

Based on this, to generalize somewhat, the ideal hydroponic substrate will have high air filled porosity, moderate water holding capacity and allow for easy drainage. Under these conditions, moisture content can be easily controlled through the irrigation strategy and adequate oxygen can be maintained in the substrate at all times.

 

This means that some substrates are more ideal than others for facilitating optimum levels of oxygen, nutrients and water. Further, some substrates provide a degree of tolerance for over or under watering while others tend to be far less forgiving. Therefore, the selection a substrate is an extremely important factor in hydroponic/soilless culture.[11]

 

One other physical property that is also worth touching on is the substrate temperature. The main functions of roots are water and nutrient uptake and synthesis of plant hormones. The substrate temperature effects water and nutrient uptake, root and shoot growth and metabolic processes. Among them, nutrient uptake is one of the most sensitive processes to temperature. Studies have shown that both water and nutrient uptake is impacted when root zone temperatures are above or below optimal.

 

Different plant species are affected more than others by suboptimal root zone temperatures. However, maintaining the substrate temperature between 20 – 25oC (68-77 oF) is fairly ideal for indoor, under light crops, For example, optimal root temperature is between 17-24oC (62.6-75.2 oF) for pepper[12] and between 20-25oC (68-77 oF) for tomato.[13]

 

Some substrates protect the roots from heat more effectively than others. For example, water acts as a conductor for heat. Therefore, if a substrate has a tendency to become saturated from top to bottom, it will conduct heat more than a substrate that tends to dry on top and hold less water in the upper regions. A couple of the substrate’s physical properties (particle size and whether the substrate is mineral or organic) determine how much heat it will conduct. Other than this, ambient air temperatures, the density of the plant canopy and pot, or slab, size and geometry also influence the conductance of heat.

 

Let’s leave the physical properties of substrates there for now.

 

Next we have the chemical properties of a substrate. These relate to such things as nutrients that are naturally found in the media, pH and ‘cation exchange capacity’ (CEC). The chemical properties have important implications to nutrient availability and uptake because chemical reactions/changes occur in the media as a result of its chemical properties. For example, nutrient availability is strongly influenced by pH because of changes in the form of the nutrient in the substrate solution relative to pH.[14]

 

CEC is also a chemical property.

 

CEC stands for ‘Cation Exchange Capacity’. CEC determines a substrates buffering capacity. Buffering refers to the resistance to change in pH or nutrient concentration in the substrate.

 

Substrate particles have negatively charged ‘exchange sites’ which attract and loosely-hold cations. Cations such as ammonium NH4+, calcium Ca++, magnesium Mg++ and potassium K+ carry a positive charge. As a result, they are attracted to the negatively-charged exchange sites by electrostatic forces.

 

Where a substrate has high CEC there are a lot of exchange sites and, therefore, large numbers of cations are held at the exchange sites for release into the substrate solution. Conversely, a media with low CEC has very few ‘exchange sites’. Substrates such as peat and coir have high CEC while substrates such as perlite and rockwool have low CEC. See following image….

 

CEC

 

A substrate can exchange nutrient cations back and forth between the exchange sites and the substrate solution. Therefore, the exchange sites act as a backup “pool” of nutrients to recharge the substrate solution when nutrient levels are low.

 

However, while this sounds quite positive, there is also something that growers need to be aware of when working with high CEC media. High CEC media such as soil can reduce, among others, calcium and iron availability, meaning these elements can become less available for plant uptake than in comparatively lower CEC growing media.

 

Where hydroponics is concerned, a very popular growing medium is coco coir which has a moderately high CEC. Nutrient binding is not a longer term problem, but growers do need to take some care during its initial period of use, while nutrients are being bound and hence are less available.

 

Many coco coir products supplied through the hydroponics industry are buffered (charged/primed) with calcium and magnesium during processing. This prepares the media for use and at this point nutrient binding is no longer an issue because the negatively charged particles of the media are preloaded with cations. Most inorganic hydroponic substrates, such as rockwool and perlite, have a very low CEC, so cation binding is not a problem in these substrates.

 

CEC also influences a media’s ability to a buffer against acidification. The lower the CEC, the faster the pH will decrease with time. The higher the CEC the more stable the pH tends to be.

 

Cations held on particles in media can be replaced by other cations; thus, they are exchangeable. For instance, potassium can be replaced by cations such as calcium or hydrogen, and vice versa.

 

There are two types of cations, acidic or acid-forming cations, and basic, or alkaline-forming cations. The Hydrogen cation H+ is acid-forming. A substrate with high levels of H+ is an acid substrate, with a low pH.

 

The cations calcium, magnesium, potassium and sodium are all alkaline cations, also called basic cations or bases. Alkaline cations increase pH while acidic cations reduce pH. Both types of cations (alkaline or acidic) may be adsorbed onto negatively charged media exchange sites.

 

Due to way the acid and alkaline cations interact in the substrate, high CEC helps to buffer pH and keep it more stable.

 

The next thing to understand is that the physical properties of a media can affect its chemical properties.

 

For example, bulk density, a physical property, influences CEC.

 

To understand bulk density let’s compare soil to coconut coir.

 

If we were to take two 5L buckets and fill one with soil and the other with coconut coir and then weigh each bucket, although we have equivalent volumes of each media, the bucket of soil would weigh far more than the bucket of coconut coir. Therefore, the soil has higher bulk density then the coconut coir. Bulk density is, thus, the weight of a unit volume of media.

 

Bulk density is strongly influenced by the particle size of any given media. For example, the bulk density of coir decreases as the particle size increases. Conversely, bulk density of coir increases as the particle size decreases. In simple terms, the smaller the particle size the higher the bulk density and vice versa.

 

Now, let’s say we took our 5L bucket of coconut coir and compacted/compressed it. Let’s say that 5L of coconut coir, after compaction, now takes up 3Ls of the 5L bucket. We have just increased the bulk density of the coir because it weighs the same as when it was 5Ls but now its volume is 3Ls. However, in increasing bulk density we have also compacted the media pushing out air space. What, in fact, has occurred is that in compacting the coir we have reduced pore space. Pore space allows the channeling of water through a substrate. As water drains from that substrate air fills a portion of the pore spaces which, in turn, provides oxygen to the rootzone.

 

Increased bulk density, therefore, comes at the expense of air space.

 

The CEC of coir on a weight basis is much higher than that of mineral soils. However, because of its low bulk density the effective CEC of coir on a volume basis, when compared to soil, is lower.

 

Because coir has a lower bulk density than soil it has lower CEC (on a volume basis) and as a result, its buffering capacity is lower than that of soil.[15]

 

This is just one of the misunderstood principles of substrate science by just a few hydroponic industry interests, some of whom promote high CEC hydroponic substrates, such as coir, with claims to the effect that coir has high CEC and as a result nutrients remain more balanced and available while pH remains more stable.

 

This claim is largely incorrect and may lead to less than ideal growing practices amongst hydroponic gardeners. For example, in research by Argo and Fisher (2008) where they compared the buffering capacity of high and low CEC hydroponic substrates, when an acidic fertilizer solution was applied to the plants grown in the different media, pH of a low CEC rockwool medium tended to be higher than for the higher CEC media. In all media, however, the pH dropped very quickly to a low of about 4 regardless of the CEC of the media. Therefore, the CEC of a hydroponic substrate has very little influence on overall pH stability.

 

In the same study, when shoot-tissue calcium was tested after four, eight, 12 or 17 weeks of growth, there was little difference between plants grown in the media with low CEC (rockwool perlite) or relatively high CEC (consumer grade peat + perlite). The media-CEC therefore did not act as a buffer to nutrient levels available for plant uptake.[16]

 

These findings were based on the differences in bulk density between soils and hydroponic substrates. I.e. soils with high bulk density and high CEC do offer buffering capacity while hydroponic substrates with moderate to high CEC and low bulk density offer little or no buffering capacity.

 

This is perhaps too much information, too early. The point being that; 1) CEC is a chemical property of growing media; 2) physical properties can influence chemical properties – i.e. bulk density and CEC/buffering capacity are interrelated; 3) the physical properties of soils and hydroponic substrates differ; 4) this has implications to the chemical properties of the two media.

 

On a practical level, where growing practices are concerned, high or low CEC hydroponic substrates largely act in the same way re pH and nutrient buffering. Therefore, growers should not assume that because they are using a high CEC hydroponic substrate (e.g. peat or coir) that this will aid nutrient status and reduce the need for pH maintenance through monitoring and adjustment.

 

On the other hand, claims have been made by some industry interests that hydroponic substrates should possess as little CEC as possible to guarantee optimal control over nutrition and pH. For example:

 

(Quote)

 

“Media with no or low Cation Exchange Capacity (CEC) guarantees complete control over nutrients available to the plant. It also helps maintaining a uniform pH, salt concentration through the root profile as well as to prevent decomposition through time.”

 

(End Quote)

 

However, when looking at the science, such claims are, at best, suspect (in this case the entity behind the claim had interests in selling a low CEC hydroponic substrate).

 

Last, but not least, we have existing nutrients in a substrate. This one doesn’t apply where the use of inert substrates (e.g. perlite, rockwool, expanded clay aggregate) are concerned. However, coco coir growers should be aware of the fact that coir products contain existing levels of electrolytes (e.g. potassium, phosphorus, sodium and chloride) and, therefore, this needs to be compensated for, in some cases, through treatment of the media (i.e. flushing and buffering) prior to use and, in all cases, through using a nutrient that is specifically formulated for coir growing. We go into far greater detail on this in my preferred method of growing on page…. so let’s leave that one there.

 

Having touched on the physical and chemical properties of growing media, let’s now go into far greater detail about the various physical properties of substrates and how these properties influence air and water holding capacity.

 

 

PARTICLE SIZE, PORE SPACE AND THE OXYGEN MOISTURE RATIO

 

 

Water retention and air content are dependent on particle size distribution of the growing medium and the pores created between these particles. Let’s begin by looking at pore space.

 

 

Pore Space – Micropores and Macropores

 

The word pore is derived from the term porous which literally means permeable by water, air etc.

 

Pores exist between and within particles and are occupied by water and air. Macropores are large pores, usually formed between particles, and are generally greater than 0.08 mm in diameter. Macropores drain freely by gravity and allow easy movement of water and air. They channel water and nutrients, hold air and allow root growth. However, while macropores aid in drainage and aeration they provide little water storage. See following image…

 

 

macropore

 

 

With diameters less than 0.08 mm, micropores are small soil pores usually found within the substrate particles. Micropores limit drainage and act like sponges. They absorb water and hold it against the pull of gravity until plants can use it. A percentage of the water in the micropores is unavailable to plants because it is tightly bound. See following image…

 

micro-macro-pores

 

Where hydroponic substrates are concerned, pore space plays a crucial role in determining optimum irrigation frequency because pores allow water, nutrients and air to flow freely through the substrate. As a general rule, a substrate with large pore space will have high air porosity and low water holding capacity while a substrate with small pore space will have high water holding capacity and low air space.

 

 

Particle Size, Pore Space and its Implications to Irrigation Frequency  

 

Shape, size, and arrangement of solid particles in a substrate decide the characteristics of pores through which water and air must flow.[17]

 

The smaller the particle size, because small particles tend to nest or settle within each other, the smaller the pore space, the less air and more water holding capacity Conversely, the larger the particle size, the larger the pore space, the more air and less water holding capacity.

 

Think of the relationship between particle size, pore space, water holding capacity and air filled porosity as this.

 

If I were to pour a given volume of water through clay, the clay would retain much of the water and what water it didn’t retain would drain very slowly. This is because clay consists of very small particles which results in small pore space due to the small particles of clay nesting and binding tightly together. This acts to reduce water and air flow. Therefore, because of these physical properties clay has high water holding capacity (WHC) but low air filled porosity (AFP).

 

Next, if we were to pour the same volume of water through expanded clay aggregate the water would flow quickly through the media and the aggregate would retain only a fraction of the water because the particles are large and therefore the pores between the particles are large also. Therefore, due to its physical properties, expanded clay aggregate has far higher AFP and far lower WHC than clay.

 

Plants growing in a media with large particles (e.g. expanded clay aggregate) could dry out very quickly and therefore would need very regular feeds to maintain an ideal oxygen moisture ratio (excessive large pores decrease the amount of water the media can hold[18]). On the other hand, plants growing in a small particle media (e.g. clay) would become waterlogged and oxygen starved very quickly if subjected to the same number of irrigation events (no doubt you’d drown any plant growing in clay very quickly applying even quarter of the irrigation that was required to maintain good WHC and AFP in expanded clay aggregate).

 

I’ve used two extremely different particle type examples here. Clay being a very small particle (the smallest particle found in soils – less than .0002mm in diameter) and expanded clay aggregate being, comparatively, a vastly larger particle at typically between 4mm to 16mm. The result of these particle size differences equates to Hydroton (expanded clay aggregate) possessing an AFP of about 42% while clay has about 3% AFP. Of course, you wouldn’t be growing hydroponically in clay (at least one hopes not!). The point for now, however, is to illustrate how particle size influences pore space, and how pore space influences the oxygen moisture (WHC and AFP) ratio of any given media. See following image comparing expanded clay aggregate particle size and pore space to that of clay.

 

clay-particle-size

 

What this boils down to is that there is a very important interrelationship between particle size, pore space and feed frequencies. Particle size governs a substrates ability to retain or release moisture while feed frequencies can take advantage of substrates moisture and release properties to realize better growth and yields.[19] As Holtman et al (2014) put it, “by far the most important factor determining oxygen concentration in the root environment is the irrigation strategy…” [20]

 

 

Particle Size Distribution (PSD)

 

Particle size distribution is important for describing the physical quality of a substrate and its suitableness for plant growth. It influences the volume of air and water held by the substrate.

 

The size of the particles that make up a hydroponic substrate are not uniform (unless it has been treated, through sieving, to create uniform size) and, therefore, particle sizes found in substrates will range between small, medium to large and be distributed across/throughout the substrate at varying percentages and ratios. The sizes of the particles and the percentage of the various sizes is measured by particle size distribution (PSD), also known as gradation, which is generally listed by percentage of the relative amount of particles present in the substrate according to size.

 

The most easily understood method of establishing particle size distribution is sieve analysis, where the media is separated on mesh sieves of different sizes. The PSD is defined in terms of differing size ranges (e.g. % of sample below 0.85mm; % of sample between 0.85 – 1.40 mm and % of sample of anything larger than 1.40 and up to 2.36mm) when sieves of these sizes are used. The PSD is usually determined over a list of size ranges that covers all the sizes present in the media. For example, a coir substrate product may be graded as small particle = <0.85 (30%), medium particle = 0.85 – 1.40 mm (50%) and large particle = 1.40 – 2.36mm (20%).

 

Given pore space influences how fast or slow water flows through a substrate, particle size distribution becomes important when discussing AFP and WHC because smaller particles can fit in between larger particles and reduce pore space. As a result, the ability for water to flow through a substrate is reduced where a high percentage of small particles are found in a mixed particle size media. See following image…

 

particle-distriibution

 

Permeability of a Media – More on Particle Size Distribution  

 

The size of pore space and interconnectivity (effective porosity) of the spaces help determine permeability, so shape and arrangement of media play a significant role in determining permeability.

 

Permeability is a measure of a media’s ability to transmit a fluid, usually water.  Often the term ‘hydraulic conductivity’ is used when discussing permeability of soils and soilless substrates.  Hydraulic conductivity is a measure of how quickly water moves through the pore spaces of the substrate from top to bottom (point of irrigation to drainage) – it is a measure of (length)/(time). The hydraulic conductivity of any media is largely determined by the intrinsic permeability of that media.

 

The larger the pore space, the more permeable the material.   However, the more poorly sorted a media (mixed particle sizes), the lower the permeability because the smaller particles fill the openings created by the larger particles. For this reason if we have high degrees of small particles included with large particle media we reduce permeability, which acts to increase water holding capacity and decrease air filled porosity. See following image….

 

particle-uniform-mixed

 

 

Effective porosity

 

When looking at the preceding image, we can see that the smaller particles in the mixed particle size media (at right) reduce effective porosity.

 

To understand effective porosity, total porosity includes all the pore spaces in a media, whereas effective porosity only includes the interconnected pores that allow water to flow through a media. In general, the larger macropores in a media provide most of the effective porosity while the smaller micropores provide very little effective porosity.

 

Therefore, a media with larger particles and, as a result, larger pore size will have higher effective porosity than a media with smaller particles and smaller pores.

 

As a result, a substrate with higher effective porosity will drain more quickly, facilitating higher air filled porosity than a substrate with lower effective porosity.

 

Applying Theory to Practice: Substrate Choice, Growing Style and Irrigation Strategy

 

Often growers will ask me: “What is the best hydroponic substrate?” The easy answer to this is there is probably no such thing.

 

Over the years I have used numerous hydroponic mediums and/or combinations of media to blend a single substrate. In most cases I have had great results whether that be working with peat, peat + perlite, perlite, perlite + vermiculite, expanded clay, coir, coir + perlite, coir + peat +perlite etc.

 

Quite simply, a plant doesn’t care about what substrate it is being grown in. All the plant cares about is that critical root zone elements (temperature, oxygen, water, nutrients) are provided within optimal ranges.

 

This leaves plenty of options where the choice of a substrate is concerned.

 

This said, some substrates offer better physical properties than others. For example, root distribution in container media can be influenced by particle size distribution. A medium with a high water-holding capacity and lower aeration may result in a concentration of roots in the top portion of the media, especially if the medium in the bottom portion of the media remains saturated for extended periods of time.[21]

 

Therefore, I do tend to recommend that the ideal hydroponic substrate should provide for high air filled porosity, moderate water holding capacity and good drainage. These properties can then be fully exploited by the irrigation strategy to ensure optimum water and air relationship in the root zone at all times (or the vast majority of the time).

 

Additionally, some substrates tend to provide more fail safes, and/or be more forgiving than others where irrigation and other factors are concerned. For example, expanded clay aggregate or perlite are very good recycling system growing substrates until a pump failure occurs and the substrate quickly dries out leaving the plants without water. If the pump failure isn’t noticed within a short space of time this can result in stressed, unhealthy plants (in a worst case scenario the plants can become so water stressed that they are unable to recover). Other than this, after many years of consulting to indoor growers I have found some substrates are far more forgiving where less than ideal environmental factors and/or growing practices present. For example, some substrates seem to protect the roots from excessive ambient air temperatures far better than others. Additionally, some substrates tend to promote healthier root growth and be more tolerant to over or under watering than others. This makes some substrates more ideal for novice indoor gardeners than others.

 

My own preference for a substrate, and one I have long promoted to novice growers, is a combination of coconut coir and perlite. There are a few reasons for this. Firstly, some organic substrates such as coir tend to offer similar security to growing in soils; however, you get the higher growth rates, faster finishing times and yields attributed to hydroponic technologies. Even better, when discussing growth rates in coir, studies have shown time and again that coir produces higher yields than inorganic hydroponic substrates such as rockwool and perlite.

 

Secondly, my preferred method of growing is run-to-waste (RTW/DTW).

 

RTW/DTW growing provides fresh, unadulterated nutrient at every feed and, therefore, compared to recycling systems, RTW/DTW growing provides a higher degree of control over the nutrition that is provided to the plants at each and every feed (see pages …. for more information);.

 

Coir, because it has high water holding capacity, is an ideal RTW/DTW growing media and this is just one reason why I initially began experimenting with coir and perlite combinations many years ago.

 

Lastly, where mixing perlite with coir is concerned, inline to research findings, I have found that providing the plants with a low EC feed solution (i.e. EC of approx 1.2 in bloom) at high fertigation frequency provides benefits to yields over and above fewer irrigations at higher EC. That is, to come back to material we covered in the nutrient section of IH, surrounding high fertigation frequency and improved nutrient status (page….), several studies have shown that high fertigation frequency results in better nutrient status in the root zone. Studies have demonstrated that increased fertigation frequency significantly increased plant yield, especially at low nutrient concentrations.[22]

 

Studies have shown that high fertigation frequency maintains higher dissolved nitrogen, phosphorus and potassium concentration in the substrate, by shortening the period during which nitrogen, phosphorus, and potassium retention takes place.[23] Phosphorus (P) nutrient status is particularly improved under high fertigation frequency. Studies have shown that yield improvement as a result of high fertigation frequency is primarily related to the enhanced nutrient uptake of P.[24] Thus, high fertigation frequency can serve as an efficient means of enhancing crop yield by improving the uptake by plants of less mobile nutrients.[25] This is particularly true during periods of the most vigorous growth where the plants have high nutrient needs.

 

However, as a result of employing high fertigation frequency I am applying water and nutrients with small intervals between irrigations. Therefore, if I were working with a substrate with high water holding and, comparatively, low air capacity there is a danger of over-wetting (water-logging) the substrate and subjecting the plants to oxygen stress.

 

On this note:

 

Cocopeat is considered as a good growing media component with acceptable pH, electrical conductivity and other chemical attributes. But it has been recognized to have high water holding capacity which causes poor air–water relationship, leading to low aeration within the medium, thus affecting the oxygen diffusion to the roots.” [26]

 

Therefore, in many cases, coir alone is not suitable as a substrate where high frequency fertigation is applied. For example, in a study conducted by the Arizona College of Agriculture (2008) different substrates and substrate mix (granulated rockwool, 100% coconut coir, 50% coco + 50% perlite, 70% coco + 30% perlite) were tested on the growth of strawberry. It was concluded that the growth and yield of strawberry plants were restricted when 100% coconut coir was used as the substrate but was greater when rockwool and a 50:50 mix of coir and perlite was used, supporting the hypothesis of the importance of oxygen availability in coir…[27] which brings us to why I recommend to blend coir with perlite (at about a 50:50 ratio). This is where understanding the theory we have covered on particle size distribution comes into play.

 

Perlite, which has a relatively larger particle size than coir, has high air filled porosity and low water holding capacity.[28] By adding perlite to coir we increase air and decrease water holding capacity. The result is a substrate that drains well and promotes both adequate levels of water and air under various irrigation strategies. I.e. a combination of coir and perlite tends to create a very user friendly, versatile medium which maintains good water and air status under both higher and lower irrigation events. .[29] The end result is a more ideal root zone environment and better yields.

 

Therefore, I mix coir and perlite to create a blended substrate that is ideal for high frequency fertigation and fewer irrigations as well. That is, I blend two media with differing physical properties to create a single substrate that has good drainage, moderate water holding and high air capacity. Of course, given the theory we have covered to date we do not need to limit ourselves to blending coir with perlite to increase the air capacity of coir. We could just as easily use another larger particle media with high air capacity to achieve the same thing. For example, when looking at the following table we can see that burnt rice hull could just as easily be used in coir to increase AFP. See following table where air filled porosity (AFP) of coco coir, burnt rice hull and perlite mixtures at 2 and 5 hours after soaking are compared.

 

 

 

Media (%)

 

Hours after soaking

 

Aiir-filled porosity (%)

100% coco coir 2

5

28.1

29.8

70%: 30% burnt rice hull 2

5

38.0

36.9

70% coco coir: 30% perlite 2

5

37.4

37.4

 

Table information extracted from Yahya Awang et al (2009) Chemical and Physical Characteristics of Cocopeat-Based Media Mixtures and Their Effects on the Growth and Development of Celosia cristata

 

Similarly, we could blend larger particles with smaller particles of the same type of media (in this case coir) to increase AFP and reduce WHC. See following table.

 

 

Effect of particle size on water holding capacity and air filled space of coco coir

 

 

Coir Particle Size Water Holding Capacity Air Filled Space

 

 

Small = <0.85mm

 

 

87.61

 

9.82

 

Medium = 0.85 – 1.40mm

 

 

65.17

 

24.64

 

Large = 1.40 – 2.36mm

 

 

41.36

 

48.69

 

Table information extracted from Damien Duggan-Jones (2012) The Effect of Coir Particle Size on Yield of Greenhouse Tomatoes (Lycoersicon esculentum Mill): Master’s Thesis, Massey University, New Zealand

 

Looking at the table we can see that the larger the particle size of coir the higher the air space and the lower the water holding capacity. Conversely, the smaller the particle size the lower the air space and the higher the water holding capacity.

 

Given this information, if we to blend larger particles with smaller particles of coir we could increase the AFP and WHC in the final substrate to be used for crop production.

 

See following table that demonstrates the effect that blending various particle sizes of coir has on water holding capacity and air filled porosity.

 

Mix of Coir Particle Sizes Water Holding Capacity

(WHC)

Air Filled Porosity

(AFP)

Small + Medium (SM) 86.11 10.75
Small + Large (SL) 75.28 21.83
Medium + Large (ML) 49.81 40.36
Small + Medium + Large (SML) 80.35 17.28

 

Mix particle sizes consist of: Small = <0.85mm, Medium = 0.85 – 1.40mm, Large = 1.40 – 2.36mm

SM = 50% S and 50% M, SL = 50% S and 50% L, ML = SL = 50% M and 50% L, SML = 33% S and 33% and M and 33%

 

Table information extracted from Damien Duggan-Jones (2012) The Effect of Coir Particle Size on Yield of Greenhouse Tomatoes (Lycoersicon esculentum Mill): Master’s Thesis, Massey University, New Zealand

 

We can see in this table that by mixing larger particles with smaller particles of coir we increase air filled porosity and reduce water holding capacity. On this note, these days some manufacturers are now providing coir substrate products that help facilitate a more ideal oxygen moisture ratio. This comes down to some manufacturers producing coir substrate products which contain a mixture of small, medium and large particle sizes at varying percentages to provide a more ideal oxygen moisture ratio than a small particle distribution coir substrate would possess. Speak to your hydroponic supplier for more information about mixed particle sized substrate options.

 

To dispel a myth of coir….over the years it has been asserted by some industry interests that it is hard to overwater coir. This claim is incorrect! For example, one study showed that coir substrate contained about 40% less oxygen than Grodan Rockwool or perlite. Quite simply, many coir substrates are prone to water logging and, therefore, accurate feed regimes are required.

 

On many occasions I have spoken to coir growers who inform me that they find that coir performs at its best when allowed to dry out between wetting/feeds. That is, they find low irrigation frequency (i.e. long intervals between irrigation events – perhaps 1-2 irrigations per day) results in better growth rates and higher yields than where higher irrigation events are used.

 

However, when I quiz them further on just what it is they are doing, in the vast majority of instances, it turns out that they are using straight coir (no added perlite). To repeat the point, coir, dependent on particle size distribution, has high water holding capacity, but less than ideal air capacity and, therefore, it requires mixing with a larger particle media (e.g. perlite) to ensure optimum water to air ratio at all times. What these growers have discovered through not adding larger particle media to coir is exactly this and in order to compensate for too much water and not enough air in the substrate they are forced to allow the media to dry out to provide more air to the root zone. However, what this means is that after irrigation the media is low in oxygen (too saturated in water) while when dried out the media is potentially low in easily available water* while holding enough air. Further, because there are long intervals between irrigation events this is probably compromising the root zone nutrient status. This could have easily been avoided had they simply mixed the coir with perlite. Either way, they have added more complexity to their feed regimes than necessary and compromised an ideal root zone environment (optimal oxygen to moisture ratio and nutrient status) in the process.

 

Author’s note: *Easily Available Water (EAW)

 

Plants can take up only those nutrients that are dissolved in the substrate solution. The term ‘substrate solution’ refers to the film moisture in the substrate together with its dissolved substances (e.g. nutrient ions). Therefore, if water that is directly available to the roots for uptake is inadequate nutrient availability is low. This is where easily available water becomes extremely important when considering nutrient availability.

 

Put simply, substrates have the ability to hold and release water; some water is readily available to the plants, while some of the water isn’t available even if the substrate seems moist.

 

The water that is available to plants is called ‘plant available water’ (PAW); the water that is unavailable to plants is called ‘plant unavailable water’ (PUW). While the science that underpins plant available and unavailable water is extremely complex, in very simple terms, the portion of the water that is unavailable is largely determined by the forces of adhesion and cohesion of a substrate which acts to bind water molecules so tightly that plants are unable to access this water.

 

This comes down to the physical characteristics of a substrate. For example, particle size plays a key role in determining plant available water because water is attracted to surfaces with a force large enough to support a relatively large mass of water against the pull of gravity. A given volume of small particles has greater surface area than the same volume of larger particles and, therefore, there is a greater force to hold water against the pull of gravity. Based on this, the smaller the particles that make up a substrate the more firm the hold on water and the less plant available water, relative to total water holding capacity.

 

Next, we also need to consider plant available water (PAW), which is the water that is so loosely held by the substrate that plants can pull it into their roots. However, only some of this water lies directly within the root zone which means a percentage of PAW is outside of the root zone, making it hard for the plant to access. This means the plant has to exert energy to access the water outside of the root zone. As a result, this increases the energy the plant has to use to access this water and starts to effect yield and quality. Most plants will undergo substantial water stress long before visible symptoms of water stress (e.g. wilting) is apparent.

 

Easily available water is the percentage of PAW that is directly in the root zone and therefore readily/immediately available for uptake by the plant. EAW is, therefore, readily extractable water that is directly available to the roots.

 

With this information we can then understand that only a percentage of the water in a substrate is EAW. For example, we may have a substrate that has 65% water holding capacity but only 25 % of this is EAW. We can also understand that the more easily available water there is, in conjunction with adequate oxygen (i.e. adequate quantities of easily extractable water and oxygen in the root zone), the more ideal things are with regards to water and nutrient availability.

 

When discussing coir specifically, research has shown that coir has relatively low EAW. For example, coir with a particle size distribution similar to peat possesses comparatively higher aeration and lower capacity to hold total and easily available water.[1]

 

Although coir is capable of holding large quantities of water, much of this water is held very tightly by the coir and is not easily available to the plant; whereas, much of the water held in peat is easily available to the plant. [2]

 

This has implications to ideal irrigation frequency between the two substrates; i.e. because peat releases water more efficiently to the plant than coir, more irrigation events per day would be required in coir than peat to provide comparative levels of EAW to the plant.

 

Actually, as a bit of a story and to hammer home the point, some 12-13 years ago I wrote about growing in coir and perlite mixes. When this material was made available in 2002 through Edition 1 of Integral Hydroponics, an Australian supplier of a multinational coir substrate product began circulating information that I had got it wrong and that coir should never be mixed with perlite.

 

However, this particular company, at the time, and for many years after, was recommending that plants grown in their coir (no added perlite) should be irrigated with multiples of small feeds over the course of the day. In other words, this company was recommending that their coir should be treated with high irrigation events to achieve optimum yields. Similarly, I had recommended that multiples of small feeds over the course of a day (high irrigation events) in a 60% coir 40% perlite mix was optimal where growth rates were concerned. One of us seemingly had it right, while the other seemingly had it wrong. After all, the water to air relationship is very different in straight coir when compared to the 60% coir, 40% perlite mix I had recommended. There was no feasible way that the same irrigation strategy recommended in very different substrates could result in the same thing (i.e. optimum yields)!

 

Flash forward, some 13 years later and as I pen this today, the same company is now informing its customers that their coir product performs best at one single irrigation event per day (perhaps two for larger plants). The key, they claim, is to allow the coir to dry out which based on their findings results in 3% more air in the substrate which, in turn, leads to yield improvements of 6-10%.[1] On this note, we can see how important the root zone environment is, in relation to yields. In this case, 6-10% yield can be gained through ensuring that 3% more air is available. Of course, this also tells us that this company’s original recommendation of multiples of small feeds over the course of a day was costing growers, for many years, 6-10% in yield (amounting to massive yield losses when considering the indoor growing scene as a whole). Conversely, had growers taken my advice they would have been in a much better position where yields were concerned…that’s not to say “I told you so”, but to instead highlight the importance of the irrigation strategy and the air to water relationship in any given substrate.

 

Additionally, this also tells us that this company’s coir product is not suitable for higher irrigation events and, therefore, based on their latest recommendation of one irrigation event per day, while this may provide more ideal aeration it will also, in all probability, leave the root zone nutrient status wanting. Other than this, directly following irrigation and for some time thereafter (with this particular coir product) oxygen in the root zone will fall below optimal and only as the media dries out would this situation change. This has some significance because, to repeat an earlier point, research has shown oxygen deficiency in the root zone has immediate effects on the water uptake while nutrient uptake is also affected within a short space of time.[2] Therefore, the longer that levels of oxygen remain low in the root zone, the longer water and nutrient uptake is potentially affected. Because this coir product has high water holding capacity, directly after irrigation air/oxygen falls below optimal and it may takes hours before adequate air/oxygen becomes available. During this period, more probable than not, water and nutrient uptake is negatively impacted. Other than this, one further impact may be that because irrigation only occurs once daily and plants consume large amounts of water, easily available water (EAW) may become low in the substrate. I.e. coir has high watering holding capacity but only a percentage of this is EAW so even when the coir appears moist it may be low on EAW. Low water in the substrate may have some impact on the mass flow of nutrients. I.e. water supply in a substrate has a strong influence on the nutrient supply. As water becomes more limited, decreased substrate water movement reduces mass flow of nutrients to the roots, and increased concentration of nutrient salts in the substrate solution such as exchangeable cations like calcium reduces the activity of anions like phosphate. This is just one reason that high fertigation frequency is shown to provide better nutrient status in the root zone. Of course, I’m dealing with hypotheticals as to what may or may not be occurring at one (small plants) to two (larger plants) irrigation event/s per day but applying theory can often tell us that one practice (higher irrigation events) may be better than another (fewer irrigation events).

 

Given this, there is a far better option than providing the plants with a single irrigation – two at most – per day. That is, 3% or far more air could be gained by simply adding perlite while also allowing growers to irrigate more frequently which, in turn, would help to maintain better nutrient status in root zone. Quite simply, most hydroponic experts assert that an ideal media for hydroponic growing should have moderate water holding capacity, good drainage and high air capacity. Many straight coir substrates do not provide these qualities and therefore can be improved with the addition of perlite. See following table which demonstrates how addition of perlite increases AFP of coir.

 

 

Growing Media Water Holding Capacity Air Porosity
Coconut Coir 88.35 23.5
Perlite 19.63 41.1
Coir/Perlite 57.40 35.0

 

Compare the numbers of each substrate type. You will note that coir alone has a WHC of 88.35% and an AFP of 23.5% while perlite, a relatively larger particle, has a WHC of 19.63% and an AFP of 41.1%. By adding the perlite to the coir (smaller particles + larger particles) we increase the coir’s AFP to 35% and reduce WHC to 57.40%. Thus, AFP is increased 11.5% and WHC is decreased 30.95% by increasing the particle size distribution of coir with perlite. The result is a hydroponic substrate that drains well and promotes both adequate levels of water and air under various irrigation strategies. That is, a combination of coir and perlite tends to create a versatile growing medium which maintains good water and air status under both higher and lower irrigation events. The coir acts to hold plenty of water under lower irrigation events (e.g. 3-6 irrigations per day during grow, stretch and late bloom) while the perlite acts to provide plenty of air under higher irrigation events (e.g. 12-15 irrigations per day during flowerset/full bloom).

 

Anyway, let’s not get too caught up in talking about coir and perlite growing… there is a whole section about just this on pages …. However, you can perhaps see the logic I am applying when considering my growing method and choosing/blending a substrate to suit. Thus, to repeat a very important point and put a bit of a spin on it, “regardless of the substrate being used, an extremely important factor in determining the oxygen and water relationship in the root zone is the irrigation strategy.” On the other hand (the spin)… if you wish to employ a specific irrigation strategy, it is imperative that you use a substrate that is suited to that irrigation strategy.

 

Keep in mind that a key underpinning principle of Integral Hydroponics, raised at the very beginning of the book, is…

 

“There is no one way of doing things. There are only sound principles to adhere to. Understanding and applying these principles is the best place to begin.”

 

Therefore, when discussing high fertigation frequency and run-to-waste growing, rockwool also makes for a reasonably good substrate choice. For example, in the study that was cited in the nutrient section, on page ….., surrounding high fertigation frequency and better nutrient status in the root zone in RTW/DTW growing, 20 feeds per day were applied. This study showed that fertigation with high fertigation 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.”[3] In this study, the plants were grown RTW/DTW in Grodan Rockwool slabs. Rockwool has high water holding capacity and a steep gradient in water content occurs from the top to the bottom of the slab following irrigation and gravity drainage. The air volume at the bottom of the slabs is only about 4% at about 1 cm above the base while the upper layers are dryer and, therefore, possess higher air. Grodan states that “GRODAN Rockwool, when allowed to drain by gravitational pull, i.e. at field capacity contains 80% solution, 15% air pore space and 5% Rockwool fibers.”[4]

 

Putting this aside (too much information), to simplify, Grodan rockwool slabs can become quite saturated, especially in the lower regions, directly after irrigation. However, the slabs drain quickly (due to plant water uptake and gravity drainage) leaving the top area dryer and creating adequate quantities of available oxygen for plants. Because of its physical properties rockwool can simultaneously hold plenty of water and air.[5] This makes rockwool a reasonably good choice for use in drip fed (multiples of very small feeds), RTW/DTW high irrigation frequency situations.

 

However, what I would add here, from my own experiences, and on a practical level, is that rockwool has the tendency to become waterlogged if too heavily irrigated and therefore a word of caution for those who choose to use rockwool in high frequency fertigation situations. For myself, I have found a coir and perlite mix more to my liking as a substrate that offers versatility and root zone security over and above that of rockwool.

 

This said, ultimately growers must make their own decisions about what methodology they apply to their growing practices. This applies to, among other things, whether you choose to grow run-to-waste or recycling and which substrate you choose to use.

 

My point here is to demonstrate that in order to maintain ideal root zone conditions (air, water, nutrients) a certain logic needs to be applied when considering the substrate’s physical properties and the irrigation strategy that will be most suited to that substrate. Quite simply, the more that you know/understand about any media’s properties the better you can anticipate how that media will act under a given set of conditions. As previously noted, substrate science is an extremely complex subject; however, in understanding some fundamentals you are now in a far better position as a grower to be able to understand how to control the root zone environment and provide ideal water, air and nutrition to the plant leading to, when other key environmental factors are in check, optimum yields.

 

For more information about substrates speak to your hydroponic supplier. Questions worth asking are: 1) What growing method do you recommend? 2) Why do you recommend this growing method? 3) What substrate is ideal for this method? 4) Why is this substrate ideal? 5) What is the recommended irrigation strategy for the substrate? And why?

 

Based on the answers that you receive, through now having an understanding of nutrient and substrate science (knowledge is power!), you will be able to determine whether the information the retailer provides is based on sound scientific principles, or otherwise.

Okay, let’s leave substrate science there.

 

References

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[2] Wessel L. Holtman, Berry J. Oppedijk, Marco Vennik and Bert van Duijn (2014) Low Oxygen Stress in Horticultural Practice – In Joost T van Dongen, Francesco Licausi (Eds) 2014. Low-Oxygen Stress in Plants: Oxygen Sensing and Adaptive Responses to Hypoxia, Springer-Verlag Wien. pp. 379

[3] Morard, P. Lacoste, L & Silvestre, J. (2008) Effect of oxygen deficiency on uptake of water and mineral nutrients by tomato plants in soilless culture

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[2] Nichols, M.A (2013) COIR: Sustainable Growing Media, in Practical Hydroponics and Greenhouses December 2013 / Issue 138

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[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] Michael F. Dowgert, Rockwool as a Growing Substrate for Hydroponic Systems retrieved from http://grodan101.com/knowledge-center/rockwool-growing-substrate-hydroponic-systems

[5] RAVIV, M. & LIETH, J. 2008. Soiless Culture Theory and Practice, London, Elseiver.