HYDROPONIC COIR SUBSTRATE SCIENCE – ISSUES TO BE AWARE OF
Why Hydroponic Growers Must use a Reputable Flushed and Buffered Coir Substrate Product
Excerpt from Integral Hydroponics Evolution
When I first published Integral Hydroponics in 2001 – 02 I had no idea that the book would go onto sell, to date, over 100,000 copies globally and become the Australian and New Zealand hydro industry bible. In simple terms my humble scribblings went on to influence many thousands of growers when, in fact, I had first written the book to supply it to my own hydroponic store customers, perhaps estimating that we’d sell and/or give away several hundred copies a year.
One impact Integral Hydroponics had was to switch many Australians and others over to coco coir substrate and RTW/DTW growing. Not a bad thing really as to this day I am yet to find a better growing methodology for novices. Quite simply, RTW/DTW growing in coco substrate is fantastic and I highly recommend it.
However, when I first penned Integral Hydroponics in 2001-02 coconut coir was a relatively new substrate to the Australian hydroponic retail market and I had advised growers to only use a quality flushed and buffered coir product. At that time, based on my own experiences, I had recommended using Canna coir substrate because it was, in those days, one of the only correctly treated (aged, graded, flushed and buffered) coir substrates available through the Australian retail hydroponics sector. Further, in IH Edition 1 I had recommended using a coir specific nutrient. This was scientifically sound advice, although I perhaps erred in not hammering home enough the importance of this information.
Let’s not make that mistake again!
THE IMPORTANCE OF USING A HIGH QUALITY WASHED AND BUFFERED COIR SUBSTRATE
The Physical and Chemical Properties of Coir
Chemical Properties of Coir
The chemical properties of coir can vary significantly between coir products or, for that matter, even vary on a batch-to-batch basis of the same product. There are several very important factors to consider when discussing the chemical properties of coir and how these influence the suitability of the product for hydroponic production. These factors are; 1) the levels and types of naturally present salts in coir; 2) the CEC (cation exchange capacity) of coir; 3) the C:N (carbon to nitrogen) ratio of coir and; 4) the presence of potentially phytotoxic phenolic compounds in coir.
Naturally Present Salts Found in Coir
Coir can contain extremely high levels of sodium (Na), potassium (K) and chloride (Cl) salts. These salts can have a major impact on plant health and growth if they are not reduced through the proper pretreatment of a coir substrate products.
Excess Na in the substrate can cause Na toxicity and Na competes with calcium (Ca) and other cations to reduce their availability for plant uptake.
Chloride (Cl) and Sodium Chloride (NaCl)
While chloride is an essential plant nutrient, excessive chloride levels can cause phytotoxicity. The toxicity results from accumulation of chloride in the leaves. Other than this, sodium and chloride combine to form sodium chloride (NaCl), or common table salt. Sodium chloride (NaCl) can be toxic at even 50ppm to some plants while tomato plants, which are classed as moderately or semi-tolerant to NaCl, can tolerate approximately 200ppm of NaCl. However, yields may be affected at much lower levels than this. For example, Heuvelink (2005) points out that tomato crops can be grown in a nutrient solution containing 100ppm of chloride without too much difficulty; however, it is more ideal that it contains below 50ppm of sodium and 70ppm of chloride.
Too much potassium, which is absorbed rapidly by plants from the substrate 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 and magnesium uptake. Large amounts of calcium are required for optimum flower development while magnesium activates the plant enzymes needed for growth.
Actually (show, don’t tell), let’s have a look at a lab analysis of an untreated coir substrate product to demonstrate just what salts/ions can be found in coir.
The lab analysis was conducted on a compressed coir product from Holland (originally deriving from Sri Lanka) which was tested after questions were raised as to why the product was seemingly killing plants. The test is related to the total salts and not salts that are immediately plant available. However, without going into great detail about different soil/substrate testing standards, what is immediately apparent is this sample of coir possesses extremely high levels of K, S, Na and Cl.
Try to remember that coconut palms grow well in areas of high salinity. This means they can uptake a lot of salt (NaCl) from their environment. Plants that are salt tolerant are able to uptake salt and then displace it into areas of the plant where it does the least harm. Seemingly, much of the salt is displaced into the coir of the coconut palm (the very thing we use as a hydroponic growing medium).
Additionally, coconut palms uptake high levels of nutrients from the soil. One study showed the amounts of macronutrients uptaken by coconut palms were 116.79 kg N, 14.02 kg P, 245.43 kg K, 40.47 kg Ca, and 33.66 kg Mg per hectare per year. Some of these nutrients invariably end up in the coir of the plant which is then used as a hydroponic substrate. In this case, based on the lab analysis, you will note 3700ppm of Potassium and 1978ppm of sulphate. This tells us that there are sometimes extreme levels of potassium and sulphate that are naturally present in just some untreated coir products.
Because coir can contain high levels of potentially harmful salts, quality coir products that are ideal for hydroponic growing need to be washed/flushed and buffered. Washing/flushing coir with fresh water leaches some of the K, Na and Cl out of the coir. However, due to its CEC properties, washing/flushing coir alone has limitations and the washing/flushing process will only remove some of the salts from the substrate. This is why buffering coir with calcium, or a combination of calcium and magnesium, is imperative.
Buffering Coir with Calcium and Magnesium
Coir is an organic substrate that possesses moderately high CEC.
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 moderately high CEC while substrates such as perlite and rockwool have very low CEC.
A substrate with high CEC 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, the CEC exchange sites of coir are naturally loaded with potassium (K) and sodium (Na) with little or no calcium (Ca) or magnesium (Mg).
As a result, if not prepared correctly, coir can bind/lock Ca and Mg meaning these elements/ions/salts can become unavailable for plant uptake until the coir has been in use for some time and is fully conditioned. Additionally, other elements such as phosphorus (P) and iron (Fe) can become problematic in coir substrates that aren’t washed/flushed and buffered correctly.
Correcting the CEC through Cal Mag Buffering
Because the cations on the exchange sites are held tightly washing coco with water does little to change the makeup of the cations on the exchange sites. The washing will reduce the EC but not the CEC.
CEC sites have a preference for some cations over others. For example, if the cations of Ca, Mg, Na and K are all present in the solution at the same concentration they will be adsorbed at different levels with calcium and magnesium being adsorbed at double the rate as they both have a double-positive charge, while potassium and sodium have a single-positive charge (Ca++, Mg++, K+, Na+). If you like, when coir is buffered with calcium and magnesium, which both possess double-positive charges, these ions knock the single-positive charged ions potassium and sodium off the CEC exchange sites and replace them. Therefore, buffering coir with calcium and magnesium acts to reduce K and Na levels through replacing K and Na with Ca and Mg on the CEC exchange sites, resulting in a substrate that is more ideal for hydroponic growing.
For example, when looking at two lab analysis of the same coir source, one of which is washed/flushed and the other of which is washed/flushed and buffered we can see some significant differences where potassium, sodium and chloride levels are concerned. See following table.
|Coir Washed||Coir Washed and Buffered|
|Chloride mg/L (ppm)||104.7||25.4|
|Sodium mg/L (ppm)||40.6||22.1|
|Potassium mg/L (ppm)||102.5||14.5|
|Phosphorus mg/L (ppm)||7.4||2.5|
You’ll note that even after washing/flushing the un-buffered substrate has high levels of plant available potassium, sodium and chloride.
“Potassium and phosphorus concentrations in coconut coir were two and eight times higher than those in rockwool. The electrical conductivity, potassium and phosphorus concentrations in coconut coir increased 2.3, 4 and 17 times, respectively compared with those in the nutrient culture solution, while the calcium concentration decreased by one-fifth in the coconut coir medium. The amount of exchangeable calcium was also reduced by more than half in the coconut coir medium through exposure to the nutrient solution. These results suggest that calcium ion was immobilized in coconut coir.” 
Therefore, the high levels salts in the ‘coir washed’ product could cause problems if a hydroponic nutrient wasn’t formulated to accommodate for the existing salts the substrate contains. For example, because our washed, un-buffered coir substrate sample contains 102.5ppm of K, a nutrient formulated for this substrate would need to possess very low K and higher Ca and Mg to replace the K on the exchange sites; i.e. the K in the coir substrate plus levels of K found in a standard hydroponic coir nutrient would result in excessive K in the root zone which could easily result in a deficiency of Ca and Mg. Furthermore, the relatively high levels of Na combined with excessive K could further exacerbate Ca and Mg deficiencies.
Basically, the ‘coir washed’ sample is less than ideal for use in a hydroponic growing situation where standard off-the-shelf hydroponic coir nutrients are used.
However, when looking at the buffered sample (lab analysis) we can see this problem has been corrected; i.e. after buffering, the coir product looks far better with much lower levels of K (14.5 ppm v. 102.5 ppm), Na (22.1 v. 40.6 ppm) and Cl (25.4 v. 104.7 ppm). Therefore, the buffered substrate is far more suitable for use in hydroponic growing.
C:N Ratio of Coir
The C:N ratio represents the ratio of the mass of carbon to the mass of nitrogen in a media. For example, a C:N of 10:1 means there is ten units of carbon for each unit of nitrogen. Organic matter such as coir is subject to decomposition and the level of decomposition is important in relation to nitrogen (N) immobilisation. Immobilisation of N is the reduction in plant available N (i.e. nitrate or ammonium N) as a result of microorganisms using this N as they decompose organic materials with a high carbon (C) content. In turn, the N extraction from the substrate solution by microorganisms lowers available nutrient supplies to plants, which leads to plant nutrient deficiencies if additional N is not added to correct the problem.
Properly composting/biodegrading coir during processing has been shown to eliminate or significantly reduce the potential for N immobilization by lowering the C:N ratio and allowing the initial breakdown, which requires high levels of N by microorganisms. Therefore, a quality coir substrate product will have been properly pretreated through adequate aging/biodegradation to ensure that N immobilization doesn’t become an issue during the crop cycle.
Phytotoxic Phenolic Compounds in Coir
Having highlighted the importance of correctly aging/biodegrading coir during manufacture to offset potential C:N ratio issues, aging/biodegradation also becomes important when considering phenolic compounds.
Coir consists of lignin 20 to 40%, cellulose 40 to 50%, hemicelulose 15 to 35% and protein 2.04%. The high degree of lignin results in phenolic compounds which are the break down products of lignin. Studies have concluded that plant health and growth rates in improperly treated coir are negatively impacted by excessive levels of these phenolic compounds. For example, one study showed fresh coir dust severely inhibited root elongation of lettuce. The phytotoxicity was attributed to the excessive phenolic compounds in the coir. In short, fresh, improperly treated coir can be toxic to plants; therefore, adequate aging/biodegradation is required to decrease the presence of phenolic compounds.
Physical Properties of Coir
One other important issue to be aware of where achieving optimum yields in coir is concerned is too often growers and industry interests apply a broad brush when discussing coir growing optimums; as if all coir substrate products are the same and thus require similar irrigation treatments to achieve the same outcomes.
…Nothing could be further from the truth!
Coir is produced in several countries and has been shown to differ in physical properties according to the quantity of fibrous particles contained. Increased fibre is generally associated to increased air filled porosity and decreased bulk density and water holding capacity.  Conversely, less fibre which results in a smaller particle sized media results in higher water holding capacity and lower air filled porosity. When comparative analysis is made of the water holding capacity (WHC) and air filled porosity (AFP) qualities of various sources of coir often significant differences are found to exist. This largely comes down to the particle size distribution/average of the media which is determined by the percentage of small coir dust particles and the percentage of larger particles such as fibre. What this means is that two different coir substrate products may require very different irrigation treatments to maintain optimum air to water ratio. For example, if a fibrous, larger particle size/average product (high air to water ratio) and a low fibre, small particle size/average product (high water to air ratio), were subjected to the same irrigation treatments this may mean the low fibre product (high water and low air capacity) is over wet, resulting in low root zone oxygen, while the fibrous product (high air and low water capacity) may dry out between irrigations leaving plenty of oxygen but low plant available water. In short, two different coir products could require two very different irrigation treatments in order to realize optimum yields. For this reason, be somewhat cautious of advice given out online via blogs and forums, through books or through industry interests about optimum irrigation strategies in coir. The person giving the advice could be talking about a coir substrate product with very different physical properties than the product that you are using.
You may have correctly noted that I myself am somewhat guilty of applying a broad brush to coir growing optimums when saying I blend coir and perlite at a 50:50 ratio, thereby implying that this is the correct way to do things no matter what coir product you are working with. Guilty as charged! Albeit, there is a method to my madness, if for no other reason than to highlight what I have just said about too many applying a broad brush when it comes to discussing coir growing optimums without considering that the physical (and indeed chemical) properties of various coir substrate products can be very different. Thus, optimum treatment will vary between coir products.
Over the years, when working with various coir products, I have adapted the perlite to coir ratio to suit the coir substrate and the irrigation strategy I am working with. Keep this in mind when refining your coir growing skills as you may want to experiment with different coir to perlite ratios. I have found side-by-side controls are the best means to do this. It is as simple as having different ratios of perlite and coir in several pots during your grow without changing anything else; i.e. all plants receive exactly the same treatment bar for the fact that the coir to perlite ratios differ in several pots. This enables you to dial things in perfectly and create the ideal root zone environment for your plants under high (or any other) fertigation frequency. Read more about side-by-side controls on page….
Conclusion and Closing Remarks on Coir Substrate Quality
You can perhaps tell there is a lot to producing a quality coir substrate that is ideal for hydroponic growing. This is something that growers need to be aware of. 15 or so years ago when I first penned Integral Hydroponics very little was known about coir substrates. In those days, often piles of coir dust were not left to decompose sufficiently and the resulting coir products sold to growers had C:N ratio issues and/or were high in phytotoxic phenolic compounds. Additionally, many unwashed (e.g. cheap coir bricks) or washed products that weren’t buffered were available through retail hydroponic stores. Ultimately, this resulted in many growers who weren’t purchasing quality washed and buffered coir substrate products encountering problems with, among others, N, Ca and Mg deficiencies. This also resulted in many growers encountering phytotoxicity issues due to excessive levels of sodium chloride (NaCl) and/or phytotoxic phenolic compounds found in substandard coir products.
A lot has changed over the past 15 years and many coir substrate products supplied through the hydroponic industry are now washed and buffered with calcium and magnesium during processing. Suppliers of high grade coir substrates also carry out regular testing of their products to check for any irregularities in supply and to correct these if and where need be.
However, while there are many good coir substrate products on the market there are still poor quality products being sold and growers need to select and only use reputable brands. I cannot stress this point strongly enough (be warned!). For example, often I will see novice growers on forums who have encountered problems when growing in coir. They’ll post something to the effect that their plants are showing deficiencies, or their plants are stunted and stressed. Other forum members then chime in with various well-meant advice on how to rectify the problem, much of which appears wrong with a few nuggets of sound information along the way. Generally, arguments ensue between self-proclaimed expert coir growers, one saying that it is a deficiency, another saying it is an excess and yet another stating the problem is related to the fact that the grower who has encountered problems is using “cheap assed coir bricks” which are typically laden in sodium chloride (common table salt). The latter possibly has it right while the others possibly have it wrong. Without lab tests it’s purely conjecture, albeit that phytotoxicity caused through excessive sodium chloride in the growing medium (often a problem with, to quote, “cheap assed coir bricks”) is expressed through what looks like nutrient deficiencies and/or excesses. Other than this, Ca and Mg deficiencies can occur in coir products that aren’t buffered correctly. However, the fact is that had these novices invested wisely in a premium buffered coir substrate product they would probably not be encountering problems with phytotoxicity or nutrient deficiencies or excesses, or all of the above (who really knows?). The only guarantee when you purchase “cheap assed coir bricks” is you are entering a world uncertainty. Not to put too fine a point on it, some coir substrate products are time bombs waiting to go off.
The bottom line…it is imperative to purchase a coir product that has been produced with quality in mind. Just some brands I have worked with and recommend to growers are Canna, Atami B’cuzz (Atami), House and Garden and Nutrifield Premium Coco. There are also many other good brands available and no doubt new premium coir products will appear on the market in the coming years. Speak to your hydroponic supplier and ask them what they recommend with regards to a high quality flushed and buffered coir substrate.
As a tip, look for substrates that list quality assurance standards such as the RHP certification standard. The Foundation ‘RHP’ (Regeling Handels Potgronden) is an organization for quality control of substrates in the Netherlands. The quality standards and requirements have been formulated by the Technical Commission of the RHP which consists of the substrate experts of the Horticultural Research Stations and other leading substrate experts in the Netherlands. The RHP certification standard includes the coir meeting the acceptable chemical requirements of the standard. A product with the RHP quality mark therefore guarantees that the buffering process has been correctly conducted.
Quality washed and buffered coir substrates tend to cost a little more than lesser quality products. This is because there are more materials and processes required to produce a high quality coir substrate. However, as a rule you typically get what you pay for. In short, studies have shown that there are significant effects on plant health, growth, yield and quality between plants grown in correctly treated and untreated coir.  This means growers can confidently invest in more expensive correctly treated products with the assurance that benefits will outweigh the initial cost.
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