While we all know coir is a by-product of the coconut processing industry, understanding its properties and potential as a horticultural growing medium can be slightly more complex than the other inert substrates…
Coconut fiber—or, coir—is a widely used and recognized hydroponic substrate. Available in a range of products, brands and grades, coir is essentially an “eco” medium. A healthy, well-run coconut fiber system will become a habitat for a diversity of beneficial microbes that enjoying the luxury of organic matter, plentiful oxygen and humic substances. This in turn creates what many of us aspire to: a more balanced approach to hydroponics where microbes and plants can live in harmony and both benefit from a disease-free and growth-promoting root zone environment. To achieve this balance however, the coconut fiber substrate needs to meet certain standards for hydroponic production. It must also be treated in a slightly different way to other mediums and growers need to be aware that not all coco products are created equal.
Coco fiber: some background
Coconut fiber for horticultural use has had a rather disappointing reputation in the past. In the early days, little was understood about the properties of this organic growing medium. It was poorly processed and had major problems with nitrogen draw down (high levels of this means nitrogen applied to the coir in the early stages of plant growth would not be available for plant growth, even if full-strength nutrient was applied), high cation exchange capacity (CEC) and retention of ions like calcium and iron, extremely high levels of sodium and a naturally occurring potassium content that growers did not take into account. As a result, many of the first trials with coco fiber did not produce particularly pleasing growth results despite the medium having almost ideal physical characteristics. However, once the properties of this medium began to be more understood, and processors realized the potential of the expanding horticultural market, correct processing started to ensure we had improved-quality coco being supplied for hydroponic use.
The coco we use in hydroponics today is processed from the outside layer (mesocarp) of coconut husks that consists mainly of coarse tough fibers, but also contains finer material known as coir dust. Harvested coconuts are first soaked in water (not seawater, however, since this leaves extremely high levels of sodium in the substrate—something which is undesirable for hydroponic use). This process is called retting and it makes the fiber easier to remove. The longer fibers are then usually removed for other uses, while the coir pith undergoes further processing and decomposition that makes it suitable as a plant growth medium. During this process, the high nitrogen draw down that occurs in coconut fiber in the early stages is modified, as are excess minerals—such as high levels of naturally occurring potassium—and the CEC of the coco (this is so the medium does not retain ions such as calcium and iron). Further processing of some high-value coco growing mediums can also occur, which includes pre-conditioning and buffering so that the substrate has minimal effect on the composition of the nutrient solution surrounding the root zone.
Because coco is a natural product, there are variations in the chemical and physical properties of this medium depending on its source and supplier. Suppliers of high-grade coco carry out regular testing of their product to check for any irregularities in supply and then correct for these. However, while there are excellent brands of coco on the market, these are also still poor-quality supplies still being sold as a cheap growing medium. As such, growers need to select and only use a reputable brand.
Getting the best from coco—different products for different uses
There are many different grades of horticultural coco and some have been specifically designed for different plants and systems. While orchids prefer a very coarse coco chip, the propagation and germination of small seeds requires a much finer grade that will hold sufficient moisture and oxygen. While the high water-holding capacity of coir dust is great in some situations, it can create problems with over saturation of the root zone, and grades of coco commonly used in grow slabs tend to consist of a mixture of longer coarse fibers, or flakes, of coco that keep the substrate open and aerated, and finer particles that hold more moisture. These grades of coco are ideal for longer term hydroponic crops, such as tomatoes, cucumbers, melons, peppers and cut flowers, because the fibers help prevent the substrate from packing down over time. These properties mean also high-quality coco fiber can be used for many successive crops before finally being recycled as a soil conditioner or mulch.
Aside from different grades, coco also comes in a range of different products—from differently sized compressed bricks to grow slabs to pre-expanded, ready-to-use bagged product. Compressed bricks of coco fiber keep the cost of shipment to a minimum—a typical 11 lb. block of compressed coco can be expanded in water to create over 17.17 gal of growing substrate. Another advantage of coco bricks is that, once expanded, the media can be used to fill any size or shape of growing bed, pot or bag (the disadvantage is that time is required for the media to fully expand and some labor is needed to fill the growing plots). Loose coco placed into growing pots or containers can be easily inspected for moisture level by checking the appearance of the top of the substrate or by feeling the moisture level of the coco just below the surface (however, this is more difficult with wrapped and expanded coco grow slabs).
Coco conditioning and nutrients
While coco fiber is relatively easy to use as a hydroponic substrate, there are some tips for getting the best from this medium. First, always buy a reliable brand and one that is specified for hydroponic use. There are coco mulch products available on the general gardening market, but these might not have been processed sufficiently for hydroponic use and might contain high levels of unwanted salts. These products are simply designed to be used as soil conditioners or mulch rather than stand alone plant growth mediums. If in doubt, a leach test can help determine if a coco product is suitable for hydroponic use. Simply run clean, distilled or RO water through a small sample of the coco several times and measure the EC of the leachate of the water. A high EC will indicate the presence of salts, usually potassium and sodium, and indicates the product is not suitable for hydroponics.
Secondly, while preconditioned or buffered coco products are a good place to start, it pays to carry out a little extra conditioning before planting out. The coco fiber should be fully expanded with clean water and then left in a warm growing environment for a few days. During this time, a dilute solution of coco nutrients can be applied to run off and any mixtures of beneficial microbes, trichoderma and organic supplements (in diluted state) can also be applied. Re-check the EC of the solution draining form the coco before planting to make sure everything is okay in the root zone (ideally the EC in the root zone should be below 1.6 at planting).
Thirdly, when using coco it important to apply the right nutrient product. Coco growing media is not like many other soilless substrates that arrive pre-sterilized, chemically inert with a low CEC and with a minimal effect on the composition of the nutrient. Coco contains naturally occurring potassium, which is considered a bonus since potassium is a major plant nutrient; however, this needs to be accounted for in the nutritional program of the plants. Coco also has other effects on the composition of the nutrient solution applied, so levels of nitrate, phosphate, calcium, magnesium and iron might need to be adjusted to allow for these properties.
Finally, remember that coco is a living substrate and it should be treated as an entire eco-system consisting of beneficial microbes that make their home in the coco particles. This beneficial microbial life plays an important role in soilless systems as many fungi have a protective effect on the plant’s root system, have been proven to suppress plant pathogens and have other possible benefits with nutrient uptake and plant growth. While other growing substrates start out as sterile, coco is best left in its original state or even inoculated with populations of beneficial microbes such as trichoderma. These populations of beneficial microbes in coco are to be encouraged and hash sterilizing chemicals, such as chlorine bleach, hydrogen peroxide and even boiling water, should not be used on coco substrates for that reason.
All growing mediums can have their problems. Although coco is generally easy to use, it still needs to be monitored for over saturation, EC and salt buildup.
Coco can look slightly dry on the surface and still be fully moist in the root zone, so checking the moisture level a few inches below the surface is recommended. Moisture should appear when the coco is squeezed between the fingers, but the surface should not appear wet—overly damp coco can attract fungus gnats, as well as reduce oxygen in the root zone.
Coco usually maintains pH within an optimal range; however, EC can build over time, so it should be checked—particularly under warm growing conditions. Because of the nature of coco growing media, the EC around the plant’s roots might be different to that in the leachate or the solution draining from the growing slabs, pots or bags. However, a quick and simple extraction sample EC test can be carried out on coco media to determine the actual EC around the root zone. For an extraction test, a small sample of coco is taken from the growing media after it has been in use for a few weeks. Then 3.38 oz. of these combined samples is measured out (the coco should be damp, but not overly saturated). These sample of coco is placed in a jar and 5.07 oz. of RO water is added and the mixture shaken 50 times. This is allowed to sit overnight to allow extraction of nutrient ions into the water. The resulting mix is then re-shaken and filtered to remove particles, and the pH and EC can be measured from the remaining liquid. The ideal pH range of the extract for most crops is between 5.5 and 6.2. Ideal EC levels vary depending on the stage of plant development, the growing environment and the crop being grown; however, a general range is between EC 1.0 and 2.5 (tomatoes can be grown at much higher EC values, particularly with commercial crops). Larger scale growers and those in commercial production will often have the coco extract sent to a lab for a complete nutrient analysis, which determines the levels and ratios of each of the elements in the nutrient solution so that fine tuning adjustments can be made.
These days, good-quality coco has been proven to be a superior growth substrate for a large number of different hydroponic crops. It also has the advantage of being from a renewable and environmentally friendly source. By treating coco as living substrate and allowing natural microbe populations to flourish, coco can become part of a more eco-friendly hydroponic system.
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