Coco coir is the name given to the natural fibre extracted from coconut husks. It has long been used in the manufacture of brushes; stuffing for automobile seats and mattresses; drainage pipe filters; twine and other products. However, the very short fibres (measuring 2mm or less) and dust that were left behind were once considered a waste product for which no industrial use had been discovered.
The first reports of using coco coir and dust as a growing medium for various plants date from the late 1940s. But it was not until the early 1990s that coco really made a break-through as a growing medium, when it was introduced into Dutch horticulture. It was used to replace potting mix but it also served as an alternative to rockwool. It had the advantage of being a high-quality and environmentally friendly growing medium and the use of coco has really taken off since it was first introduced. What makes coco so special for growing? This article will focus on coco as a growing concept and explain some of its physical and chemical properties.
Picture 1 – Example of different coco fractions. 1. Fine Coco dust. 2. Coarse Coco coir with high fibre content. 3. Coarse Coco coir with high chip content.
What is coco coir?
At first glimpse, coco coir looks like soil, but it is actually a by-product of processing coconut husk fibre. Coco coir is the name given to the fibrous material that can be found in the middle layer of the coconut fruit (Cocos nucifera).
Three main horticultural coir products can be obtained from this husk; coir chips, coir fibre or coir pith/dust (see picture 1). The coir dust retains water well, while the fibres and chips help to create pockets of air and drainage. Coco coir consists primarily of particles in the size range 0.2-2.0 mm (75-90%), and unlike a medium like sphagnum peat, for example, it is free of extraneous organic matter like wood or leaves.
Physical and chemical characteristics
Coco coir, mainly coco dust, is the major constituent of most coir products and it is made up of millions of capillary micro-sponges that absorb and retain up to nine times their own weight in water. As a substrate, it is well-aereated and compaction after drying is minimal. It has a natural pH of around 5,7 to 6,5, plus a high cation exchange capacity or ‘CEC’ which makes it good but challenging substrate for hydroponic cultivation.
Now what exactly do we mean by ‘CEC’? It all comes down to how easily the substrate releases nutrients that can then be taken up by the plant’s roots. In order to understand this, we should think of the substrate as a storehouse for plant nutrients. Many nutrients, such as calcium and magnesium, may be supplied to plants from reserves held in the substrate. Others, like potassium, are added regularly to the substrate as fertilizer so that they can be taken up by the plants as they are needed. The relative ability of the substrate to store a particular group of nutrients – called cations – is called the cation exchange capacity or CEC of that substrate (see picture 2).
Picture 2 – schematic overview of the cation ion exchange in coco substrate.A coco substrate may consist of coir chips, coir fibre and coir pith.
Chips are the biggest particles of coco used, and the dust is made up of the smallest. The coco substrate can be seen as a mix of microscopic particles which carry a negative charge. Each particle attracts positively charged ions. This is why natural, untreated coco particles have high amounts of sodium (Na+) and potassium (K+) ions. During cultivation, these ions are released making them available for the plants. Unfortunately, bivalent ions like calcium (Ca2+) and magnesium (Mg2+) bind to the coco substrate in their place. This means that these nutrients are less available for the plants, so there may be a shortage of calcium or magnesium. When we use a ‘washed and buffered’ coco substrate (right), the coco has been pre-treated with calcium fertilizer to enhance the potassium-calcium exchange.
Coco substrate that has been pre-treated in this way is unlikely to release excess potassium or bind the calcium and magnesium that the plants need. The result: healthy plants with no symptoms of nutrient deficiency.
Washing and Buffering
Coco coir, in its unprocessed form, tends to be high in both sodium and potassium compared to sphagnum peat. We know that the sodium can easily be leached from the material using irrigation. Potassium is also released and becomes available for the plants. Unfortunately, bivalent ions like calcium and magnesium then bind to the coco particles instead. This may result in excess levels of potassium, and a simultaneous deficiency of calcium or magnesium.
Of course, this is something that growers need to avoid and to prevent it from happening the coco substrate is often washed thoroughly to leach excess sodium and chloride. It is then pre-treated with a calcium fertilizer such as calcium nitrate to enhance the potassium-calcium exchange. This process is called buffering and it produces a coco substrate that is not likely to release excess potassium or bind the calcium and magnesium required for plant growth.
Properties of coco coir during cultivation
Plants grown in coir can develop large roots, stems and blooms. Unlike regular potting soil, which can easily become compacted, coco coir retains plenty of air pockets leaving room for the root system to grow. This gives rise to a healthy aerobic rhizosphere – essential for good uptake of nutrients and water. Some crops like tomato or sweet chili may tend to grow more vegetatively; meaning that the plants will look healthy and grow fast and strong; but will actually produce relatively fewer flowers and fruits. For growers, this is something to be avoided, of course; because they will want to maximize their harvest. As you can read in our other article on coco on page 26; small adjustments in the fertilization regime can resolve these problems.
Coco coir has a naturally high lignin content which encourages beneficial micro-organisms around the roots and discourages decomposition, making it an ideal growing media for reuse.
Picture 3 – Cross section of a coconut including husk.
It is also thought that the occurrence of favourable micro-organisms acts as ‘protection’ against plant pathogens. This was demonstrated in several experiments conducted in vitro which showed that coco coir suppressed the growth of soil-borne plant pathogens. One of these experiments showed that mycelia growth in the fungus of Phytophthora capsici was strongly inhibited when the growth medium was adjusted with an unsterilized coir suspension.
On the other hand, the growth of P. capsici in the same medium was amended with a filter-sterilized coir suspension.
Other experiments have demonstrated that the growth of Fusarium solani (picture 3) on water agar amended with non-sterile coir was completely inhibited. When the coco coir was completely sterilized, the growth of F. solani on water agar amended with autoclaved coir was no longer inhibited. Similar results were also obtained for Aspergillus terreus; a plant pathogenic fungus that produces toxins which inhibit pollen development.
The results from these studies suggest that coir can suppress soil-borne plant pathogens in vitro and this ability is largely due to micro-organisms associated with the substrate. We now know that the famous Trichoderma fungus plays a key role in the ability to supress plant pathogens; but the benefits of these micro-organisms during crop cultivation are still largely unknown.
Current studies are focusing on the resilience of coco coir during crop cultivation trials which aim to increase the plants’ vitality and maximize harvests. Nowadays, the increased sustainability of coco coir is emphasized; and particularly the fact that the coco substrate can be safely re-used. But as promising as substrate re-use may sound, poor-quality coco coir can lead to some unexpected nutrient problems. A well-known issue that sometimes occurs is nitrogen-binding as the coco coir (or fibres) decompose. In some experiments it was found that a small amount of nitrogen draw-down occurred with coir dust. Nitrogen draw-down is when nitrogen is retained in the substrate during decomposition of organic matter and is thus unavailable for crops. This is usually caused by the micro-organisms involved in the decomposition process; which compete for plant-available nitrogen.
Coco coir as an alternative for peat
In this article we have focused on coco coir as a stand-alone substrate. However, coco coir is also used as a replacement for sphagnum peat in peat-based substrates.
For almost a quarter of a century; coco coir has also been tested as a candidate to replace sphagnum peat; sedge peat and to a large extent rock wool plugs as a germination and nursery medium for broccoli; tomato and lettuce seedlings. It was found that germination occurred earlier in coir and tomato seedlings grew bigger and more uniformly.
It is actually very probable that some countries will ban the use of peat as a substrate within the next few decades; because our supplies of peat are not endless and extracting it is not an environmentally friendly process.
The coco substrate as a growing concept
The whole concept of coco substrate; both its benefits and its disadvantages; will require adjustments to the nutrient composition and regime. CANNA was among the first manufacturers of specialized coco nutrients to come up with solutions for the release and binding of plant nutrients.
These days, coco coir is a very promising substrate. It is sustainable, it can be re-used easily; and ample research has shown that coco coir has a natural ability to suppress plant diseases. The other side of the coin; however; is that coco coir requires specific nutrient adjustments to compensate for the natural binding of calcium and magnesium and the release of mainly potassium. Unfortunately; we still know little about the benefits of disease-suppressing properties during crop cultivation.
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