COIR: Sustainable Growing Media

Coir is a natural fiber extracted from the husk of coconut and used in a wide range of domestic products including floor mats, door mats, brushes and mattress filling. However, in recent years, there has been an increasing interest in the use of this renewable resource as a horticulture growing medium.

by MIKE NICHOLS

Soil is not a good medium in which to grow plants. It is either too wet (and thus does not provide adequate aeration) or else it is too dry (with good aeration), but insufficient water for optimum plant growth.

In the field there is little that can be done economically to overcome these problems, but in intensive greenhouse production (controlled environment agriculture), improving the root environment not only becomes feasible but necessary.

For some 50 years, greenhouse crop producers in developed countries have sought an economic, disease-free medium in which to grow their plants. Until the late 50s, soil was the standard medium in which plants were grown with the exception being plants in pots which were grown in a specially concocted 'potting medium'. In the late 19th Century, this was a confidential formulation held tightly by the chief propagator, but in the 1950s, the John Inns Institute in the UK developed a number of standardized loam + peat + sand-based media, while the University of California developed a series of standardized growing media based primarily on peat and sand (the UC System).

Nevertheless, the majority of crops (such as tomatoes) were still grown in soil in the greenhouse. The soil structure might be improved by the addition of farmyard manure, and in the case of cucumbers, by the construction of a raised bed consisting primarily of turfy loam and rotted horse manure. However, growing the same crops in the same soil year after year lead to the inevitable build-up of soil borne diseases (such a verticilium) and pests (such as nematodes) and necessitated the expensive (but essential) development of soil sterilization (actually pasteurization) by steam or fumigation by chemicals such as methyl bromide or chloropicrin.

In North Otago (New Zealand), for example. it became the tradition to replace the soil in the tomato house to a depth of 30 cm every year with, of course, no guarantee that the new soil did not contain disease or pests anyway.

Over this period, the only products to gain general acceptance have been Rockwool and peat. Perlite and pumice and even vermiculite have been and are still used in some situations, but most greenhouse crops are grown in the soil, or in Rockwool, or in peat at this present time. Weight of the medium and its disposal after use now all play an important role in determining the suitability of a growing medium.

Peat is now considered to be a limited resource, and there are strong environmental pressures to reduce its use in horticulture, while the production of Rockwool (an artificial medium produced by melting granite rock, and spinning it into a candy floss-like slabs) is energy demanding. Also, a major problem with Rockwool is disposal after use. In many countries it is now a requirement that the used material is returned to the manufacturer for melting down into briquettes-a further expense.

In recent years, there has been a steadily increasing interest in the use of coir dust (or coco peat) as a growing medium. Some 20 years ago, I was invited by the Food and Agricultural Organization of the United Nations (FAO) to write a project proposal to develop coco peat for horticultural purposes. This proposal was rejected by the Indian Government, because they advised that they already knew how to use coco peat in horticulture. History has proved them wrong; because we still do not fully understand how to use this medium for best results, but this is now all water under the bridge.

Coir dust is a product from the coconut palm tree (Cocos nucifera), which grows throughout the tropics. The coconut palm is frequently described as the tree of life, because of the large number of useful products that can be derived from it. These include wood products from the trunk, copra and a variety of drinks from the fruit, fiber and coir dust from the husk, charcoal products from the fruit shell, roofing material from the leaves and even dye from the roots. Commercially, the main product from the coconut palm has been copra, and more recently considerable interest has been shown in the potential for coconut water as a health drink for athletes. Coconut water comes from immature nuts before the coconut 'meat' has developed.

Some 50 million tons of coconuts are harvested worldwide every year. The major producing countries are the Philippines, Indonesia and India, which produce between them some 90% of the world's coconuts. About 25% of this weight is the mesocarp or husk (coir). Coir comprises of fiber and coir dust in a ratio of approximately 1/3:2/3. In those countries where coir fiber is produced, the coir dust is (or has been in the past) a waste product, and huge heaps of it exist. Coir dust consists of a mixture of short fibers and cork-like particles ranging in size from granules to fine dust. It is also known as coco peat.

Traditionally, coir fiber was obtained by retting the husks in water for up to 10 months, after which the fiber was separated from the coir dust by beating with wooden mallets. For white coir, the retting was in slightly saline water, and for brown coir the retting was in fresh water. The coir fibers were then dried, cleaned, and the white fibers spun into yarn to make rugs and rope, while the brown fibers were used in mattresses. The coir dust was considered to be a waste product, and for many years dumped in piles, which provided a source of pollution and also a health risk from disease carrying mosquitoes, breeding in the stagnant pools of water caused by the heaps of coir dust. It is only in the past 25 years that a market been developed for coir dust as a horticultural growing medium.

When coir dust (coco peat) first appeared in the early 1990s, the largest stockpiles were the first to be exploited and these were the oldest. Some of these coir dumps were reputedly over 100 years old. Now that this old material is becoming scarce, only freshly processed coir is now available. Many of the early efforts to develop the horticultural use of coco peat as a growing medium foundered, because it was found that some coir dust contained excessively high levels of sodium chloride. Why this should occur is unclear. It may be that the coir dust had been obtained from coir, which had been retted with saline water, or it may be that the coir dust had been obtained from coconuts grown near the sea, which had absorbed high levels of sodium. Whatever the reason, it resulted in coir dust as a horticultural growing medium getting very poor press.

To overcome this, buffered coir dust was introduced. This is 'manufactured' by using a calcium nitrate solution to leach out any sodium from the coir dust, and then leaching out excess calcium nitrate with fresh water. At the same time, this will also remove any excess potassium.

If all the world's coir was used to produce coir fiber, then there would be approximately eight million tons of coir dust produced annually. In fact, much of the coir is not processed at the present time.

About 23t of coir medium is required for every hectare of greenhouse use, which means that the current world supply of coconuts could provide the equivalent of 350,000ha of greenhouses. The world greenhouse industry (controlled environmental agriculture) is about 500,000ha, so there is still plenty of coir to go round. Not everyone will use it, and it is sometimes used to grow two or three crops before being discarded, so currently there is plenty available; in any case, only a small amount of husk is converted into fiber, and the rest disposed of in other ways, such as fuel etc.

As stated earlier, coir dust (coco peat) is obtained from the husk of the coconut, and is essentially a waste product. To obtain the coir fiber (which finds a ready use for making ropes and for stuffing mattresses etc) requires decorticating machinery to break up the husk, and then to separate the fiber from the dust. The coir dust is initially sieved to remove any lumps and fiber, and then left for up to six months to 'mature' outside in large heaps, exposed to the weather. After drying, the coir fiber is then used for manufacturing rope, or for filling mattresses.

Coco peat is then usually (but not always) treated with calcium nitrate solution to buffer it and remove sodium. This is done by applying a standard quantity of calcium nitrate solution to a fixed volume of coco peat, and then after a few days, applying water to wash out any excess calcium nitrate. It is now necessary to dry the coco peat prior to compressing it. This is usually carried out by the sun on large concrete pads in areas of low rainfall; but where rains can occur, it requires artificial methods. Even in arid climates, the occasional rainfall can prove a problem, as coco peat readily absorbs moisture. Under these circumstances the coco peat is rapidly heaped up, so that only the top gets wet, and when the rain stops, it is spread out again on the concrete drying pads.

There are many different varieties of coconut worldwide, and presumably each variety has subtle differences in coir dust characteristics. However, I suspect that at this stage of development of the industry the important thing is the size of the individual particle, and the size mix of these particles.

In fact, there is only one published account of the physical characteristic of coir from different sites, and this is virtually of no value, as it does not compare coir of similar particle sizes.

The figures also point to the difficulty of comparing the performance of coir dust with other media, without defining the specific physical characteristics of the medium. This huge variation in physical attributes must question the value of any such comparisons.

Rockwool is still very much the 'gold' standard as a growing medium, but rockwool has to be treated with a chemical during manufacturing to make it hydrophilic, because industrial (insulation) Rockwool is hydrophobic (water repellent). Of course, it is possible to manufacture Rockwool to precise standards, but in the final analysis it is a completely inert material, which does not absorb nutrients or water, unlike both peat and coir, which absorb both water and nutrients, and thus provide an important buffering capacity. Rockwool's buffering capacity is limited solely to the quantity of nutrients and water held within the pore space of the medium.

Freight costs are also an important part of the equation. Coir can be transported in a compressed form, and on adding water expands (by about five times) up to its required volume; whereas, Rockwool must be transported in its 'to be used' form, which comprises about 93% air space.

There are some important differences between peat and coir, which must be emphasized. Firstly, coir is hydrophilic (that is, it absorbs water easily, even when dry), whereas peat is hydrophobic (that is, when it is dry it very is difficult to re-wet). Secondly, once peat dries it tends to shrink, and when wet tends to slump; whereas, coir tends to retain its basic structure when wet or dry. This is important in that when peat slumps, it does not recover its air-filled porosity.

Another important difference between coir and peat is that peat has a superior water release curve-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. When compared with Rockwool, most of the water is also easily available to the plant.

Coir products, however, have the potential to provide a range of different water-holding and air-filled space by mixing different particle sizes and ratios together. Another important factor in its favor, is that after use it can be disposed of by being incorporated into the field as an organic amendment, and thus improve soil structure etc.

The area of greenhouse produced crops in the world is steadily increasing. This is because:

  • There is a steady increase in the demand for higher quality produce from greenhouses compared with poorer quality produce grown outside. This is apparent in Australia, but is occurring worldwide, particularly in the tropics and in less developed (but rapidly developing) countries such as PR China.
  • Water is going to be a scarce resource in the future, and greenhouse crops with recirculating hydroponic systems are four times more efficient in producing product than outdoor irrigated crops per unit of irrigation water.
  • The opportunity to use biological control or integrated pest management systems in protective cultivation is greater than outside in the field.
  • 1665

    producers

    Within Australia there are some 1665 producers of greenhouse crops on 1341ha. Many of these growers still use soil, old greenhouses and old technology. This is going to change (Smith, 2005).

  • 2000

    producers

    In New Zealand, there are 2000 producers of greenhouse crops on 905ha (Ivicevich, 2004). The majority of these growers use hydroponics. Individual properties have increased rapidly in size, and the number of growers has fallen over the past five years and is continuing to do so.

  • 53,000ha

    Cultivation

    In New Zealand, coir is replacing Rockwool and pumice as a medium for the hydroponic production of tomatoes, cucumbers, capsicums. In Japan, there are some 53,000ha of protective cultivation, but only 1348ha in soilless culture, and few of these use coir.

  • 200,000ha

    Greenhouses

    There are currently some 200,000ha of greenhouses in the Mediterranean region (Baudoin, 1999), and PR China has been reported to have a similar area.

Although much of these are low-tech soil-base systems, eventually and inevitably high-tech hydroponic systems will become established, and coir is likely to become the preferred medium.

Greenhouse vegetable crops growing in hydroponic media (Rockwool, pumice, perlite, peat, coir) generate two tons per hectare of 'waste' media per year (Stanghellini et al., 2003). Assuming only 10% of the greenhouse industry uses coir, then the requirements will be in excess of 100,000 t/year worldwide.

Much of the greenhouse production worldwide is still undertaken in soil, and with improvements in the control of the aerial environment there will be a need to improve the root environment in order to take full advantage of the potential of an improved aerial environment.

    Key points regarding coir are:

  • A renewable resource
  • Large potential world market
  • Low freight costs
  • Crop specific moisture and aeration characteristic can be manufactured
  • High yield potential
  • Easy disposal of medium after use.
  • Environmentally friendly.
  • The possibility for coir grown crops to be certified organic.