Article Title

by Dr. Mike Nichols

Introduction
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 the past 50 years greenhouse crop producers in developed countries have sought an economic, disease free medium in which to grow their plants. Until the late 50’s soil was the standard medium in which plants were grown with the exception of 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 1950’s the John Inns Institute in 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 farm yard 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 as a growing medium have been rock wool 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 it’s disposal after use now all play an important role in determining the suitability of the growing medium.
Peat is now considered to be a limited resource, and the production of rock wool (an artificial medium produced by melting granite rock, and spinning it into a “candy floss like” slabs) is energy demanding. A major problem now with rock wool 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 cost.
Some 12 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. The proposal was rejected by both the Indian and Sri Lankan Governments, because they advised that they already knew how to use coco peat in horticulture. In recent years there has been a steadily increasing interest in the use of coir dust (or coco peat) as a growing medium, but we still struggling to use it to best effect!

The world scene
There are some 50 million tonnes of coconuts fruits produced annually world wide. About 25% of this weight is the mesocarp or husk (coir). Coir comprises fiber and coir dust in a ratio of approximately 1: 2. 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 fibres and cork like particles ranging in size from granules to fine dust. It is also known as cocopeat.
If all the worlds coir was used to produce coir fiber, then there would be approximately 8 million tones 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 for the equivalent to 350,000 ha of greenhouses. The world GH industry (controlled environmental agriculture) is about 500,000 ha so there is still plenty of coir to go round, as it can be used for 2/3 years, and not everyone will use it.
There are in fact 2 types of coir fiber; white and brown. White is obtained by retting the husk in saline water, while brown is obtained by using either fresh water, or solely by mechanical means.
When coir dust first appeared in the 1990’s it ranged in age from fresh to up to 100 years old, but now much of the old material has been used, and only freshly processed coir dust is appearing in the market.
It is unclear why salinity should be a problem with coir dust, unless the coir dust was obtained as a bi-product of white fiber production. Nevertheless coir dust has the reputation of being suspect with regards to salinity.

Future needs to establish coir as a reliable medium
We need to have a number of pieces of information to enable us to make the best use of coir dust.
1. Source
It is essential that we know the source of the coir being used. Is it young or old coir? Does it matter (we do not know).
Was the coir produced with saline water or with fresh water? Is this the sole reason for the salinity problem?
Does the geographic home for the coconut effect the quality of the coir? eg is Sri Lanka coir different from Brazilian coir. It certainly is likely to have been derived from a different variety of coconut. There are many different varieties of coconut world-wide, and presumably each variety has subtle difference 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.

2. Particle size
How does particle size effect the performance of coir? Moisture holding and aeration characteristics?

3. Particle size mixture?
How does the mix of different size particles effect the moisture holding and aeration characteristics.

Research results
A review of the literature shows very mixed and muddled results from the use of coir. That it has potential as a greenhouse medium is undeniable, but the results of trials leave a lot of unanswered questions. For example Kang et al (2004) show a wide range of physical differences between 4 samples of coir. This poses the question of what were the ages of the different media? What were their size grades fractions? Similarly the work of Shinohara et al (1999); Mazuela et al ( 2004); and Noguera et al (1997), although comparing coir dust with other media do not define either the source, age, particle size (or size grade fractions) of the coir.
Coir products with a range of different water holding and air filled space can now be developed, and should have a number of major advantages over rock wool, particularly because of it’s organic nature it is able to absorb water, and has a base exchange capacity which doe not occur with inert materials such as rock wool. A key factor, however is that not only does it perform at least as well as a other growing media but after being used in the greenhouses it can be “disposed of” by being incorporated into the field as an organic amendment, and thus improve soil structure etc.
Freight costs are also an important part of the equation. Coir can be transported in a compressed form, and on adding water it expands (by about 5 X) up to its required volume, whereas rock wool must be transported in it’s to be used form (which comprises about 93% air space!).
There are many different varieties of coconut world-wide, and presumably each variety has subtle difference 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. For peat the relationship between particle size (is demonstrated in Tables 1 and 2.)
To improve the physical characteristics of your coir it would be desirable to study the influence of different sized coir particles on water holding and aeration characteristics.

Table 1. Effect of particle size on water holding capacity and air filled pore space of peat.

Table 2. Effect of particle size mixtures on EAW and air filled space of peat.

Rock wool is still very much the standard, but rockwool has to be treated with a chemical during manufacturing to make it hydrophilic, because industrial (insulation) rockwool is hydrophobic (water repellant). Of course it is possible to manufacture rockwool to precise standards of fibre diameter and air filled space, but in the final analysis it is a completely inert material, which does not absorb either 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.
There are some major differences between peat and coir, which should be emphasized. Firstly coir is hydrophilic (that is that it absorbs water easily even when dry), whereas peat is hydrophobic, ie when it is dry it very is difficult to re-wet. Secondly once peat dries it tend to shrink and when wet tends to slump, whereas coir tends to retain it’s basic structure when wet or dry. This is important in that when the peat slumps, it does not recover it’s air field porosity.

Table 3. Physical properties of some coir dust samples (after Kang, Lee and Kim, 2004).

The figures in Table 3 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 such comparisons. This points to the need to have some form of quality assurance (QA) for any coir dust.

My suggestions are:

1. That there is a need to ensure that coir dust is all of the same age. Coir dust is a light tan colour but darkens with age to a chocolate brown. When coir first appeared in the early 1990’s, 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 available. ie one should use only coir dust with a known age. (preferably young).
2. When first produced, coir dust contains significant amounts of sodium chloride and potassium which are naturally present at high concentrations in the mesocarp. If the husks have been soaked in brackish water, the levels of these and other salts (particularly sodium) may be higher still. There is a need to ensure that the coir dust is regularly monitored for Ec level. Provided that the husk is not soaked in saline water (i.e. for the production of white coir), I can not see any basic reason why husks should not be obtained from near the seacoast, provided that they are fresh.
3. The importance of determining the appropriate particle size grade and mix is likely to be critical. Is there any research data on this?
   

In the past comparisons between coir and other potting media have not been very illuminating because the source and physical characteristics of the coir have not been clearly defined.
One further problem with coir dust is that it is normally size graded when dry, and yet is (obviously) used in a moist state. There is a clear need to understand the relationships between moisture holding and aeration characteristics and particle size when wet and when dry. The other question is that coir dust is not of fixed dimensions. It is thus very different from sand or pumice or perlite.

Conclusions
Coir dust would appear to have tremendous potential as a growing medium for controlled environment agriculture. There is, however, a clear need to establish some clear set of standards to ensure that the physical characteristic of coir can be defined.
We need to have a clear understanding of the effect of particle size (and size grade fractions) on medium aeration/ water holding characteristics, and also how mixing the coir with other media (eg pumice/perlite/peat) can influence such parameters.

References
Kang, J-H., Lee, H-H., & Kim, K-H. (2004) “Physical and chemical properties of organic substrates used in Korea” Acta Hort., 644, 231-235.
Manzuela, P., Urrestarazu, M., Salas, M. C., Guillen, C., & Sanchez, J. A. (2004) “Comparison between different fertigation parameters and yield using pure compost and coir waste fibre in tomato (Lycopersicon esculentum cv Pitenza) by soilless culture”, Acta Hort., 659, 653-656.
Noguera, P., Abad, M., Puchadea, R., Noguera, V., Maquiera, A. & Martinez, J. (1997) “Physical and chemical properties of coir waste and their relation to plant growth.” Acta Hort., 450, 365-373.
Prasad, M. (1997) “Physical, chemical and biological properties of coir dust.” Acta Hort., 450, 21-29.
Shinohara, Y., Hata, T., Maruo, T., Hohjo, M. & Ito, T (1999) “Chemical and Physical properties of the coconut-fibre substrate and the growth and productivity of tomato (Lycopersicon esculentum Mill.) plants” Acta Hort., 481, 145-149.