Alternative Compost Materials

Alternative compost materials

Alternatives to sand or grit are usually more expensive and more difficult to obtain. Various advantages for them may be claimed and interest in them varies. These materials may be naturally occurring or they may be synthetic (man-made). Usually they are light, non-toxic, sterile, and slow to break down.



This is produced from a naturally occurring material called mica which is a glassy type of mineral consisting of flat layers or wafers. It is mined, mainly in South Africa. When it is heated to a temperature of about 1,400°C it expands considerably, thus providing a very large surface area for water retention and rooting. It also has a good capacity for retaining the plant foods added to the compost and itself contains high levels of magnesium, calcium and potassium. Thus it is a useful additive at approximately 10% by volume to composts, but it can be expensive, so its advantages have to be weighed against the extra cost. Because of its light weight it has been widely used as a base for the production of lightweight fertilizers.



Alternative Compost Materials Although very different from vermiculite this mineral is processed in a similar manner by heat. The basic raw material is a dense, glass-like rock formed by volcanic action in much the same way as pumice stone. The rock is crushed and heated so that it expands like popcorn to twenty or more times its original volume. This gives it water and air-retaining properties and, as in the case of vermiculite, lightens it. Unlike vermiculite, it naturally contains few nutrients and its capacity to absorb water is less. Perlite has been used as a base for lightweight fertilizers but its use in composts is extending considerably because of the excellent air/moisture balance it gives a compost. It is also used for plant culture.



Factories producing polystyrene products such as ceiling tiles and insulation boards have a waste problem that is an embarrassment to them because of difficulties of disposal of rejects and off-cuts. A small but increasing use for this waste has been in the addition of the broken-down pieces to composts. Various claims have been made as to the benefits obtained, such as improved aeration and drainage, even to increasing the temperature of the compost by 1-2°C but these are still early days and there is little experimental evidence to substantiate these claims. The material is very light and this causes problems as it tends to “float” to the surface of the compost during the mixing. There are little or no plant foods present and the capacity to retain added plant foods is very low.

Polystyrene may be added to compost at up to 20% by volume but the compost must not be heat sterilized, for example by steam, after incorporation, as the polystyrene will melt.


Pulverized fuel ash (PFA)

Large amounts of clinker-type materials must be disposed of by the large furnaces of power stations and one of the uses of this material is as a compost additive. The clinker is broken up (pulverized) and used as a replacement in whole or in part for the sand fraction of the compost. It is lighter than sand although heavier than the alternative materials mentioned above, and the PFA itself has some pore space within the particles. It is used with success by some growers but there have been reservations from certain authorities on its use because of possible excess levels of some trace elements.


Other materials

Synthetic foams have been used in the manufacture of composts as well as on land that is being reclaimed and for conserving water in dry areas. The most useful for horticultural purposes are the urea—formaldehyde foams as these have a cotton-wool-like texture. Shredded polyurethane foam can be obtained as a waste product of the cushion and bedding industry quite cheaply. Ideally, it should be shredded as finely as possible for addition to compost. Its addition is claimed to improve aeration and heat retention in the compost; roots appear to be attracted to the foam.

Polyurethane blocks are also now produced for rooting cuttings. The blocks are available in sheet form, each block being a cube 3-4cm. They are wetted before use and the cuttings inserted into the blocks. Advantages claimed are that the rooting medium is light, sterile and uniform, maintaining the correct balance of air and water, giving even root development. It is likely that this type of medium will find increasing usage in the future. Indeed, the more these synthetic foams are used the more advantages are found for them. Mineral rockwool is also used in block form as a growing substrate.


Storage period for composts

All compost formulae containing plant foods in any of their various forms do deteriorate or change with storage. The higher the temperature, generally speaking, the quicker fertilizers are rendered into more available form. Generally speaking it is better to mix or buy composts in quantities which allows for their use within a reasonable period, and store the unused composts in a cool place.


Compost strengths

Plants vary in their tolerance of the fertilizer level in the compost. In a ready-made compost the level of fertilizer is kept reasonably low so that the least tolerant plants may be grown. But problems arise when the gardener wishes to make up his own compost and has to decide on the stength of fertilizer to be used.

Seed is sown or cuttings rooted in the standard composts detailed earlier. The plants in the first group are pricked out or potted into a compost with a strength of approximately John Innes Potting 1- or the equivalent in soilless. The final potting of these plants is done at John Innes Potting No 1 strength. In the second group the plants are pricked out into John Innes Potting No 1, and potted on into John Innes Potting No 2. All plants may be liquid fed after the roots have filled the pots (gently tap out the root ball to examine). Certain of these plants will tolerate higher levels of plant foods, but for the sake of simplicity the groupings have been kept to two.

Plants that are grown in the early stages in a greenhouse and later planted out in an outside soil, eg bedding plants, chrysanthemums, cucumbers, should be pricked out or potted on into John Innes Potting No 1 or its soilless equivalent. If the plants are to stay in the compost longer than approximately four weeks, some supplementary feeding should be given.



Various systems of culture involving the growing of plants in nutrient solutions contained in tanks, as shallow films in troughs (Nutrient Film Technique), rock-wool, perlite or other aggregates have been widely adopted by commercial growers in the last few years.

All plants take up their nutrients in liquid form under any system of culture, but with hydroponic systems no soil or compost is involved. A specialized approach to such systems is required and details are available from suppliers of equipment.


03. April 2011 by Dave Pinkney
Categories: Greenhouse Gardening, Organic Gardening, Soil Cultivation | Tags: , , , | Comments Off on Alternative Compost Materials


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