Raw Materials in Soilless Composts

Soilless compost: raw materials


Raw Materials in Soilless Composts

By far the most important raw material is peat, and sphagnum moss peat is ideally structured for soilless compost, with good water retention and aeration characteristics. As it is naturally highly sterile and relatively free from pests, diseases and weed seeds, sterilization is not necessary. Composts consisting of moss-sedge peat mixes or even sedge peat alone (Carex spp, not the dark, well-decomposed peat popularly referred to as sedge) have, however, been used successfully. Note that sphagnum peat is acid in nature and sedge peat much less so, which affects quantities of lime to be added when mixing a compost.

Because of environmental pressures, organic materials other than peat are increasingly being used as compost ingredients. These include coir (coconut pith), composted timber and paper waste, brewery waste and other composted materials. Readers can be assured that as the properties of coir and other peat alternatives are gradually evaluated by researchers, growers and gardeners, compost formulae will be subject to modifications. Some adjustment in watering and feeding techniques seem presently required when peat alternatives are used.


Composted or pulverized wood bark

There is increasing use of composted or pulverized wood bark in gardening spheres — and this includes its inclusion in composts.

Early problems which arose when using this material related to the varying degrees of composition. If the bark was not sufficiently well decomposed, a shortage of the element nitrogen arises in the plants being grown in it.


Sand or grit

Soilless composts may be made with peat alone but these are not as popular as the composts which have other materials added. Sand or grit is the most widely used additive, making mixing the nutrients easier and the structure of the peat less critical, although still important. Naturally, the compost is heavier with the inclusion of sand but the weight is still only half that of a John Innes compost. The increase in weight is helpful in that it improves the stability of pots.


Rate of addition

With different formulations of compost there are different proportions of peat to sand. The most common ratios are three parts peat to one part sand, 2 parts peat to 1 part sand, 1 part peat to 1 part sand; all these are based on volume not weight, ie. 1 bushel of peat to 1 bushel (36 litres) of sand, or 1cu yd of peat to m3/cu yd of sand, or even 1 bucket of peat to 1 bucket of sand. Generally speaking, as the amount of sand is increased, the retention of water and nutrients will decrease. It is also reasonably safe to say that the aeration of the compost will be increased as the proportion of sand is increased. This is known as ‘opening up’ the compost.



The particle size of the sand has a great influence on aeration and water retention, permitting variations to suit the type of plant being grown. Three points should be considered in selecting particle size. Firstly, the fine sands as recommended in the original UC compost should be avoided. They were very successful in California but are not generally suitable for the moister and cooler climate in Britain. Secondly, if a coarser sand is being used, the peat should always be moistened before mixing to prevent separation of the peat and sand, which might give variabilities in the compost. Lastly, if the peat is fine, balance this by using a coarser sand; if the peat is coarse a finer sand may be included. However, such variability is not to be recommended as a general practice.

It is preferable to standardize soilless compost by using a medium grade sphagnum moss peat (ie. neither too fine nor too coarse) and a medium grade grit (say particles as in the John Innes recommendations of 3mm / 1/8in down). This combination will give a free drainage compost that will retain adequate water but will not be easily waterlogged. In general practice, three parts peat to 1 part sand by volume is the preferred proportion of peat to sand.


Lime content

Sand should be lime-free for most subjects, and this is essential for growing the lime-hating plants such as heathers and azaleas. Compost formulations often contain lime as calcium carbonate and/or dolomitic limestone. This assumes that sand is lime-free, usually with a pH between 6.0 and 6.5. As the pH of the sand rises, so the amount of lime added to the compost should be reduced, until with an alkaline sand having a pH of 8.3-8.5 (not normally recommended), no lime should be added to the mix.

It may be difficult for the amateur to assess the pH of the sand, unless the supplier is able to state it. The sand may be simply tested by using hydrochloric acid, which may be obtained in small quantities from a local chemist, but because it is a strong acid it should be handled with care. Hydrochloric acid reacts chemically with calcium carbonate (lime, chalk etc) to release carbon dioxide gas and this chemical reaction is used to test the sand for the presence of calcium carbonate. About one dessert-spoonful of sand is placed in a non-corrosive (eg. plastic) container and about a teaspoonful of hydrochloric acid is added to it. If no fizzing occurs (ie. no carbon-dioxide gas is released), the sand will contain little (less than 0.25%) or no calcium carbonate and it will thus be suitable for use in the compost. If fizzing occurs, then the sand contains over 0.25% lime and an alternative supply should preferably be found.

Occasionally, sands from the seashore are sold for horticultural use. These may contain common salt from the sea water, which is harmful to plants. However, generally speaking, these sands are washed and dried before sale, and provided they are structurally satisfactory they may be used in composts.

04. April 2011 by Dave Pinkney
Categories: Greenhouse Gardening, Organic Gardening, Soil Cultivation | Tags: , | Comments Off on Raw Materials in Soilless Composts


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