Physical Properties of Soil-Based and Soilless Composts

Differences in the physical properties of soil-based and soilless composts

An important difference between soil-based and soilless composts is in the supply of nutrients. Soil-based composts contain relatively large quantities of nutrients supplied at rather infrequent intervals, but soilless composts contain smaller quantities of nutrients which need to be supplemented more frequently. Incorrectly this is put down to the poor absorption of nutrients by the peat in the soilless compost. It is in fact because of the lower fixing capacity (storage) in peat.

physical properties of soil based and soilless composts

This may be best explained by a brief description of clay particles in the soil. These are very fine particles that contain nutrients within themselves which are not exchangeable in the normal soil solution. The exchangeable nutrients are held on to the surface of the clay particles by electrical charges and are removed into the soil solution to be taken up by the plant roots. This leaves a gap on the outside of the clay particle which is replaced from the nutrients within the clay particle. Thus the clay particle acts as a reserve or storage of nutrients. It is this reserve that peat itself lacks. Peat may be compared to a sponge that absorbs water and nutrients and releases them on demand, but once all the nutrients previously given have been removed, there is no reserve supply. The peat itself will gradually decompose but this breakdown is too slow to meet the needs of rapidly growing plants.


This is both an important weakness and strength in a soilless compost. Because of the lack of a reserve a wider range of nutrients must be added and added more frequently. But the grower has a tighter control over the growth of the plants because the plants will respond more readily to changes in nutrition without the reserve of plant foods in the background.

A final difference between the two composts may aptly be mentioned now. Nearly all forms of peat suitable for use in compost have a higher water-holding capacity than loam. Again peat may be likened to a sponge as it contains very large quantities of space for holding water. In a soilless compost, which usually contains peat as the largest proportion of all the ingredients, this may result in over-watering. This is dangerous because the compost will become poorly aerated and this in turn will quickly lead to death of the roots. To overcome this, several precautions are usually taken when using a soilless compost in place of a soil-based compost. The pot should be loosely filled with soilless compost whereas a pot of soil-based compost should be pressed well in. Pressing down a soilless compost will result in immediate trouble due to the lack of air. It should be emphasized just how important the air is in a soilless compost. To prevent over-watering, the pot is filled to the top with compost, whereas with a soil-based compost approximately 0.6-1.25cm (¼—1/2in) is left at the top of the pot to allow for watering.

The compost may be made free draining by adding a very coarse sand or grit to ‘open up’ the compost structure but it is not advisable to add too much otherwise the compost will require frequent watering and nutrients will be leached away, making high demands on the supplementary feeding. When watering a soil-based compost, the normal procedure is to allow the compost to become dry before watering well. Following this procedure with soilless compost will cause many problems, especially as the peat could be very difficult to re-wet. For soilless compost, the procedure is to water little and often, always keeping the compost more moist than a comparative soil-based compost.

Lastly, to keep the aeration good, the peat used should not be too fine or too decayed. Either would make the compost structure too close and result in a lower proportion of air to water.

27. March 2011 by Dave Pinkney
Categories: Greenhouse Gardening, Organic Gardening, Soil Cultivation | Tags: , , | Comments Off on Physical Properties of Soil-Based and Soilless Composts


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