The dead organic matter has an important effect on the soil. The fresh, still recognizable material physically ‘ opens up ’ the soil, improving aeration. Active micro-organisms gradually decompose this material until it consists of unrecognizable plant and micro-organism remains. This finer material has less physical effect, but usually improves the water holding capacity of the soil.
In general, succulent (‘green’, leafy) organic matter decomposes very rapidly, so long as conditions are right, so has only a short-term physical effect, but yields nutrients, especially nitrogen compounds. The fibrous or woody (‘brown’) plant material tends to decompose very slowly so its physical effect persists, but nutrient contributions are low. The distinction between the ‘green’ and ‘brown’ organic matter is a crude but useful one when composting.
This process of decomposition continues until all the organic matter is reduced to carbon dioxide, water, minerals and humus. The humus arises from a small proportion of the fibrous (‘brown’) organic matter which is highly resistant to decomposition; the lignin and other resistant chemicals form a collection of humic acids which forms a black colloidal (jelly-like) material. The humus coats soil particles and gives topsoil its characteristic dark colour.
This colloidal material has a high cation exchange capacity and therefore can make a major contribution to the retention of exchangeable cations, especially on soils low in clay (see sands). It also adheres strongly to mineral particles, which makes it a valuable agent in soil aggregation. In sandy soils it provides a means of sticking particles together, whereas in clays it forms a clay-humus complex that makes the heavier soils more likely to crumble. Its presence in the soil crumbs makes them more stable, i.e. more able to resist collapse when wetted, and it increases the range of soil consistency. Bacteria eventually decompose humus so the amount in the soil is very dependent on the continued addition of appropriate bulky organic matter.
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