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