Once soil has been cultivated a distinct boundary between topsoil and subsoil is developed as the concentration of organic matter in the surface layers is evened out (see Figures 18.5 and 17.5). On first cultivation the increased aeration and nutrients stimulate micro-organisms and a new equilibrium with lower soil organic matter levels prevails. Once under cultivation grasses and high-producing legumes tend to increase organic matter levels, but most crops, particularly those in which complete plant removal occurs, lead to decreased levels. Only large, regular dressings of bulky organic matter such as compost, straw, farmyard manure or leaf mould can improve or maintain the level of soil organic matter on cultivated soils.
Organic matter can accumulate under grass and form a mat on the surface where the carbon cycle is slowed because of nutrient deficiency usually induced by surface soil acidity or excess phosphate levels. This is part of the reason for the development of 'thatch' in turf (see Figure 18.6).
While all soils contain some organic matter, most are classified as mineral soils. However, at levels above about 15 per cent (when the organic matter present dominates the soil properties) they become classified as 'organic soils', e.g. organic clay loam. They develop where decomposition is slow because the activity of micro-organisms is reduced by cold, acidity or waterlogged conditions. Peaty soils are those where organic matter content is greater than 50 per cent; if content is more than 95 per cent the soil is considered to be a peat.
Peat is formed from partially decomposed plant material. This usually develops in waterlogged conditions where decomposition rates are low. There are great differences between peats because of the range of species of plants involved, which in turn depends on the conditions where they occur. Some peat is formed in shallow water, as found in poorly drained depressions or infilling lakes. In such circumstances the water drains from surrounding mineral soils and consequently has sufficient nutrients to support vegetation, often dominated by sedges, giving rise to sedge peat. As the waterlogged area, pond or lake becomes full of humified organic matter it forms a bog, moor or fen. In wetter areas, sphagnum moss, which is able to live on the very low nutrient levels that prevail, grows on top of the infilled wet land.
The dead vegetation becomes very acid and decomposes slowly. It builds up to form a high moor; sphagnum moss growing on top of very slowly decomposing sphagnum moss.
Some of the peatlands that are enriched with minerals prove very valuable when drained, e.g. the fenlands of eastern England. They are easily worked to produce vegetables and other high value crops. Unfortunately the increased aeration allows the organic matter to be decomposed at a rate faster than it can be replenished. Furthermore, when the surface dries out, the light particles are vulnerable to wind erosion. Consequently the soil level of these areas is falling at a rate of about three metres every hundred years. This can be checked by keeping the water table as high as possible and providing protection against wind.
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