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.