Geoscience Reference
In-Depth Information
Some soil types consist entirely or predominantly of organic matter; for example,
shallow organic soils may overlie hard unweathered rocks in high rainfall areas, and
deep peats occupy basin sites more widely. For most soils, however, it is the mineral
component which is dominant. This develops through processes acting either directly
on weathered rocks or on unconsolidated superficial geological deposits such as gla-
cial drifts or alluvium. These parent materials differ greatly in mineralogical compos-
ition, and the processes of soil development also vary greatly in response to landform,
climate and management history (
chapter 2
). The resultant diversity of mineral soil
types and properties is thus even greater than that found within the litter and humus
layers.
The relative proportions of different size particles are a key influence on the
physical character of soils. Particular attention is conventionally given to the 'fine
earth' fraction of the soil - particles less than 2mm diameter which are divided into
sand, silt and clay grades. Although gravel and stones may make up a large part of
the total soil, it is these finer fractions which are more important in determining soil
texture, water movement and chemistry, and in influencing the conditions for soil or-
ganisms.
The coarse to medium sands come at the visually distinct range, from 2 to 0.2mm
in diameter, their size and angularities rendering them resistant to movement within
the soil. They are derived from the the physical weathering of the rocks of the earth's
crust: through the shattering effects of heat and cold, from the grinding of rock upon
rock in glaciers and rivers, and by the blast of other sand grains transported by the
wind.
Very fine sands and silts, between about 0.06 and 0.002mm, are chemically
similar but their small size renders them more mobile. In the words of V.I.Stewart,
they “readily flow when saturated, clogging pores between larger particles, in-filling
drains and lubricating the erosion of soil downslope… behaving”, as he puts it “like a
pile of minute billiard balls”.
