Geography Reference
In-Depth Information
draw water into the plant. If the soil is coarse, generally consisting
of lots of large particles and large pore spaces between the particles,
then it will not be able to hold as much water as a finer soil with
smaller particles and smaller pore spaces. This explains why sandy
soils cannot hold much water and are not as good at supporting
plant growth as finer textured soils which have more small pores.
Soil formation takes place over thousands of years. The main
input of soil material comes from the weathered rock below the
soil. Mineral particles are released by weathering and contribute to
the lower layers of the soil. Surface accumulation of organic matter
from plants and animals is also important, as is dissolved material in
water and particles carried by precipitation and the wind. The main
losses of material from soils occur through wind and water erosion,
plant uptake (but this usually gets returned to the soil after death if
the system does not have crop material removed from site) and
leaching
. Leaching is the removal of dissolved soil material. The
leaching process is most rapid where there are large water inputs to
the surface and where the soils are well drained (e.g. in an irrigated
agricultural field with coarse soils with under-drainage installed).
The percolating water carries dissolved substances downwards,
depositing some in lower layers but some of the dissolved material
may be completely washed out of the soil.
The factors that influence soil formation include climate, the
'parent material' (i.e. the weathered rock matter), the slope and
organisms. The most influential factor is climate since it determines
the moisture and temperature conditions for soil development;
maps of major soil types often follow the climate zones. Soils in
high latitudes are often very shallow and develop slowly while soils
that are several metres deep are typical of tropical areas. Parent
material influences soil formation through the influence of weath-
ered material on soil processes while slope steepness, aspect and
altitude all affect the local climate as well as drainage and erosion
conditions. The type of vegetation influences the type and amount
of litter that is returned to the soil while different soil types support
different vegetation communities. In conifer forests there will be a
deep litter layer of thin waxy needles which only decompose
slowly. Vegetation also protects the soil from water and wind
erosion by intercepting rainfall, decreasing the role of rainsplash
(see section on erosion above).
