Geography Reference
In-Depth Information
Soils are often classified by the nature of their vertical profile. If
you dig a hole through a typical soil, from the surface to the
bedrock, the soil will be made up of a series of horizontal layers
known as
soil horizons
. In some soils, the horizons have very dis-
tinctive colours and clear boundaries while in others the changes
with depth may be gradual with rather uncertain boundaries. Soil
horizons are given letters according to their formation and their
relative position in the profile. The uppermost O horizon is domi-
nated by an accumulation of fresh and partially decomposing
organic matter. Below this the dark A horizon contains humus and
minerals. The typically pale E horizon is dominated by leaching or
removal of material by plant roots while the underlying B horizon
is often a zone of accumulation of matter from leaching above and
weathering below. There is usually a transition from the B horizon
into the C horizon which is mainly weathered parent material
known as the
regolith
. The bedrock below this is typically desig-
nated as the R horizon. Not all soils contain all of the layers
described above.
Podzolisation
' (producing soils called podzols) may occur
where there is strong leaching often where there is plentiful rainfall
and good drainage, typically under forests or heaths. Organic acids
from litter are washed through the soil and react with iron and alu-
minium compounds that are transported downwards from the E
horizon by percolating water and deposited in the B horizon.
Podzols tend to be rather unproductive for agriculture as fertilisers
are readily washed away and they are acidic. In waterlogged con-
ditions there are reactions with iron products in the soil driven by
microorganisms in a process called
gleying
. The soil becomes grey
or bluish.
Laterisation
(ferralitisation) occurs in tropical soils
where high temperatures and plentiful rainfall result in fast rates of
weathering and leaching so that there are horizons depleted in bases
(e.g. calcium, magnesium, potassium and sodium) and enriched in
silica and oxides of aluminium and iron. In arid and semi-arid areas,
water is drawn to the soil surface and as the water evaporates salts
are left behind at or near the surface.
There are several different soil classification systems and each
system contains around 15 to 30 different soil types, often with
subcategories for main types. Often the words used in one classifi-
cation are different for the same soil in another classification
