Environmental Engineering Reference
In-Depth Information
depends upon the parent material and the degree of leaching in the soil profile. In freely
drained soils with intense leaching, silica tends to be removed, thus tending to produce
low silica 1:1 clay minerals such as kaolinite. Soil age may also be a factor here, as
kaolinite is the most resistant clay mineral and tends to accumulate over prolonged
periods of weathering (e.g. tropical Ferralsols soils). In poorly drained soils, especially
where calcium and magnesium occur, there is usually enough silica and bases to form
montmorillonite and vermiculite (e.g. in vertisols and chernozems). Under moderate
leaching and a reasonable supply of bases, illite and chlorite are often the dominant clay
minerals. These are the commonest clay minerals in British soils. Illite, which has a mica-
type structure, may also be formed directly from mica minerals in parent materials. In
acid soils such as podzols, the increase in clay-size material in the B horizon is due to the
formation of hydrated oxides of iron and aluminium rather than clay minerals. This
results from the total loss of silica from the soil profile under the influence of organic
acids. In this case the B horizon would be termed Bs, the spodic (podzolic) B.
The second case, the movement in suspension of discrete clay particles by water
percolating to lower levels in the soil, produces clay enrichment in the B to give the Bt,
the luvic (argillic) B. This form of clay leaching is a dominant process in luvisols and
acrisols (argillic brown earths) and is variously termed clay translocation, clay eluviation,
clay illuviation (in-washing) or
lessivage
(its French term). However, it can also occur in
other soil types. Various factors favour clay translocation. It operates best in slightly acid
conditions when the clay particles are dispersed and not flocculated by the presence of
calcium. The clay is usually precipitated in dry periods and moved in wet periods. Thus
areas with climates of marked wet and dry seasons, e.g. in Mediterranean, savanna and
continental regions, favour clay translocation. This may also explain why the process is
common in southern and eastern England but not in western Britain and Scotland. The
clays are deposited as oriented clay or coats (called
cutans
) on the surface of structural
aggregates or in pores and around stones. These are sometimes visible to the naked eye
but often they need to be identified as thin cutans by the study of thin sections under a
petrological microscope (see Colour Plate 18 between pp. 400 and 401). When the
individual clay plates are deposited they become oriented parallel to each other; the entire
cutan has the property of birefringence, hence the term 'birefringent clay'.
The typical horizon sequence for luvisols (argillic brown earths) is A-E-Bt. This is
illustrated in Figure 19.10, an uncultivated soil from a beechwood in the Chilterns,
England. The parent material is colluvium with many chalk fragments. The leaching of
calcium carbonate from the upper horizons will have taken place before the clay minerals
were translocated.
GLEYING
Soils which are affected by temporary or permanent waterlogging have very distinct
profiles. When pore space is occupied by water rather than air, a series of reduction
processes replace the oxidative processes in well aerated soils. One of the main reduction
reactions is that involving iron oxides which reduce from ferric to ferrous compounds
according to the equation:
