Soils in their environment - Fundamentals of the Physical Environment - page 440

aluminium rather than clay minerals. This is a result of
the total loss of silica from the soil under the influence of
organic acids.
The second soil-forming process, 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 horizon, also known as the argillic or luvic
B. This form of clay leaching is a dominant process in
argillic brown earths (FAO: Luvisols and Lixisols) and is
known by various terms: clay translocation, clay leaching,
clay eluviation, clay illuviation (in-washing) or lessivage
(its French term). It occurs in many soil types. The factors
which favour clay translocation are:

1 Slightly acid conditions, so that clay particles are

dispersed and not flocculated by the presence of
calcium ions.

2 Climate with distinct wet and dry seasons, so that

clay can be precipitated in dry periods and moved
in wet periods. Thus the process is common in
Mediterranean, savanna and continental regions. This
factor also explains why the process is common in
southern and eastern England but not in western
Britain and Scotland.

The clays are deposited as coatings called cutans on the
surface of structural aggregates, along channels or pores,
and around stones. These are often visible to the naked
eye, but are especially clear under a petrological micro-
scope. When the individual clay plates are deposited they
become oriented parallel to each other giving the entire
cutan the property of birefringence, hence the term
'birefringent clay' ( Plate 18.12 ).
The typical horizon sequence for luvisols (argillic
brown earths) is A-Eb-Bt, as in Figure 18.9 , an unculti-
vated soil under beech woodland in the Chilterns,
England. The parent material is chalky colluvium, and the
leaching of calcium carbonate from the upper horizons
will have taken place before the clay minerals were able to
be translocated.

Plate 18.12 Photomicrograph of clay illuviation horizon (Bt)
in Argillic Brown Earth (FAO: Orthic Luvisol). The round black
object to the left is organic but the pores to the right are black
under cross-polarized light. The bright yellow colours along the
pores denote birefringence from the coatings of clay. Frame 2
mm wide.
Photo: Ken Atkinson

Fe (OH) 3 + e -

H + + = Fe (OH) 2 + H 2 O

ferric electron from ferrous

hydroxide organic hydroxide

matter

The process of ferric iron reduction to more mobile
and grey ferrous iron compounds is partially chemical,
partially carried out by anaerobic micro-organisms and
partially carried out by the products of decomposing
organic matter. The process is known as gleying . When the
soil or horizon is permanently gleyed, it has a uniform
grey or blue-grey colour. Where the soil or horizon is only
gleyed temporarily or seasonally, and reoxidation can take
place intermittently, the soil shows reddish-orange mottles

Anaerobism and gleying

Soils which are affected by temporary or permanent
waterlogging have very distinct profiles. With pore space
occupied by water rather than air, reduction processes
replace the oxidative processes in well aerated soils. One
of the main reduction reactions is that involving iron
oxides which reduce ferric to ferrous compounds
according to the equation: