nizable in the field. These features, particularly specific layers called ‘soil hori-
zons’, are the basis for soil classification. An idealized soil profile, i.e. a vertical
section, is shown in Fig. 4.21. Soil horizons are described using an internation-
ally agreed system of abbreviations that are shown on Fig. 4.21 and used in the
descriptions below.
The classification of soils is potentially complex. Two systems of soil classifi-
cation are common, that of the US Department of Agriculture (USDA) and that
of UNESCO’s Food and Agriculture Organisation (FAO). In this book we mostly
use the USDA’s first tier of classification known as soil orders (Fig. 4.22), which
can be related to factors such as degree of weathering, type of parent material
and climate. Where possible the USDA name is followed by the equivalent FAO
name in brackets. In some cases, for example vertisol, the name is the same in
both systems. Most of the order names have the common ending -sol, from the
Latin solum, meaning soil. Although soils are classified using stable features devel-
oped over time, the soils formed under dynamic conditions. It is variations in
soil-forming processes that give rise to the vast number of different soil types.
Three contrasting soils with distinctive diagnostic horizons are shown in Plate
4.2 (facing p. 138). Clearly a range of dynamic influences have controlled the
development of each of these soils, and these are discussed separately below,
emphasizing the role of soil chemistry.
4.9.1 Soils with argillic horizons
The term ‘argillic’ indicates that a soil has a clay-rich horizon (Bt in Fig. 4.23).
The downward percolation of water through the soil controls most of the sig-
nificant processes in the development of the clay-rich (argillic) horizon (Bt). The
movement of water causes; (i) the leaching of calcium ions (Ca^2 +) from the A
horizon; (ii) the washing of materials down profile (eluvation); and (iii) the depo-
sition of these ‘washed-down’ materials at depth (illuvation) (Fig. 4.21). The
leaching (decalcification) of Ca^2 +that had bound together clay particles with
excess negative charge (Fig. 4.23), causes destabilization of clay aggregates, allow-
ing them to fall apart (deflocculate). The disaggregated clay particles are them-
selves then susceptible to translocation down profile (eluvation). The clay
particles will re-flocculate lower in the profile, to form the argillic horizon, where
sufficient divalent cations are present to re-bind them (Fig. 4.23). In the soil
shown in Plate 4.2 the Ca^2 +is supplied from weathering a CaCO 3 -rich ‘C’ horizon
below the Bt horizon. Note that the chain of events leading to the development
of the argillic horizon is analogous to those that translocate clays over larger scales
in the formation of vertisols (Section 4.7). Argillic horizons can form in a number
of soil types, for example ultisols (acrisols), mollisols (chernozems and kas-
tanozems) and alfisols (luvisols).
4.9.2 Spodosols (podzols)
Spodosols (podzols) contain separate horizons from which material has been
both removed and deposited (Fig. 4.24). However, spodosols form under very
The Chemistry of Continental Solids 113