Environmental Engineering Reference
In-Depth Information
Soils affected by salinity and alkalinity have low fertility. High concentrations of soluble
salts are harmful to plants. The salinity of the soil is measured by the
electrical
conductivity
(EC) of a saturation paste or of water extracted from a soil which has just
been saturated with water (the
saturation extract
). The units are mS cm
−1
, millisiemens
per centimetre. Table 19.2 shows typical values. The alkalinity of soil is measured by
three properties: the pH, the
exchangeable sodium percentage
and the
sodium
adsorption ratio
(the ratio of sodium ions to the square root of the calcium plus
magnesium ions):
Table 19.2 Significance of EC values
EC mS cm
−1
Effect on crops
0-2
Negligible
2-4
Sensitive crops reduced yield
4-8
Many crops reduced yield
8-16
Only tolerant crops yield satisfactorily
>16
Only few very tolerant crops yield satisfactorily
LATERIZATION
Tropical regions are well known for the speed and intensity of processes of weathering
and soil formation. Where abundant moisture is available to match the prevailing high
temperatures, the soils are the product of intense tropical weathering which removes all
the geochemically unstable elements (potassium, sodium, magnesium and calcium) from
the soil and concentrates sesquioxides (oxides of iron, aluminium and manganese) and
silicon. The rapid breakdown of rock minerals and the thorough leaching of base
elements in tropical environments has been termed
katamorphism
.
The resulting soil profile is shown in Figure 19.15. At the surface, organic matter is
rapidly decomposed so that the humus Ah horizon is very thin. Some surface eluviation
may produce a thin, eluvial layer (Ae) but this is usually masked by the red iron colours.
The surface horizons are commonly high in concretionary iron particles about the size of
a large pea. They thus appear gravelly, but the gravels are pedologically formed, not
alluvially formed. This is the horizon of concretionary pisolithic ironstones (A/B) and
may be between 1·0 m and 1·5 m thick. Below this is the main laterite (
plinthite
) horizon
with a
