Physical Resources / 117mediate layer, called ‘gley’, is wet, sticky, and blue because of the reduced iron compounds it contains,
and compression forces it upward through cracks and mixes it with the overlying material in the
process of ‘gleying’ (or in the United States ‘gleyzation’).
Oxisols, the predominant soils of the humid tropical lowlands, are the deepest of all soils. In many
places the total soil depth, from surface to bedrock, may be 10 m and all the lower horizons are very
thick. Scale apart, they are similar to Spodosols, except that their surface horizons may be badly
eroded, so the B horizon is close to the surface, and they contain very little humus, because of the
rapidity with which organic matter decomposes under the humid tropical climate, its nutrients being
reabsorbed by plants. Almost all plant nutrients are contained within the living vegetation itself and
soluble compounds have been leached from the soil, leaving it acid and inherently infertile. Clays,
mainly comprising kaolinite (an aluminium-silica mineral, Al 2 Si 2 O 5 (OH) 4 ) and ferric and aluminium
oxides and hydroxides, may accumulate near the top of the B horizon, cemented together to form
nodules or more extensive layers of ‘laterite’. Laterite is extremely hard and impermeable and, in
soils prone to it, laterization, the process by which it forms, is sometimes accelerated by clearing
vegetation and leaving the ground exposed to heavy rain and thus to increased leaching.
Given the close association between climate and pedogenesis, it is not surprising to discover that the
global distribution of soil orders broadly conforms to climatic zones. As Figure 3.14 illustrates,
Oxisols are found in the humid tropics, Alfisols in temperate regions, Mollisols in the prairies,
pampas, and steppes, and Spodosols in a belt around northern America and Eurasia.
Simply because an apparently deep, dark soil occurs in a climate favourable for agriculture, however,
it does not necessarily follow that the land will sustain farming. Farmed soils are ‘domesticated’ by
years of careful management and are markedly different from the ‘virgin’ soils that preceded them.
Early farmers settled on the most promising land and when, after a few seasons, their crop yields
began to decline, they moved elsewhere and started again. There is only a limited store of plant
nutrients in any soil and it is depleted by the removal of crops, which reduces the amount available
for recycling. Fertilizers and lime (to restore leached calcium) replenish the store, but if they are
unobtainable (or unknown) farmers may have no alternative but to adopt some form of shifting
cultivation. This still remains a common type of farming in many parts of the tropics.
Soil fertility is not determined by the amount of plant nutrients contained within reach of plant roots,
because presence and proximity do not guarantee access. The roots must be able to absorb the nutrients
they require and their ability to do so depends on the chemical characteristics of the soil.
Humus and silicate clays consist of masses of microscopic soil particles, each about 2 μm across.
They are called ‘colloids’, because they change between a gel-like consistency and liquid according
to the chemical environment around them. Soil colloids have negatively charged surface sites on to
which cations (positive ions) can be adsorbed. Humus has many more of them than clay, which in
turn has far more than sand. The commonest soil cations are calcium (Ca2+), magnesium (Mg2+),
sodium (Na+), potassium (K+), and aluminium (Al3+), but they are changing constantly as one replaces
another, often in the order Al → Ca → Mg → K → Na. Fertilizers add cations, such as ammonium
(NH 4 +), and lime adds calcium.
Anions (negatively charged ions) are also exchanged, but to a much lesser extent. Several im portant
plant nutrients commonly occur as anions, including sulphate (SO 4 2-), nitrate (NO 3 - ), phosphate (H 2 PO 4 -
or HPO 4 2-), and molybdate (MoO 4 2-). These are not held at cation-exchange sites, but dissolve in soil
water and are absorbed by plants directly from solution.
While the colloid remains saturated with exchangeable cations it retains its structure, but as
these are replaced by hydrogen (H+) the structure weakens. The soil becomes more acid (a measure