Physical Resources / 107is simpler. Adequate drainage must be installed and irrigation suspended until the water table has
been lowered.
As the demand for food intensifies it is likely that the total area of irrigated land will increase. Some
say it may double between about 1990 and the early part of the next century (PIEL, 1992, p. 216). In
Asia, where such an increase is likely to be concentrated, this will allow two or even three crops to
be grown each year on land that presently produces only one. The advantages will prove enduring,
however, only if irrigation schemes are planned with care to avoid the hazards attendant on them.
26. Soil formation, ageing, and taxonomy
From the moment it is exposed at the surface, rock is subjected to persistent physical attack. Water
fills small fissures and when it freezes it expands, exerting a pressure of up to 146 kg cm-2, which
is sufficient to split the toughest rock (DONAHUE ET AL., 1958, p. 28). In summer, the rock
warms during the day and cools again at night, expanding as it warms and contracting as it cools,
but it is heated unevenly. The surface is heated more strongly than rock beneath the surface; some
parts of the surface are exposed directly to sunlight, others are in shade. As a consequence, some
parts of the rock expand and contract more than others. This, too, causes the rock to break. Often
flakes are loosened or detached from the surface, a process called ‘exfoliation’. Detached particles
then grind against one another as they are moved by gravity, wind, or water. This breaks them into
still smaller pieces.
The smaller any physical object, the greater its surface area in relation to its volume: a sphere with a
diameter of 4 units has a surface area of 50 units^2 and volume of 33.5 units^3 , giving an area:volume
ratio of 1:0.7; if the diameter is 2, the surface area is 12.5 units^2 , volume 4.2 units^3 and ratio 1:0.3. As
the rock particles grow smaller, therefore, the total surface area exposed to attack increases. Still
vulnerable to abrasion, they are now subject to chemical attack.
This takes several forms. Some of the chemical compounds of which they are composed may be
soluble in water; wetting dissolves and drainage removes them. Other compounds may react chemically
with water. The process is called ‘hydrolysis’ and can convert insoluble compounds to more soluble
ones. Orthoclase feldspar (KAlSi 3 O 8 ), for example, a common constituent of igneous rocks, hydrolyses
to a partly soluble clay (HAlSi 3 O 8 ) and very soluble potassium hydroxide (KOH) by the reaction:
KAlSi 3 O 8 + H 2 O → HAlSi 3 O 8 + KOHHydration is the process in which compounds combine with water, but do not react chemically with
it. The addition of water to a compound’s molecules makes them bigger and softer and so increases
their vulnerability to breakage. Oxidation also increases the size and softness of many mineral
molecules and may also alter their electrical charge in ways that make them react more readily with
water or weak acids. Reduction, which occurs where oxygen is in short supply, also alters the electrical
charge on molecules and may reduce their size.
Compounds may also react with carbonic acid (H 2 CO 3 ), formed when carbon dioxide dissolves in
water. This reaction, called ‘carbonation’, forms soluble bicarbonates. Barely soluble calcium
carbonate (CaCO 3 ), for example, becomes highly soluble calcium bicarbonate (Ca(HCO 3 ) 2 ).
Physical and chemical processes thus combine to alter radically the structure and chemical composition
of surface rock. How long it takes for solid rock to be converted into a layer of small mineral
particles depends on the character of the original rock and the extent of its exposure; in arid climates