Physical Resources / 119of the hydrogen ion concentration) and when it is nearly saturated with hydrogen the colloid breaks
down into its constituent compounds, which move downward through the soil.
The amount of exchangeable cations in a unit weight of dry soil is known as the ‘cation exchange
capacity’ (CEC) of that soil and it provides an important measure of soil fertility. It is measured in
milliequivalents (me) per 100 g (a milliequivalent being that quantity chemically equal to 1 mg of
hydrogen). Sandy soils typically have a CEC of 1–5 me 100 g-1, loams 5–15 me per 100 g, and clays
more than 30 me per 100 g. The CEC for a typical Mollisol ranges from just over 24 me per 100 g at the
surface to 25.7 me per 100 g in the lower part of the B horizon. The proportion of the cation-exchange
sites occupied by calcium, magnesium, potassium, and sodium ions is known as the ‘percentage base
saturation’; the lower its value the more acid the soil. Most crop plants prefer a base saturation of 80 per
cent or more, producing a neutral soil (pH 6.0 or higher), but some grow best in more acid soils, with
a lower percentage base saturation (FOTH AND TURK, 1972, pp. 171–175).
Those cations that are plant nutrients are absorbed by roots while dissolved in soil water. Some are
replaced from other exchange sites in the soil colloid, but these are generally insufficient to replace
all of them, and cation-rich fertilizers must replenish the store, the frequency and amount of fertilizer
application depending on the CEC of the soil. In other words, soil fertility can be expressed as CEC
and sandy soils (low CEC) generally need more fertilizer more often than clay soils (high CEC).
Cation exchange also makes soils with a high CEC excellent purifying filters for water percolating
through them. Positively charged pollutants, such as lead (Pb2+) and cadmium (Cd2+), are quickly
adsorbed to exchange sites and thus immobilized, so water contaminated with them is purified by the
time it reaches the ground water.
It is technologically possible to grow any plant anywhere in the world by supplying it with the
environment it requires. Construct and heat a glasshouse, supply it with an appropriate soil, illuminate
it artificially to achieve a suitable light intensity and length of day, and tropical crops could be raised
in Greenland. It is possible, but hardly sensible when tropical crops can be produced so much more
easily and cheaply in the tropics. Soils appropriate to particular plants are most likely to develop
where the climate also suits those plants or their close relatives, but there are dangers. Soil fertility
is usually reduced by cropping and must be replenished, and some soils, especially in the tropics, are
much less fertile than the luxuriant vegetation they naturally sustain makes them appear.
29. Soil erosion and its control
Central Belgium has large areas of loess soils and it is estimated (KUPCHELLA AND HYLAND,
1986, p. 447) that each year these are losing between 10 and 25 tonnes of soil from each hectare by
erosion. The United States is losing about 18 t ha-1 yr-1 and the Yellow River basin in China 100 t ha-
(^1) yr-1. In 1939, wind-blown soil from Texas fell in Iowa, 800 km away. It landed on top of snow, so
the soil could be collected, weighed, and analysed. The amount deposited was 450 kg ha-1 and the
deposit contained more than three times more organic matter and nitrogen than the soil from which
it had been removed (HUDSON, 1971, p. 257). Clearly, soil erosion is a serious problem affecting
cultivated land in almost every part of the world. As was mentioned earlier (Section 2) there is
reason to hope that the introduction of modern farming methods may reduce the rate of soil erosion
dramatically. Using herbicides to clear the ground of weeds as an alternative to ploughing is the most
promising method. This makes it possible to reduce tillage to a minimum and to sow seeds directly
into land covered with, and protected by, dead weeds. At the same time, increasing the productivity
of the best land reduces the need to cultivate the poorer, more erosion-prone land.