Physical Resources / 115accustomed. Following a series of dry years, in 1934 and 1935 the crops failed and soil reduced to
little more than powder was carried away by the wind, creating the Dust Bowl (ALLABY, 1998,
pp. 76–82). This brought tragedy to countless American farming families in the 1930s, most of
whom were already impoverished as a result of the economic depression. After that experience it
was recognized that the land was unsuitable for arable farming and much of it has been returned to
grassland. To take another example, unexpected floods destroy crops and livelihoods, and may
cause the deaths of livestock and people. Yet the disasters may also bring benefits to those farming
the fertile loess or alluvial soils.
Such events occur naturally. Clearly, the soils of the Dust Bowl were unsuitable for the type of
farming practised on them. By removing the natural vegetation cover and cultivating the soil, farmers
reduced the soil to a condition in which it would blow away, but the drought was a natural occurrence.
Human activities can interfere more directly. When a river is dammed, for example, silt will accumulate
behind the dam. This progressively reduces the volume of water held in the artificial lake, but it also
interrupts the natural sedimentation process further downstream. On the flood plain or delta, farmers
may depend on the seasonal floods for the silt they bring, rich in the plant nutrients that have drained
into it the entire length of the river’s course. Deprived of their ‘natural fertilizer’, farmers may be
forced to buy factory-made fertilizer, which in many cases they can ill afford, and the farming
techniques they have developed may be inappropriate for alluvial soils that are not regularly
replenished. Soil structure and fertility may deteriorate. Similarly, the clearing of vegetation from
upper slopes may increase the transport of sediment to lower levels, polluting rivers.
28. Soil, climate, and land use
Climate is by far the most important factor controlling the development of soils (pedogenesis) from
their parent materials, but it is not the only one. Pedogenesis is also affected by the type of vegetation
(which is also climatically governed), the activities of animals including humans, the parent material,
and topography. Most of the chemical reactions by which the mineral constituents of rock are modified
do not commence until the temperature rises above 10°C, and their reaction rates double for every
further 10°C rise above that. It follows that, provided water is available, soils develop and age faster
in warm than in cold climates and it is in warm climates that biological organisms are more active.
Some of the early schemes for classifying soils were based on climate, grouping soils into cold,
cool-temperate, subtropical, and tropical zones, within which many of the names were descrip-tive.
In the cold zones, for example, are tundra and mountain meadow soils, and in the cool-temperate
zones prairie soils (CRUICKSHANK, 1972, pp. 155–166). This led to the concept of ‘zonal soils’,
typical of the zones in which they occur. The zones might also contain soils formed under the influence
of some local factor and therefore atypical. These were called ‘intrazonal soils’. Soils that had not
developed at all might be found in any climatic regime, and were called ‘azonal’.
Such classifications were based on examinations of only the A and C soil horizons, the B horizon
being considered merely transitional between those above and below it. In time, soil scientists
came to realize that zonal schemes were classifying not so much the soils as the environments in
which they form. Modern soil taxonomy is based on the soils themselves, defined in terms of
more than 20 surface and subsurface ‘diagnostic horizons’, called ‘epipedons’ and ‘endopedons’
respectively. An anthropic epipedon, for example, is a surface horizon formed where people
have lived for a long time or have grown irrigated crops; cultivation over many years might lead
to the formation of an agric endopedon just beneath the ploughing depth, where clay and organic
matter have collected.