Geoscience Reference
In-Depth Information
limestone, the rooting depth might be only half as much; in this case the available wa-
ter content might be expressed as 75mm in 450mm.
Taken together with the average rainfall of a district, the total available water
content gives a good measure of the cropping potential of the land. Thus in Eastern
England, with rainfall of less than 625mm (24 in.), sugar-beet and cereals will yield
twice as much, and potatoes three times as much, on soils with high AWC as on soils
with low AWC. In areas with higher rainfall this discrepancy will be less. As a gen-
eralization, medium-textured soils have better characteristics for supplying water for
plant growth than either coarser- or finer-textured soils. This is attributable to the fact
that good soil structure is easiest to create in clay loams.
Farmers expend enormous efforts in cultivation, drainage and irrigation to pro-
duce the optimum balance of air and water in the soil. Optimum, that is, for the range
of arable crops which society demands. The flora in Mesolithic times, before farmers
started to cultivate the land, would have been perfectly well adapted to a quite differ-
ent range of soil conditions. Life in the soil has had an unbroken history since the last
glaciation, and even what we consider to be semi-natural communities in this coun-
try have themselves been modified by direct or indirect forms of land management
over the centuries. Some of these management influences are considered in the
next
tribution of soil types.
