All in all, the composition of arid soils is very much more
favorable to plant growth than that of humid soils. As will be shown
in Chapter IX, the greater fertility of arid soils is one of the
chief reasons for dry-farming success. Depth of the soil alone does
not suffice. There must be a large amount of high fertility
available for plants in order that the small amount of water can be
fully utilized in plant growth.
_Summary of characteristics.--_Arid soils differ from humid soils in
that they contain: less clay; more sand, but of fertile nature
because it is derived from rocks that in humid countries would
produce clay; less humus, but that of a kind which contains about
3-1/2 times more nitrogen than the humus of humid soils; more lime,
which helps in a variety of ways to improve the agricultural value
of soils; more of all the essential plant-foods, because the
leaching by downward drainage is very small in countries of limited
rainfall.
Further, arid soils show no real difference between soil and
subsoil; they are deeper and more permeable; they are more uniform
in structure; they have hardpans instead of clay subsoil, which,
however, disappear under the influence of cultivation; their
subsoils to a depth of ten feet or more are as fertile as the
topsoil, and the availability of the fertility is greater.
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