_Hardpan.--_Instead of the heavy clay subsoil of humid regions, the
so-called hardpan occurs in regions of limited rainfall. The annual
rainfall, which is approximately constant, penetrates from year to
year very nearly to the same depth. Some of the lime found so
abundantly in arid soils is dissolved and worked down yearly to the
lower limit of the rainfall and left there to enter into combination
with other soil ingredients. Continued through long periods of time
this results in the formation of a layer of calcareous material at
the average depth to which the rainfall has penetrated the soil. Not
only is the lime thus carried down, but the finer particles are
carried down in like manner. Especially where the soil is poor in
lime is the clay worked down to form a somewhat clayey hardpan. A
hardpan formed in such a manner is frequently a serious obstacle to
the downward movement of the roots, and also prevents the annual
precipitation from moving down far enough to be beyond the influence
of the sunshine and winds. It is fortunate, however, that in the
great majority of instances this hardpan gradually disappears under
the influence of proper methods of dry-farm tillage.
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