In arid regions, on the other hand, the rainfall penetrates the soil
only a few feet. In time, it is returned to the surface by the
action of plants or sunshine and evaporated into the air. It is true
that under proper methods of tillage even the light rainfall of arid
and semiarid regions may he made to pass to considerable soil
depths, yet there is little if any drainage of water through the
soil into the standing ground water. The arid regions of the world,
therefore, contribute proportionately a small amount of the
substances which make up the salt of the sea.
_Alkali soils.--_Under favorable conditions it sometimes happens
that the soluble materials, which would normally be washed out of
humid soils, accumulate to so large a degree in arid soils as to
make the lands unfitted for agricultural purposes. Such lands are
called alkali lands. Unwise irrigation in arid climates frequently
produces alkali spots, but many occur naturally. Such soils should
not be chosen for dry-farm purposes, for they are likely to give
trouble.
_Plant-food content.--_This condition necessarily leads at once to
the suggestion that the soils from the two regions must differ
greatly in their fertility or power to produce and sustain plant
life.
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