Clinging to every
tree, stone, or animal tissue is a small quantity of moisture
varying with the temperature, the amount of water in the air, and
with other well-known factors. It is impossible to rid any natural
substance wholly of water without heating it to a high temperature.
This water which, apparently, belongs to all natural objects is
commonly called hygroscopic water. Hilgard states that the soils of
the arid regions contain, under a temperature of 15 deg C. and an
atmosphere saturated with water, approximately 5-1/2 per cent of
hygroscopic water. In fact, however, the air over the arid region is
far from being saturated with water and the temperature is even
higher than 15 deg C., and the hygroscopic moisture actually found
in the soils of the dry-farm territory is considerably smaller than
the average above given. Under the conditions prevailing in the
Great Basin the hygroscopic water of soils varies from .75 per cent
to 3-1/2 per cent; the average amount is not far from 12 per cent.
Whether or not the hygroscopic water of soils is of value in plant
growth is a disputed question.
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