Under
dry-farm conditions it frequently occurs that the draft upon the
water of the soil is so great that nearly all the water is quickly
and so completely abstracted from the upper few inches of soil that
they are left as an effective protection against further
evaporation. For instance, in localities where hot dry winds are of
common occurrence, the upper layer of soil is sometimes completely
dried before the water in the lower layers can by slow capillary
movement reach the top. The dry soil layer then prevents further
loss of water, and the wind because of its intensity has helped to
conserve the soil-moisture. Similarly in localities where the
relative humidity is low, the sunshine abundant, and the temperature
high, evaporation may go on so rapidly that the lower soil layers
cannot supply the demands made, and the topsoil then dries out so
completely as to form a protective covering against further
evaporation. It is on this principle that the native desert soils of
the United States, untouched by the plow, and the surfaces of which
are sun-baked, are often found to possess large percentages of water
at lower depths.
Pages:
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149