King
states that the largest amount of capillary water that can be held
in sandy loams varies from 17.65 per cent to 10.67 per cent; in clay
loams from 22.67 per cent to 18.16 per cent, and in humus soils
(which are practically unknown in dry-farm sections) from 44.72 per
cent to 21.29 per cent. These results were not obtained under
dry-farm conditions and must be confirmed by investigations of arid
soils.
The water that falls upon dry-farms is very seldom sufficient in
quantity to reach the standing water-table, and it is necessary,
therefore, to determine the largest percentage of water that a soil
can hold under the influence of gravity down to a depth of 8 or 10
feet--the depth to which the roots penetrate and in which root
action is distinctly felt. This is somewhat difficult to determine
because the many conflicting factors acting upon the soil-water are
seldom in equilibrium. Moreover, a considerable time must usually
elapse before the rain-water is thoroughly distributed throughout
the soil. For instance, in sandy soils, the downward descent of
water is very rapid; in clay soils, where the preponderance of fine
particles makes minute soil pores, there is considerable hindrance
to the descent of water, and it may take weeks or months for
equilibrium to be established.
Pages:
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108