This means that the total surface of the soil grains
contained in a column of soil 1 square foot at the top and 10 feet
deep is approximately 10 acres. When even a thin film of water is
spread over such a large area, it is clear that the total amount of
water involved must be large It is to be noticed, therefore, that
the fineness of the soil particles previously discussed has a direct
bearing upon the amount of water that soils may retain for the use
of plant growth. As the fineness of the soil grains increases, the
total surface increases' and the water-holding capacity also
increases.
Naturally, the thickness of a water film held around the soil grains
is very minute. King has calculated that a film 275 millionths of an
inch thick, clinging around the soil particles, is equivalent to
14.24 per cent of water in a heavy clay; 7.2 per cent in a loam;
5.21 per cent in a sandy loam, and 1.41 per cent in a sandy soil.
It is important to know the largest amount of water that soils can
hold in a capillary condition, for upon it depend, in a measure, the
possibilities of crop production under dry-farming conditions.
Pages:
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107