Ordinarily, only capillary water is found in
well-tilled soil, and the force causing evaporation must be strong
enough to overcome this attraction besides changing the water into
vapor.
The less water there is in a soil, the thinner the water film, and
the more firmly is the water held. Hence, the rate of evaporation
decreases with the decrease in soil-moisture. This law is confirmed
by actual field tests. For instance, as an average of 274 trials
made at the Utah Station, it was found that three soils, otherwise
alike, that contained, respectively, 22.63 per cent, 17.14 per cent,
and 12.75 per cent of water lost in two weeks, to a depth of eight
feet, respectively 21.0, 17. 1, and 10.0 pounds of water per square
foot. Similar experiments conducted elsewhere also furnish proof of
the correctness of this principle. From this point of view the
dry-farmer does not want his soils to be unnecessarily moist. The
dry-farmer can reduce the per cent of water in the soil without
diminishing the total amount of water by so treating the soil that
the water will distribute itself to considerable depths.
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