EVAPORATION FROM SOILS 137 
out. This part of the soil is the hygroscopic moisture 
spoken of in the last chapter. 
Moreover, it must be kept in mind that evapora- 
tion does not occur as rapidly from wet soil as from 
a water surface, unless all the soil pores are so 
completely filled with water that the soil surface 
is practically a water surface. The reason for this 
reduced evaporation from a wet soil is almost self- 
evident. There is a comparatively strong attraction 
between soil and water, which enables the moisture 
to cling as a thin capillary film around the soil par- 
ticles, against the force of gravity. 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, respec- 
tively, 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- 
