Moreover, it must be kept in mind that evaporation 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 particles, 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, 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.  This brings into prominence again the practices of fall plowing, deep plowing, subsoiling, and the choice of deep soils for dry-farming.

Very much for the same reasons, evaporation goes on more slowly from water in which salt or other substances have been dissolved.  The attraction between the water and the dissolved salt seems to be strong enough to resist partially the force causing evaporation.  Soil-water always contains some of the soil ingredients in solution, and consequently under the given conditions evaporation occurs more slowly from soil-water than from pure water.  Now, the more fertile a soil is, that is, the more soluble plant-food it contains, the more material will be dissolved in the soil-water, and as a result the more slowly will evaporation take place.  Fallowing, cultivation, thorough plowing and manuring, which increase the store of soluble plant-food, all tend to diminish evaporation.  While these conditions may have little value in the eyes of the farmer who is under an abundant rainfall, they are of great importance to the dry-farmer.  It is only by utilizing every possibility of conserving water and fertility that dry-farming may be made a perfectly safe practice.

Loss by evaporation chiefly at the surface