SOIL MOISTURE AND DRY FARMING 87 



while crops are being grown upon them. The farmer 

 in dry areas is usually spared loss from this- source. 



In some instances, nevertheless, soils may be ex- 

 cessively wet, as when, for instance, seepage waters flow 

 into depressions from a subterranean source. If these 

 are possessed of much clay, they usually become hard 

 when the water evaporates sufficiently to admit of til-ling 

 them. When plowed they turn up cloddy, and much 

 labor is involved in pulverizing them. If plowed when 

 wet they bake. Moreover, they are much liable to contain 

 substances that are injurious to vegetation, as an excess 

 of salts. These soils are undesirable, as has been al- 

 ready shown (see p. 73). 



Loss of moisture by evaporation. The chief in- 

 fluences that lead to the loss of soil moisture by evapo- 

 ration are: (1) sunshine; (2) dry and warm air, and 

 (3) wind. The sun shining down on moist soil turns the 

 moisture near the surface into vapor, which rises and 

 mingles with the air. The rapidity of the process is 

 proportionate to the heat of the sunshine and the de- 

 gree of moisture in the soil. The influence of sunshine 

 as a factor in removing moisture by evaporation is 

 readily seen by comparing the quick drying of the sur- 

 face soil after rain when exposed to bright sunshine 

 with the slow drying of soil in a similar condition on 

 a cloudy day. Sunshine is the most powerful factor in 

 thus removing moisture. In dry areas such removal 

 should be specially guarded against, because of the great 

 abundance of the sunshine. 



Air penetrates the soil to a greater or lesser depth, 

 according to its density or porosity. It more readily 

 penetrates between the soil grains in a newly cultivated 

 soil. As it passes between these, more or less of the mois- 

 ture which adheres to the soil grains becomes incorpor- 

 ated with the air thus diffused in the soil, and escapes 

 with it into the atmosphere as a result of constant move- 



