26 DRY FARMING IN THE GREAT BASIN. 



The water required to cover 1 square foot to the depth of 1 inch 

 weighs about 5i pounds, while an acre-inch of water weighs about 

 113 tons. One per cent of moisture per cubic foot of soil is equivalent 

 to 0.84 pound of water, while 1 per cent per acre- foot of soil equals 

 about 18 tons of water. 



The preceding table shows the relation between rainfall and soil 

 moisture as stated in percentages. These figures are useful in com- 

 puting the potential effect of rains of various amounts, assuming 

 that all of the rain enters the soil. 



Estimating that a wheat crop, for instance, can use moisture 5 

 feet deep in the soil, it will be seen that a sandy soil would be able 

 to hold available onh' about 540 tons of water per acre, while a 

 heavy loam would hold available about 1,350 tons per acre, or a dif- 

 ference of more than 800 tons of water per acre in favor of the heavy 

 soil. These figures show the imi:)ortance of selecting only the heavier 

 soils for dry farming where the storage of water from one season to 

 another is necessary. 



Under their virgin conditions the heavier and better soils of a 

 semiarid region are rarely wet to a depth of more than 3 or 4 feet — 

 often 2 feet or less. With the surface soil undisturbed so that this 

 moisture can move upward and dissipate by direct evaporation, and 

 with the natural vegetation using large quantities, it is only a short 

 time after the rains cease before all the available water is lost from the 

 soil. If this same land is broken by the plow to a depth of 5 or 6 

 inches, more of the rain water or melting snow soaks into it and pene- 

 trates far below the limit of virgin conditions. If, in addition to the 

 first plowing, the surface soil is stirred frequently, a surface mulch 

 is formed which is nearly as effectual in checking direct evaporation 

 as a coat of oil on the surface of water. The moisture thus held 

 in the soil continues downward, wetting soil that in some cases has 

 not been wet for centuries. Since the total annual rainfall in many 

 dry-farming regions is less than 15 inches, of which but little more 

 than half can be secured and held even under favorable conditions, 

 there is ample storage capacity in deep and heavy soils for all the 

 normal rainfall of two years. 



In short, the soil of a semiarid region may be regarded as a reser- 

 voir which under natural conditions is shallow, with a surface diffi- 

 cult to penetrate and favoring the quick loss of water by evaporation. 

 Under proper methods of tillage the reservoir may be greatly deep- 

 ened and provided with a cover that will effectually prevent evapora- 

 tion, so that its supply may be held for the use of crop plants. It 

 is the aim of interseason tillage to accomplish this result. With 

 crops that do not permit intertillage during the growing season the 



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