DRY-LAND AGRICULTURE 



245 



of non-tilled crops. The air enclosed in masses of vegetation, such 

 as wheat, oats, millet, clovers and similar crops, has a comparatively 

 high humidity, so that evaporation from the soil is retarded and 

 probably almost entirely prevented during a large part of the day. 

 Jt is a matter of common observation that the dew remains in 

 heavy oats or wheat many hours after sun-up and is deposited again 

 several hours before sunset. This will effectively prevent much 

 evaporation from the soil. While the humidity of the air in these 

 crops of semi-arid regions would not be as high as in humid ones, 

 yet the difference would be suflicient to lessen the evaporation. 



With tilled crops, shading aids to some extent, but the mulch 

 is the important factor. When the crop has grown to such she that 

 the roots are well distributed through the soil, moisture has very 

 little chance of reaching the surface, because of the network of 

 roots which are absorbing all moisture that comes within reach. 



Tillage. The best mean* for preventing loss of water is by 

 tillage, by which a mulch is maintained. Various experimenters 

 have found that cultivation will save from 22 to 55 per cent of the 

 water that would otherwise evaporate. 



(a) Depth of Tillage. Tillage produces conditions in the soil 

 that permit very slow capillary movement by forcing soil particles 

 apart so that the films of water cannot pass freely from one to 

 another. As a general rule, the deeper the mulch the more effec- 

 tive it is in preventing evaporation. In arid regions the plowing 

 is one of the most fundamental operations, since it plays two very 

 important functions, first, in producing a loose mulch for retarding 

 capillary movement, and, second, in forming a deep stratum for 

 absorbing the rainfall and retaining it afterward. The Utah Sta- 

 tion has conducted a number of experiments upon depth of plowing 

 and the results show that eight to ten inches is the best depth. 

 When increases for greater depths arc obtained they are usually 

 too low to cover the additional expense. 



Yields of Wheat for Different Depths nf Plnirinq. Utah Station. Iin*hcls 



Per A i-rt- 5 



