SOIL CHANGES DUE TO IRRIGATION 71 



too loose, it will be brought to the condition of natural 

 packing by excessive irrigations. A soil properly irrigated, 

 that is, one which contains, after each irrigation, the 

 optimum percentage of water (approximately with the 

 field water capacity saturated) will, in time, under this 

 law of natural packing by successive irrigations, acquire 

 a structure best fitted, considering the nature of the soil, 

 for the support of plant-life. The top soil, only, which is 

 over-saturated at each irrigation, and thoroughly dried 

 out at each cultivation, needs mechanical means to be 

 kept in the best structural condition. 



When the soil is in the condition of natural packing, 

 the soil-water film is continuous, and water can move 

 through it rather freely from soil crumb to soil crumb. 

 From the surface of such a soil, if allowed to remain 

 uncultivated, the water stored in the lower depths may 

 readily escape by evaporation from the top. Under irri- 

 gated conditions, where water economy is paramount, 

 the top soil must be kept much looser than in the con- 

 dition of natural packing. For that reason, as was 

 emphasized in the preceding chapter, it is necessary to 

 follow every irrigation with a thorough cultivation, so 

 that the top soil may always be a dry, loose mulch to 

 prevent evaporation. 



49. Soil temperature. The temperature of the soil 

 is often of very high importance, especially in the spring 

 at the time of germination and early growth. It is of 

 importance, also, at all ages of plant-growth. Patten 

 has made elaborate investigations to determine the 

 quantity of water that will permit the most ready trans- 

 mission of heat in the soil. He found that a medium 

 quantity of water, not far removed from that which 

 corresponds to the j)oint of easy penetration and largest 



