SOIL AS WATER RESERVOIR 33 



can be broken only by explosives, and, usually, in the 

 beginning are impervious to water. 



As the practice of irrigation continues, the hardpan 

 formed by the natural precipitation is softened, the 

 materials of which it is made are distributed over larger 

 soil depths, and frequently it wholly disappears, 

 v Under vast areas of the soils of arid regions, and not 

 far from the surface, are found layers of shale or other 

 stone. These were deposited by geological forces upon 

 the soils then existing, and later, through ages, new soils 

 were deposited upon these layers. In other cases, the 

 original rock is only a few feet below the surface. Such 

 hindrances to the free descent of water cannot, of course, 

 be removed by frequent irrigation./ 



An impervious layer a short distance below the sur- 

 face, whether of a temporary or permanent nature, 

 establishes conditions which change the laws of distribu- 

 tion of water in soils as outlined previously in this chap- 

 ter. When the irrigation water, in its descent, encounters 

 the hardpan, the downward movement stops, the soil- 

 moisture film thickens; if more water is added, water 

 accumulates on the hardpan and fills the soil pores, thus 

 producing an undesirable habitat for the plant-roots, 

 and leads to serious crop injury. 



A soil underlaid with hardpan is always in danger of 

 being water-logged, for the tendency is to apply as much 

 water to such soils as to deeper lands. True, as will be 

 shown later, in wet soils plants use more water than in 

 dry ones. Yet, ordinarily, more water is added than 

 plants can use. Moreover, the excess of water in the soil 

 is a positive hindrance to successful plant-growth. Soils 

 underlaid with hardpan should be irrigated more mod- 

 erately and more frequently than deeper soils. It is often 

 c 



