398 SOILS: PROPERTIES AND MANAGEMENT 



after irrigation has been practiced for a few years. This 

 is due to what is known as a " rise of alkali," and comes 

 about through the accumulation, near the surface of the 

 soil, of salts that were formerly distributed throughout 

 a depth of perhaps many feet. Before the land was 

 irrigated, the rainfall penetrated only a slight depth into 

 the soil, and when evaporation took place, salts were 

 drawn to the surface from only a small volume of soil. 

 When, however, irrigation water is turned on the land, 

 the soil becomes wet to a depth of perhaps fifteen or 

 twenty feet. During the portion of the year in which 

 the soil is allowed to dry, large quantities of salts are 

 carried toward the surface by the upward-moving capil- 

 lary water. Although these salts are in part carried 

 down again by the next irrigation, the upward movement 

 constantly exceeds the downward one. This is because 

 the descending water passes largely through the non- 

 capillary interstitial spaces, while the ascending water 

 passes entirely through the capillary spaces. The smaller 

 spaces, therefore, contain a considerable quantity of 

 soluble salts after the downward movement ceases and 

 the upward movement begins. In other words, the 

 volume of water carrying the salts downward in the 

 capillary spaces is less than that carrying them upward 

 through these spaces. Surface tension causes the salts 

 to accumulate largely in the capillary spaces, and it is 

 therefore the direction of the principal movement through 

 these spaces that determines the point of accumulation 

 of the alkali. 



There are large areas of land in Egypt, in India, and 

 even in France and Italy, as well as in this country, that 

 have suffered in this way, and not infrequently they have 

 reverted to a desert state. 



