ALKALI SOILS 339 



reason of its greater water-holding capacity and absorptive 

 power, will contain more alkali without injury to plants than 

 will a sandy soil. Certain of the alkali salts exert a deflocculat- 

 ing action on clay soils and effect an indirect injury in that 

 way. 



In irrigated regions the injurious effects of alkali are in 

 many cases developed only 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 por- 

 tion of the year in which the soil is allowed to dry large quan- 

 tities of salts are carried toward the surface by the upward- 

 moving capillary 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 almost entirely through the capillary 

 channels. The smaller spaces, therefore, contain a consider- 

 able 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 capil- 

 lary 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 direc- 

 tion of the principal movement through these spaces that de- 

 termines 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 suf- 



