124 TH E SOII < SOLUTION 



water would be changed. Alterations in the composition of the 

 drainage water furnish the readiest as well as the best guides 

 as to the changes and the nature of the changes taking place 

 in the soil during the process of reclamation. As a practical 

 matter it should be borne in mind that the persistence of the 

 several salts of the alkali mixture does not mean necessarily 

 that they are evenly distributed in the soil ; while yet determining 

 the composition of the water entering the drain, they may have 

 disappeared from the upper soil layers which then may hold a 

 solution of quite different character, suited to the support of 

 crops. In the case just cited the soil contained, before drainage 

 operations were commenced, upwards of 2.7 per cent, of readily 

 soluble salts and would not support any growth other than salt- 

 bushes and similar halophilous plants. Four years later the soil 

 contained less than 0.3 per cent, soluble salts and yielded a very 

 satisfactory crop of alfalfa. In such cases, however, the land 

 cannot be considered as finally reclaimed until a material change 

 in the composition of the drainage water shows that there has 

 been a complete removal of some of the solid salts from that 

 portion of the soil feeding the drains. 



The rate at which alkali can be leached from a soil is de- 

 pendent in a large measure upon the absorptive properties of 

 the soil, and to some extent upon the nature of the salts com- 

 posing the alkali. The leaching is more rapid from sandy than 

 from clay soils, and white alkali is leached more readily than 

 is black. In general, however, the same laws hold here as in 

 any leaching of a solute from an absorbent, and it has been shown 

 that even in the case of black alkali, the rate of removal under 

 a constant leaching follows the law dx/dt = K (A x). 1 In 

 practice, the water does not percolate through the soil under a 

 constant "head," but the flow is intermittent, so that the value 

 of the above formula is mainly academic. On the other hand, 

 if the drainage between floodings is thorough, this procedure 

 should be more efficient than any other for causing a rapid re- 

 moval of the alkali salts, if, as is generally the case, a limited 

 quantity of water is available. 



1 The removal of "black alkali" by leaching, by F. K. Cameron and 

 H. E. Patten, Jour. Am. Chem. Soc., 28, 1639 (1906). 



