RECLAMATION OF ALKALI LAND 315 



of soil. When, however, irrigation water was turned 

 upon the land, the soil became wet for perhaps fifteen 

 or twenty feet in depth. During the portion of the year 

 in which the soil is allowed to dry, large quantities of 

 salts are carried to the upper soil by the upward-moving 

 capillary water. These salts are in part carried down again 

 by the next irrigation, but the upward movement con- 

 stantly exceeds the downward one. This is because the 

 descending water passes largely through the non-capillary 

 interstitial spaces, while the ascending water passes en- 

 tirely through the capillary ones. The smaller spaces, 

 therefore, contain quite a quantity of soluble salt after 

 the downward movement ceases and the upward move- 

 ment commences. In other words, the volume of water 

 carrying downward the salts 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 that determines 

 the point of accumulation of the alkali. 



There are large areas of land in Egypt, 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. 



There are a number of methods that have been used 

 with more or less success to reclaim alkali land. 



180. Underdrainage. Of the various methods for 

 removing an excess of soluble salts the use of tile drains 

 is the most thorough and satisfactory. When this is 

 used in an irrigated region, heavy and repeated appli- 

 cations of water must be made, to leach out the alkali 



