120 SOILS AND FERTILIZERS 



Table 28. — Resistance of Crops to Alkali - 



151. Irrigation and alkali. — Frequently the injurious 

 presence of alkali in an irrigated region has been discovered 

 only after irrigation has been practiced for a number of years. 

 This is due to what is termed " 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 alkali was distributed through a great depth of soil, but 

 after water was turned on, this was dissolved, and later 

 brought to the surface, as the soil was allowed to dry out. 

 The upward movement in such cases exceeds the downward 

 because the descending water passes largely through the 

 non-capillary pore spaces, while the ascending water passes 

 entirely through the capillary spaces. The alkali accumu- 

 lates principally in the capillary spaces and hence is swept 

 to the surface in large quantities by the upward movement 

 of capillary water. 



152. Removal of alkali. — There are several ways in 

 which alkali may be removed from soil, among which are 

 the following: (1) leaching with underdrainage ; (2) correc- 

 tion with gypsum ; (3) scraping ; (4) flushing. 



The first of these consists in laying tile drains, much as is 

 done for draining land in humid regions, then flooding the 

 land with large quantities of water, which dissolves the alkali 



