SOIL DEFICIENCIES 26l 



rises and evaporates, it leaves there all the alkali which it held in 

 solution. Thus the alkali originally distributed through the soil 

 may be concentrated near the surface, thereby causing injury to 

 plants. Checking evaporation by cultivation, mulching or shad- 

 ing the land by crops, will check the rise of alkali. It sometimes 

 happens that the alkali is concentrated in the eleventh or twelfth 

 foot of the soil. Under moderate irrigation, the water will not 

 penetrate to this depth, but excessive irrigation will carry water 

 to such depths as to dissolve the alkali in the depths of the soil, 

 and evaporation may then bring it near the surface, so as to cause 

 injury. Soils have the power of elevating water to some dis- 

 tance, through the small spaces between the particles. 



Irrigation waters, so necessary in arid regions, always contain 

 dissolved salts. These are left behind when the water evaporates. 

 If the water is of poor quality, only a few applications may be 

 sufficient to charge the soil with alkali. Even a good irrigation 

 water may give rise to alkali if all the salts it contains are allowed 

 to accumulate in the soil. 



Excessive irrigation, without under-drainage, gives rise to 

 alkali. 1 This is evident first in the low-lying lands. The excess 

 of water flows off into them, and raises the level at which the 

 water saturates the soil (known as the water table), until in some 

 cases, the water comes to the surface. The alkali is washed from 

 the higher ground, and the water evaporates in these low places, 

 leaving the alkali near the surface. The land is thus converted 

 into alkali flats. ' Even where the water does not come to the sur- 

 face, whenever it comes within such a distance of the surface that 

 the capillary action of the soil grains has the power to bring it 

 to the evaporating point, alkali will accumulate. The constant 

 rise of the water containing salts and the evaporation of the 

 water, leaving the salts behind, will accumulate alkali even if the 

 water in the soil does not contain much alkali. Usually, how- 

 ever, such water contains some alkali. 



Alkali will accumulate at any point where the water constantly 

 evaporates, as on the sides of irrigation canals. 

 1 Dorsey, Bulletin 34, Bureau of Soils. 



