314 THE PRINCIPLES OF SOIL MANAGEMENT 



have an important bearing on its effect upon plants. 

 Young plants and shallow-rooted plants may be entirely 

 destroyed by the concentration of alkali at the surface, 

 when the same quantity evenly distributed through the 

 soil, or carried by moisture to a lower depth, would have 

 caused no difficulty. 



A loam soil, by reason of its greater water-holding 

 capacity, will carry more alkali without injury to plants 

 than will a sandy one. 



Certain of the alkali salts exert a deflocculating 

 action upon clay soils, and effect an indirect injury in 

 that way. 



178. Reclamation of alkali land. The alkali salts, 

 being readily soluble, are carried by the soil-water where 

 there is any lateral movement, as frequently occurs 

 where land slopes to some one point. Low-lying lands 

 adjacent to such slopes are thus likely to contain con- 

 siderable alkali, and the "alkali spots" of semi-arid 

 regions and the large accumulations of alkali in many 

 of the valley lands of arid regions are traceable to this 

 cause. 



179. Irrigation and alkali. In irrigated regions, 

 the injurious effect of alkali is frequently discovered 

 only after irrigation has been practised 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 



