SALTS IN THE SOIL 191 



becomes much more concentrated ; and when this concen- 

 tration reaches a point exceeding the solvent power of water, 

 the salts previously held in solution will be deposited in 

 a solid form. As the evaporation of water takes place from 

 the surface of the soil, it is on the surface that deposits 

 of salts from solution chiefly occur. If the soil water is 

 brought to the surface by capillary action, an accumulation 

 of salt may form on the surface in dry weather, and be 

 visible to the eye as a delicate white crust or efflorescence. 

 In the case of fertile soils, these white films will be of rare 

 occurrence, unless superphosphate, or other manure supplying 

 sulphate of calcium (a salt of little solubility), has been 

 applied to the land. In the alkali lands of India and 

 California, however, the accumulation of salts at the surface 

 reaches serious dimensions, and may suffice to destroy all 

 plant life. The salts present in alkali lands are principally 

 the sulphate, chloride and carbonate of sodium. 



It is evident from what has been already stated that the 

 soluble salts in a soil are subject to a variety of vicissitudes. 

 They may be concentrated in a solid form, or as a strong 

 solution, at the surface ; this being especially the case after 

 active nitrification, after drought, or especially, and to the 

 greatest extent after the application of a dressing of saline 

 manure. Opposed to this tendency to accumulation at the 

 surface we have the action of rain, which washing the sur- 

 face soil with nearly pure water tends to carry all soluble 

 salts into the subsoil. Equally opposed to accumulation is 

 the action of diffusion, which, in a moist soil, tends to equalize 

 the proportion of salt throughout the whole mass. 



The movements of salts in the soil are thus determined 

 by two agencies partly by the diffusibility of the salt in 



