SOILS OF THE ARID AND HUMID REGIONS. 



395 



two, or somewhat less, is maintained in the average propor- 

 tion of soda to potash in both regions ; but this does not by any 

 means hold good in detail, very high potash-percentages being 

 often accompanied by figures for soda very much below the 

 above ratio. This is the result of an important difference in 

 the chemical behavior of the two alkalies, which has already 

 been alluded to in connection with the discussion of the zeo- 

 lites. (See chapter 3, p. 38). 



The process of " kaolinication," being that by which clays 

 are formed out of feldspathic minerals and rocks such as 

 granite, syenite, trachyte, etc., results in the simultaneous form- 

 ation of solutions of carbonates and silicates of potash and 

 soda. These coming in contact with the corresponding com- 

 pounds of lime and magnesia, also common products of rock 

 decomposition, are partly taken up by the latter, forming 

 complex, insoluble, hydrous silicates (zeolites). In these, 

 however, potash whenever present takes precedence of soda; 

 so that when a solution of a potash compound is brought in 

 contact with a zeolite containing much soda, the latter is par- 

 tially or wholly displaced and, being soluble, tends to be washed 

 away by the rainfall into the country drainage. Hence potash, 

 fortunately for agriculture, is tenaciously held by soils, while 

 soda accumulates only where the rainfall or drainage is in- 

 sufficient to effect proper leaching, and in that case manifests 

 itself in the formation of what is popularly known as " alkali 

 soils;" namely those in which a notable amount of soluble 

 salts exists, and is kept in circulation by the alternation of rain- 

 fall and evaporation, the latter causing the salts to accumulate 

 at the surface and to manifest themselves in the form of saline 

 crusts or efflorescenses. Alkali lands are a characteristic 

 feature of all regions of scanty rainfall, and arc found more or 

 less on all the continents. The substances composing the alkali 

 salts are retained not only in their soluble form, but by their 

 continued presence influence profoundly, in several ways, the 

 processes of soil formation. A more detailed discussion of 

 this important subject is given in chapters 22 and 23. 



Arid Soils arc Rich in Potash. One of the most important 

 practical conclusions flowing from the comparison of the pot- 

 ash contents of the humid and arid soils respectively is that 

 while in the former, potash is usually among the firs' sub- 



