CHEMICAL NATUBE OF SOILS 359 



in which we find the soil minerals of the arid regions is intel- 

 ligible on that ground alone. But it must not be forgotten 

 that lime carbonate, though less effective than the corresponding 

 alkali solutions, nevertheless is known to produce, by long- 

 continued action, chemical effects similar to those that are more 

 quickly and energetically brought about by the action of 

 caustic lime. In the analysis of silicates we employ caustic 

 lime for the setting free of the alkalies and the formation of 

 easily decomposable silicates by igniting the mixture; but the 

 carbonate will slowly produce a similar change, both in the 

 laboratory and in the soils, in which it is constantly present. 

 This is strikingly seen when we contrast the analyses of calca- 

 reous clay soils of the humid region with the corresponding 

 non-calcareous ones of the same. In the former the propor- 

 tions of dissolved silica and alumina are almost invariably much 

 greater than in the latter so far as such comparisons are prac- 

 ticable without assured absolute identity of materials." 



It is evident from the above that, provided the amount of de- 

 composition be the same, the soil of an arid region may contain 

 a larger proportion of desirable constituents than one in a region 

 of considerable annual precipitation. It may, also, and for the 

 same reasons, contain a larger proportion of constituents that 

 are positively deleterious. This is particularly true of arid and 

 semi-arid regions of poor drainage, like the Great Basin regions 

 of the United States, where salts of sodium accumulate to such an 

 extent as to render the land sterile and barren in the extreme. 



The primary origin of the sodium in these salts lies in the 

 soda-bearing silicate minerals forming the rocks of the region 

 and from which they have been set free through their decom- 

 position. 



It should be stated, however, that the so-called "alkali" is 

 not composed wholly of sodium compounds, but contains also 

 salts of magnesia, lime, iron and potash. Nor is the form under 

 which the salts exist at all constant. As a rule, the larger por- 

 tion of the alkali is in the form of sulphate of soda, though a 

 considerable portion may exist as carbonate or chloride, and 

 smaller proportions in the form of nitrates. Concerning the 

 formation of these carbonates, Hilgard says:^ 



*^ There seems to be a consensus of opinion that the carbona- 

 tion of the soda is connected in some way with the presence 



^ Bull. No. 3, Weather Bureau, U. S. Dept. of Agriculture, 1892. 



