394 SOILS - 



derived from these rocks, up to nearly two per cent. 1 But almost tht 

 entirety if this substance is present in the form of an insoluble, basic 

 iron compound, difficultly soluble even in acids, and rendering it wholly 

 unavailable to vegetation. So that actually the most pressing need of 

 most of these soils is phosphate fertilization. The same is probably 

 true of some of the highly ferruginous soils of California and of the 

 Cotton States. 



Sulfuric Acid. From the absence of the leaching process 

 in the soils of the arid region, we should expect that sulfates 

 would be more abundant in them than in the soils of the humid. 

 This is certainly true in the case of the alkali soils, which are 

 characteristic of the regions of deficient rainfall. See below, 

 chapter 22. 



Hence the showing made in the general table, indicating that sulfates 

 are equally abundant in the soils of the humid than in those of the arid 

 regions, is surprising in view of the efflorescences of alkali sulfates so 

 frequently observed in the latter. This is obviously due to the fact 

 that the majority of such alkali soils has, on account of their local nature 

 and usually heavy lime content, been excluded from the comparison ; 

 which otherwise would have made a very different showing. 



Potash and S\a!n. The compounds of the alkali metals 

 potassium and sodium, being on the whole much more soluble 

 in water, even without the concurrence of carbonic acid, than 

 those of calcium and magnesium, the leaching process that 

 creates such pronounced difkTences in the case of the two 

 earths must affect the alkali compounds very materially. 

 Comparison of the soils of the two regions in this respect 

 shows, indeed, very great differences in the average contents 

 of potash and soda. For potash the ratio is .216 to .670 per 

 cent, on the general average, and .187 to .670 per cent., in the 

 average by states; for soda, .140 per cent, to .350 per cent. 

 on the general average, and . 1 10 per cent, to .420 per cent, in 

 the average by states. For Ixith, therefore, the general aver- 

 age ratio is as one to between three and four for the humid as 

 against the arid region. 



It is curious that an approximation to the ratio of one to 



1 See table, chapter 19, p. 256. 



