i7o THE CHEMICAL ANALYSIS OF SOILS [chap. 



The proportion of phosphoric acid in soils is not so 

 variable as the proportion of nitrogen ; it ranges from 

 about 006 per cent, to 02 per cent. ; the lower amounts 

 occur generally on the sands and clays, the higher on 

 loams and soils well provided with calcium carbonate. 



The proportion of potash shows extreme variations, 

 a clay soil may yield one per cent, of potash to strong 

 hydrochloric acid, a sand only one-tenth as much. It 

 has already been pointed out that "clay" is chiefly 

 the result of the weathering of felspars and kindred 

 minerals containing potash ; this weathering is never 

 chemically complete, so that all soils containing any 

 considerable admixture of clay are necessarily rich in 

 potash. The amount dissolved out by hydrochloric 

 acid is also somewhat of an accidental figure, as it 

 depends very much on how far the previous treatment 

 of the soil has forwarded the weathering process, for 

 there remains in all soils rich in potash much material 

 that will not yield potash to strong hydrochloric acid 

 even after forty-eight hours' digestion. For example, 

 the soil from one of the plots in the Broadbalk wheat- 

 field at Rothamsted only yielded 0-5 per cent, of potash 

 to hydrochloric acid, but when completely broken up 

 by ammonium fluoride it was found to contain 2-26 per 

 cent, of potash. 



Of all the soil constituents calcium carbonate shows 

 the widest fluctuations ; it may constitute 40 or 50 per 

 cent, of some of the thin soils resting on the chalk, 

 or it may sink on some of the sands and clays to such 

 small proportions as only to be detected by the most 

 refined analysis. 



The importance of the calcium carbonate lies not 

 in the calcium that it supplies for the nutrition of 

 plants, but in that it acts as the chief base, maintain- 

 ing the neutrality of the soil. Many plant diseases, 



