84 SOILS AND FERTILIZERS 



the larger portion of the potash in the soil is insoluble 

 in hydrochloric acid. A soil may contain from 2 to 3 

 per cent of total potash, and 90 per cent or more may be 

 in such firm chemical combination with aluminum, sili- 

 con, and other elements, as to resist the solvent action 

 of plant roots. The larger portion of the phosphoric 

 acid of the soil is soluble in hydrochloric acid. In some 

 soils, however, from 20 to 40 per cent is present as the 

 third class of compounds. When a soil is digested with 

 hydrochloric acid, the insoluble residue is usually a fine 

 gray powder. Some clay soils retain their red color 

 even after treatment with acids, showing that the iron 

 is in part in chemical combination with the more com- 

 plex silicates. 



In order to decompose the insoluble residue obtained 

 from the treatment with hydrochloric acid, fluxes, as 

 sodium carbonate and calcium carbonate, are employed 

 which, at a high temperature, act upon the complex sili- 

 cates and produce silicates soluble in acids. Plants, 

 however, are unable to obtain food in such complex 

 forms of chemical combination. 



100. Action of Organic and Dilute Mineral Acids upon 

 Soils. Dilute organic acids, as a I per cent solution 

 of citric acid, have been proposed as solvents for the 

 determination of easily available plant food. It has 

 been shown in the case of the Rothamsted soils which 

 have produced 50 crops of grain without manure, and 

 which are markedly deficient in available phosphoric 



