70 SOIIvS AND FERTILIZERS 



part in chemical combination with the more complex 

 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 act upon the complex silicates at a high tem- 

 perature, and produce silicates soluble in acids. Plants, 

 however, are unable to obtain food in such complex 

 forms of chemical combination. 



91. Action of Organic 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 manures, and which are 

 markedly deficient in available phosphoric acid, that 

 a i per cent, solution of citric acid dissolved only 0.003 

 per cent, of phosphoric acid while the soil contained a 

 total of o.i 2 per cent. In the case of an adjoining 

 plot which had received phosphate manures until the 

 soil contained a sufficient amount of available phos- 

 phoric acid to produce good crops, there was present 

 0.03 per cent, of phosphoric acid soluble in a i per 

 cent, citric acid solution. 23 



Dilute organic acids are, to a certain extent, capable 

 of showing deficiency of plant food. A soil which 

 shows 0.03 per cent, of potash or phosphoric acid sol- 

 uble in i per cent, citric acid is, as a rule, well stocked 

 with available phosphoric acid. Prairie soils of high 

 fertility yield from 0.03 to 0.05 per cent, of both pot- 



