8o SOILS AND FERTILIZERS 



50 crops of grain without manures, and which are 

 markedly deficient in available phosphoric acid, that 

 a 1 per cent, solution of citric acid dissolved only 0.003 

 per cent, of phosphoric acid while the soil contained a 

 total of 0.12 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 1 per 

 cent, citric acid solution. 22 



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 1 per cent, citric acid is, as a rule, well stocked 

 w 7 ith available phosphoric acid. Prairie soils of high 

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

 ash and phosphoric acid soluble in dilute organic 

 acids ; soils which are deficient in these elements usu- 

 ally contain less than 0.01 per cent. 



The action of a single organic acid of specific 

 strength cannot be taken as the measure of fer- 

 tility for all soils and crops alike, because different 

 plants do not have the same amount or kind of organic 

 acid in the sap. Of the various organic acids, citric 

 possesses the greatest solvent action upon lime, 

 magnesia, and phosphoric acid, while oxalic acid has 

 the strongest solvent action upon the silicates. Tar- 

 taric acid appears to be less active as a solvent than 



