THE CHEMICAL COMPOSITION OF SOILS 85 



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 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 

 phosphoric acid to produce good crops, there was pres- 

 ent 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 

 has 0.03 per cent of potash or phosphoric acid sol- 

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

 with these elements in available forms. Prairie soils 

 of high fertility yield from 0.03 to 0.05 per cent of 

 both potash and phosphoric acid soluble in dilute or- 

 ganic acids ; soils which are deficient in these elements 

 usually contain less than o.oi per cent. 



The action of a single organic acid of specific 

 strength cannot be taken as the measure of fertility 

 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 pos- 

 sesses the greatest solvent action upon lime, magnesia, 

 and phosphoric acid, while oxalic has the strongest 

 solvent action upon the silicates. Tartaric acid ap- 

 pears to be less active as a solvent than either citric 

 or oxalic acid. The combined use of dilute organic 

 acids, as citric with hydrochloric (sp. gr. 1.115), w i^ 

 generally give an accurate idea of the character of a 



