COMMERCIAL FERTILIZERS 519 



of experimentation, that the availability is greater in acid 

 soils than in those strongly basic. If acidity of the soil 

 is due to the presence of free acid (positive acidity), it is 

 conceivable that the availability may be due to the sol- 

 vent action of the soil acid on the calcium of the trical- 

 cium phosphate, producing the dicalcium salt which ap- 

 pears to be fairly readily available to plants. When, 

 however, soil acidity is due to a lack of basicity (apparent 

 acidity), the case is different. Gedroiz 1 explains this 

 on the basis of the absorptive properties of the apparently 

 acid soil. He regards rock phosphate, not as a chemical 

 compound, but as a solid solution of dicalcium phosphate 

 with lime. It is this excessive basicity of the phosphate 

 which is responsible for its unavailability. Absorption of 

 the excess calcium would leave the phosphate in a more 

 readily available condition by forming the dicalcium salt, 

 and this is brought about in an apparently acid soil. 



Gedroiz experimented with a highly basic soil that did 

 not respond to fertilization with rock phosphate. He 

 subjected this soil to repeated washings with distilled 

 water charged with carbon dioxide. After such treatment 

 the soil gave a marked increase in crop with rock phos- 

 phate as compared with the same soil untreated. Accord- 

 ing to Gedroiz the greater availability of the phosphate 

 after treatment with carbonic acid was due to the removal 

 of bases and the greater absorptive power of the soil 

 brought about thereby. This was further corroborated 

 by the fact that the treated soil responded to a test for 

 unsaturation while the untreated soil did not. Without 



1 Gedroiz, K. K. Soils to which Rock Phosphates may 

 be Applied with Advantage. Jour. Exp. Agronomy (Russian), 

 Vol. 12, pp. 529-539, 811-816. 1911. The authors are in- 

 debted to Dr. J. Davidson for the translation. 



