460 NATURE AND PROPERTIES OF SOILS 



explains this on the basis of the absorptive properties of the 

 so-called acid soil. He regards rock phosphate, not as a chemi- 

 cal compound, but as a solid solution of di-caleium phosphate 

 with lime. According to Gedroiz 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 di- 

 calcium salt. 



The presence of certain salts has been found to influence the 

 availability of difficultly soluble phosphates. The subject has 

 been investigated by a large number of experimenters, and it 

 will be possible to summarize their results only in part and 

 very briefly. It has been found, for example, that calcium 

 carbonate decreases the availability of raw rock phosphate 

 and bone-meal. Sodium nitrate reduces the availability of 

 the tri-ealcium phosphates, while the ammonium salts increase 

 their availability. Iron and aluminum salts decrease avail- 

 ability. The influence of other salts has not been so well 

 worked out. Prianischnikov,^ as the result of his extended 

 experiments on the subject, holds that salts from which plants 

 absorb acid radicals in larger amounts than they do bases 

 decrease availability, or at least do not affect it, while salts 

 from which plants absorb the bases in the greater quantity 

 have a tendency to render the phosphate more available be- 

 cause of the hydrogen ion concentration. 



There has been great differences of opinion among investi- 

 gators as to the effect of the decomposition of organic matter 

 on the availability of the phosphorus of tri-caleium phosphate. 

 The contention that the availability is increased probably 

 originated with Stoklasa,^ whose experiments with bone-meal 



* PriaBisclinikov, D., tder dm Einfluss von KoMensmrm EalJc aiif die 

 WirMng von VerscMedenen Fhosphaten; Landw. Vers. Stat., Band 75, 

 Seite 357-376, 1911. 



*Stoklasa, X, Duehacek, F., and Pitra^ J., Pher den Einfims der Bah- 

 terien auf die Enochenzersetsunff ; Centrlb, f. Bakt., II, Band 6, Seite 

 526-535, 554-558, 1900, 



