PHOSPHATIC MANUEES 221 



Action of Superphosphates as Manure. When super- 

 phosphate is applied to the soil, the monocalcic phosphate 

 probably passes into solution, and on contact with calcium 

 carbonate is reprecipitated in the tricalcic form, thus : 

 CaH 4 (P0 4 ) 2 + 2CaC0 3 = Ca 3 (P0 4 ) 2 + 2C0 2 + 2H 2 



Monocalcic Calcium Tricalcic Carbon Water, 



phosphate. carbonate, phosphate. dioxide. 



The product is, by this means, thoroughly disseminated 

 through the soil, in a state of infinitely fine division 

 far finer than can be produced by any mechanical process 

 of grinding and is more easily soluble than the original 

 phosphate. The superiority of superphosphates, as com- 

 pared with the untreated native phosphates, must be 

 attributed to these conditions. It is obvious that if pre- 

 cipitation take place before the substance is applied, the 

 phosphate cannot be so intimately mixed with the soil, 

 and, therefore, even if it were equally soluble, could not 

 be expected to produce such good results. 



Some years ago a patent was taken out for the pre- 

 paration of a new manure called basic superphosphate 

 which was expected to compare favourably with basic 

 slag owing to the more ready solubility of the phosphate. 

 It was prepared by mixing a quantity of lime with 

 ordinary superphosphate, but it seems obvious that it 

 would be more advantageous to apply the lime separately 

 to the soil so that the precipitation might take place within 

 the soil and not before application. 



In the absence of a sufficient amount of lime in the soil 

 the soluble phosphate would probably be converted into 

 phosphates of iron and alumina (p. 133). These com- 

 pounds, it has been said, are less readily soluble in dilute 

 acids than phosphate of lime when freshly precipitated, 

 and they probably undergo gradual dehydration and 

 become still less soluble. The best effects are therefore 

 obtained from superphosphates when applied to soils 



