PHOSPHORIC ACID IN BASIC SOILS 283 



available phosphoric acid as is found in the ordinary acid phos- 

 phate. The reaction takes place according to the following' 

 formulas : 



(1) Ca 3 (P0 4 ) 2 H-4H 3 P0 4 + 3 H 2 0^3[CaH 4 (P0 4 ),H 2 0]. 



(2) Ca 3 (P0 4 ) 2 +2H 3 P0 4 +i2H 2 0=3[Ca 2 H 2 (P0 4 ),4H 2 0]. 

 In each case the water in the final product is probably united as 



crystal water with the calcium salts produced. The monocal- 

 cium salt formed in the first reaction is soluble in water, and the 

 dicalcium salt in the second reaction, in ammonium citrate. 

 Where fertilizers are to be transported to great distances, there 

 is a considerable saving of freight by the use of such a high- 

 grade phosphate, which may, at times, contain over 40 per cent, 

 of available acid. The phosphoric acid used in this process is 

 made directly from the mineral phosphate by treating it with an 

 excess of sulfuric acid. 



244. Fixation of Phosphoric Acid in Basic Soils. The problem 

 of holding phosphoric acid in the soil probably does not come 

 within the scope of this manual, except as incident to the character 

 and time of its application. This subject has been studied by 

 Crawley. 40 



Experimentally, the determination of the holding powers of 

 the soil for the phosphoric acid obtained, depends upon the same 

 methods as are described for the absorption of salts by soils. 41 

 In the irrigated soil with which Crawley worked, it was 

 found that, when the application of fertilizer containing water- 

 soluble phosphoric acid was followed immediately by irrigation, 

 more than one-half of the soluble phosphoric acid remained in 

 the first inch of the soil and more than nine-tenths in the first three 

 inches, and practically the whole of it within the first six inches, 

 of the surface. Crawley concludes from the results of his investi- 

 gations that the water-soluble phosphoric acid does not become 

 so widely distributed, in the case of heavy rains or irrigation 

 beneath the surface, as has been expected. In this connection 

 however, attention should be called to the fact that the Hawaiian 



40 Journal of the American Chemical Society, 1902, 24 : 1114- 

 11 Principles and Practice of Agricultural Analysis, and Edition, 1906, 

 1 : 133- 



