456 NATURE AND PROPERTIES OF SOILS 



Moisture, organic matter, etc 5.06 



Ca 3 (P0 4 ) 2 77.76 



FeP0 4 and A1P0 4 1.50 



CaC0 3 4.43 



MgC0 3 50 



CaF 2 + CaCl, 6.11 



FeS 77 



Fe 2 3 and A1 2 3 3.87 



Rock phosphate undergoes the same change in the soil as 

 bone-meal but generally much more slowly, unless the soil is 

 very high in organic matter. Mixing the rock with manure 

 seems to hasten its availability to plants. 



256. Acid phospthate x (CaH 4 (P0 4 ) 2 +).— Acid phosphate 

 is a dry material of a browning gray color, partially soluble in 

 water, and has a characteristic acrid odor. It is intensely 

 acid to litmus, as it contains certain acid salts. It carries 

 from 14 to 16 per cent, of available P 2 5 and small amounts 

 of insoluble P 2 5 . It is made by treating raw rock with sul- 

 furic acid under the proper conditions. 2 



Ca 3 (P0 4 ) 2 + 2H 2 S0 4 = CaH 4 (POJ 2 + 2Ca S0 4 

 ( insoluble ) ( water soluble ) 



The acid is never added in amounts capable of quite com- 

 pleting this reaction. Some di-calcium phosphate [Ca 2 H 2 

 (P0 4 ) 2 ], spoken of as citrate soluble or reverted phosphoric 

 acid, is thus produced. 



Ca 3 (P0 4 ) 2 + H 2 S0 4 = Ca 2 H 2 (P0 4 ) 2 + CaS0 4 

 (insoluble) (reverted) 



Acid phosphate consists mostly of gypsum and mono-cal- 

 cium phosphate with some di-calcium phosphate and impuri- 



1 Chemically, three forms of phosphoric acid are recognized by the 

 fertilizer industry: (1) insoluble (Ca B (P0 4 ) 2 ), (2) reverted or citrate 

 soluble (Ca a H a (P0 4 ) a ), and (3) water soluble (CaH 4 (P0 4 ) 2 ). The 

 water soluble and citrate soluble phosphates are rated as available to 

 plants. The insoluble form is considered as unavailable. 



2 Waggaman, W. H., The Manufacture of Acid Phosphate; U. S. Dept. 

 Agr., Bui. 144, 1914. 



