456 NATUEE AND PEOPERTIES OF SOILS 



Moisture, organic matter, etc 5.06 



CagCPOJ^ 77.76 



FePO^ and AlPO. . . / 1.50 



CaCOs " 4.43 



MgCO, 50 



CaF^ + CaCl. 6.11 



FeS : 77 



Fe^Os and Al^Og 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 phospliate ^ (CaH4(P04)2+)- — ^Aeid 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 PgOg and small amounts 

 of insoluble P2O5. It is made by treating raw rock with sul- 

 furic acid under the proper conditions.- 



Ca3(P04)2 + 2H,S0, - CaH,(P0j2 + 2Ca SO4 

 ( insoluble ) ( water soluble ) 



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

 pleting this reaction. Some di-calcium phosphate [CaaHs 

 (P04)2], spoken of as citrate soluble or reverted phosphoric 

 acid, is thus produced. 



Ca3(P04), + H2SO, = CaA(P0Jo + CaSO^ 

 (insoluble) (reverted) 



Acid phosphate consists mostly of gypsum and mono-cal- 

 cium phosphate with some di-calcium phosphate and impuri- 



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

 fertilizer industry: (1) insoluble (Ca,(POi)2), (2) reverted or citrate 

 soluble (Ca2Ha(P04)2), and (3) water soluble (OaH4(P04)2). The 

 water soluble and citrate soluble phosphates are rated as available to 

 plants. The insoluble form is considered as unavailahle. 



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

 Agr., Bui, 144j 1914. 



