2O2 SOILS AND FERTILIZERS 



phate rock, an excess of the rock is used so there will be 

 no free acid in the fertilizer to be injurious to vegetation. 

 As stated above, the usual form in which calcium phos- 

 phate is found in nature is tricalcium phosphate, 

 Ca 3 (PO 4 ) 2 , and unless associated with organic matter 

 or salts which render it soluble, it is of but little value as 

 plant food. When tricalcium phosphate is treated with 

 sulphuric acid, monocalcium phosphate, CaH 4 (PO 4 ) 2 , is 

 formed, which is soluble in water and directly available 

 as plant food. When tricalcium and monocalcium phos- 

 phate are brought together in a moist condition, dical- 



cium phosphate is produced. 



* 



Ca 3 (P0 4 ) 2 + CaH 4 (P0 4 ) 2 = 2 Ca 2 H 2 (PO 4 ) 2 . 



Another form of phosphate of lime, met with in basic 

 phosphate slag, is tetracalcium phosphate, (CaO) 4 P 2 O 5 . 



229. Reverted Phosphoric Acid. When mono- and 

 tricalcium phosphate react, the product is known as re- 

 verted phosphoric acid, which is insoluble in water, but 

 is not in such form as to be unavailable as plant food ; it 

 is generally considered available. Reverted phosphoric 

 acid may also be formed by the action, upon mono- 

 calcium phosphate, of iron and aluminum compounds 

 present as impurities in the phosphate rock. As it is 

 soluble in a dilute solution of ammonium citrate, it is 

 sometimes spoken of as citrate-soluble phosphoric acid, 

 and is not all equally valuable as plant food because of 

 the different phosphate compounds that may be dissolved 



