176 SOILS AND FERTILIZERS 



phate rock an excess of the rock is used so that there 

 will be no free acid to be injurious to vegetation. 



213. Different Forms of Calcium Phosphate. — 



The usual form in which calcium phosphate is found 

 in nature is tricalcium phosphate, Ca (PO ) 2 . 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 (PO ) 2 , is formed. This 

 compound is soluble in water and directly available as 

 plant food. When tricalcium and monocalcium phos- 

 phate are brought together in a moist condition, di- 

 calcium phosphate is produced. 



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



Another form of phosphate of lime, met with in basic 

 phosphate slag, is tetracalcium phosphate, (CaO) P 2 . 



214. Reverted Phosphoric Acid. — When mono- 

 and tricalcium phosphate react, the product is known 

 as reverted phosphoric acid, which is insoluble in 

 water, but is not in such form as to be unavailable as 

 plant food. It is generally considered that the re- 

 verted phosphoric acid is available as plant food : it is 

 soluble in a dilute solution of ammonium citrate, and 

 is sometimes spoken of as citrate-soluble phosphoric 

 acid. Citrate-soluble phosphoric acid may also be 

 formed by the action upon the monocalcium phos- 

 phate of iron and aluminum compounds present as 



