168 SOILS AND FERTILIZERS 



Ca s (P0 4 ). + CaH 4 (P0 4 ) 2 = 

 Another form of phosphate of lime, met with in basic 

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



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 reverted 

 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 

 impurities- in the phosphate rock. This process is a 

 fixation change, as described in Chapter VI. In an 

 old fertilizer there may be present citrate-soluble phos- 

 phoric acid in two forms, as dicalcium phosphate and 

 as hydrated phosphates of iron and aluminum. The 

 citrate-soluble phosphoric acid in fertilizers is not all 

 equally valuable as plant food because of the different 

 phosphate compounds that may be dissolved by this 

 solvent. 



215. Available Phosphoric Acid. As applied to 

 fertilizers, the term available phosphoric acid includes 

 the water-soluble and citrate-soluble phosphoric acid. 

 These solvents do not, under all conditions, make a 

 sharp distinction as to the available and unavailable 

 phosphoric acid when it comes to plant growth. 

 Some forms of bones which are insoluble in an am- 



