COMMERCIAL FERTILIZERS 211 



which it is present. In No. 3 the nitrogen is in inert 

 forms like leather, while in No. 2 it is largely in the 

 form of dried blood, and No. 1 has mainly ammonium 

 compounds. Each of these fertilizers, as explained in 

 the chapter on nitrogenous manures, has a different 

 plant food value. 



270. Phosphoric Acid. — There are three forms of 

 phosphoric acid in commercial fertilizers: (1) Water- 

 soluble, (2) citrate-soluble, and (3) insoluble. The 

 water- and citrate-soluble are called the available phos- 

 phoric acid. In most fertilizers the phosphoric acid is 

 derived from dissolved phosphate rock, and is in the 

 form of monocalcium phosphate. The citrate-soluble 

 is mainly dicalcium phosphate with variable amounts 

 of iron and aluminum phosphates in easily soluble 

 forms. The insoluble phosphoric acid is tricalcium 

 and other phosphates which are soluble only in strong 

 mineral acids. The insoluble phosphoric acid in fer- 

 tilizers is considered as having but little value. As 

 in the case of nitrogen three fertilizers may have the 

 same total amount of phosphoric acid and yet have 

 entirely different values. 



Water-soluble phosphoric acid 



Citrate-soluble 



Insoluble " " 



Total 10.00 10.00 10.00 



No. 3 is of but little value; the fertilizer contains in- 



