COMMERCIAL FERTILIZERS AND THEIR USE 



239 



nitrogen. The relative values of the different forms 

 of nitrogen are discussed in Chapter IV. Three fer- 

 tilizers may have the same amount of total nitrogen 

 and still have entirely different crop-producing powers. 



In purchasing fertilizers it is important to know not 

 only the amount of nitrogen, but also the form in 

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

 inert form as in leather, while in No. 2 it is largely in 

 the form of dried blood, and No. I has mainly am- 

 monium compounds. Each of these fertilizers, as ex- 

 plained in the chapter on nitrogenous manures, has a 

 different plant food value. 



287. Phosphoric Acid. There are three forms of 

 phosphoric acid in commercial fertilizers: (i) 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 



