COMMERCIAL FERTILIZERS 2OI 



Nitrogen is present in commercial fertilizers in 

 three forms : (i) Ammonium salts, (2) nitrates, and 

 (3) organic nitrogen. The organic nitrogen is divided 

 into two classes : (a) available, and () unavailable. 

 Pepsin and also potassiu mpermanganate are used as 

 solvents for determining the availability of the organic 

 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. 



No. i. No. 2. No. 3. 



Nitrogen as : Percent. Percent. Percent. 



Ammonium compounds-... 1.75 0.25 o.io 



Nitrates. 0.15 0.15 o.io 



Organic nitrogen : 



Soluble in pepsin o.io 1.25 0.55 



Insoluble in pepsin 0.35 1.25 



Total 2.00 2.00 2.00 



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 like 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. 



273. 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 



