NITROGEN, NITRIFICATION, NITROGENOUS MANURES I2/ 



tivation, because the tendency of ordinary cropping is to 

 reduce the supply of soil nitrogen. 



137. Organic Nitrogen of the Soil. - - In ordinary soils 

 the nitrogen is present mainly in organic forms com- 

 bined with the carbon, hydrogen, and oxygen as humus, 

 and to a less extent with the mineral elements, forming 

 nitrates and nitrites. The organic forms of nitrogen, 

 it is generally considered, are incapable of supplying 

 plants with nitrogen for food purposes until the process 

 known as nitrification has taken place. The nitrogenous 

 organic compounds in cultivated soils are derived mainly 

 from the undigested protein compounds of manure and 

 from the nitrogenous compounds in crop residues, and 

 are present mainly as insoluble proteids. 85 When de- 

 composition occurs, amides, organic salts, and other 

 allied bodies are without doubt produced as interme- 

 diate products before nitrification takes place. The or- 

 ganic nitrogen of the soil may be present in exceedingly 

 inert forms similar to that in leather, as in many peaty 

 soils where there are large amounts of inactive organic 

 compounds rich in nitrogen. In other soils the nitrogen 

 is less complex. The organic nitrogen of the soil may 

 vary in complexity from forms, like the nitrogen of urea, 

 which readily undergo nitrification, to forms like that in 

 peat, which nitrify with difficulty. 



138. Amount of Nitrogen in Soils. The fertility 

 of any soil is dependent, to a great extent, upon the 



