112 SOILS AND FERTILIZERS 



probably the free nitrogen of the air. All of the 

 ways in which the free nitrogen of the air has been 

 made available to plants of higher orders which require 

 combined nitrogen, are not known. It is supposed, 

 however, that this has been brought about by the 

 workings of lower forms of plant life, and by micro- 

 organisms. Whatever these agencies have been they 

 do not appear to be active in a soil under high cultiva- 

 tion, because the tendency of ordinary cropping is to 

 reduce the supply of soil nitrogen. 



123. Organic Nitrogen of the Soil. — In ordinary 

 soils the nitrogen is present mainly in organic 

 forms combined with the carbon, hydrogen, and 

 oxygen; and to a less extent with the mineral ele- 

 ments forming nitrates. 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 takes 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. When decomposition occurs, amides, 

 organic salts, and other allied bodies are without 

 doubt produced as intermediate products before nitrifi- 

 cation takes place. The organic nitrogen of the soil 

 may be present in exceedingly inert forms similar to 

 leather. In fact in many peaty soils there are large 

 amounts of inactive organic compounds rich in 



