7 



Transformation of Nitrogen in Soil; 

 Nitrate Formation and Nitrate Reduction 



Nitrogen is added to the soil in organic and inorganic forms. The 

 organic forms are found largely in plant residues, such as stubble, 

 weeds, leaves, and pine needles; green manure crops; stable manures 

 and composts; and special organic fertilizers, such as dried blood 

 and cottonseed meal. The inorganic forms of nitrogen are added 

 largely in the mineral fertilizers; these include mostly ammonium 

 salts, cyanamide, and nitrates. Both organic and inorganic fertiliz- 

 ers are subject in the soil to various changes brought about by 

 microorganisms. 



Decomposition of Proteins and Their Derivatives 



The amount of protein in organic material is usually measured 

 by multiplying the total nitrogen, as determined by the Kjeldahl 

 method, by the factor 6.25. The protein content of plant and ani- 

 mal residues added to the soil varies from 1.5 per cent or even less, 

 as in cereal straw and wood shavings, to 15 or even 20 per cent, as 

 in legume plants, young rye plants, and certain animal manures, such 

 as chicken manure. The protein content of certain defatted meals, 

 such as cottonseed meal, or of certain animal residues, such as dried 

 blood, may run to 30 or even 60 per cent. 



The primary stage of the breakdown of proteins by microorgan- 

 isms consists in the formation of smaller units known as peptides, 

 and these in turn break down into the individual component amino 

 acids. The enzymes produced by the organisms and responsible for 

 these reactions are known as proteases or proteolytic enzymes. The 

 amino acids are then reduced to certain derivatives, depending on 

 the organisms and the conditions of decomposition, and finally to 



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