A. G. Norman 177 



synthesis of new microbial protein and therefore ammonia begins to 

 appear. As a rough generalization, this may take place when the car- 

 bon-nitrogen ratio of the decomposing residues has narrowed to 20:1. 



In the decomposition of the more mature low-nitrogen crop residues 

 it can be assumed that most, if not all, of the plant nitrogen is con- 

 verted to microbial nitrogen. Only when the nitrogen content of the 

 material is initially in excess of 1.5 per cent is there likely to be apprecia- 

 ble release of ammonia by deamination and then only after several 

 weeks. For immediate ammonia liberation, the initial nitrogen content 

 must be in the neighborhood of 2.5 per cent or the carbon-nitrogen 

 ratio about 18:1 (//). This is usually only realized by young green 

 plants or leguminous residues. 



Although nitrogen is quantitatively the most important plant nu- 

 trient the supply of which in the soil is substantially affected by mi- 

 crobial synthesis, other elements also are temporarily immobilized in 

 an organic or inorganic form in the tissues of soil organisms and only 

 become again available to plants on the decomposition of these tissues. 

 Inorganic phosphates may be incorporated in nucleic acids and phos- 

 pholipids, sulfates in sulfur-containing amino acids and sulfonic esters, 

 minor elements such as boron and manganese may be retained in com- 

 binations not at present known. Eventually it may be possible to express 

 quantitatively the requirements of microorganisms for these elements 

 in some such general terms as now can be done for nitrogen. The 

 essential point to be made, however, is that nutrient elements which 

 are required by soil organisms are temporarily immobilized through 

 synthesis, and that this fact must not be overlooked in considering 

 the availability and supply of these elements to the crop. In the case 

 of some elements and in some soils, the amounts so immobilized may 

 be small relative to the supply available to the crop from alternate 

 sources. In other circumstances microbial immobilization may represent 

 an important step in the transformations affecting availability. 



THE NITROGEN CYCLE 



The nitrogen cycle in soil is almost exclusively a biological cycle. 

 It is not established that there are transformations of importance ac- 

 complished by purely chemical means. There is no mineral reserve of 



