A. G. Norman 179 



perhaps been given to practices that may supplement the supply of 

 soil nitrogen than to other less obvious steps that, through the agency 

 of the soil population, may affect the seasonal distribution of the nitro- 

 gen released. For example, effects on the succeeding crop caused by the 

 incorporation of crop residues may differ according to the time of 

 plowing. 



THE PHOSPHORUS AND SULFUR CYCLES 



Organic phosphorus transformations in soil, previously largely 

 ignored, are now recognized as being of real significance in relation 

 to the supply of phosphorus to the crop. In some respects the soil 

 phosphorus cycle is far more complicated than the nitrogen cycle (12). 

 There is in most soils a substantial reserve of inorganic phosphorus 

 which may be present in one or more of a number of different forms 

 that vary considerably in availability or potential availability. There 

 are, moreover, inorganic reactions that result in the fixation or con- 

 version of soluble phosphates to unavailable forms. The inorganic 

 chemistry of phosphorus and phosphorus compounds in soil is, there- 

 fore, of great complexity. Superimposed on this there are the synthetic 

 activities of plants and microorganisms that result in the immobiliza- 

 tion of phosphorus in both inorganic and organic combinations. The 

 soluble phosphate utilized by the crop may, therefore, originate in less 

 soluble inorganic sources, or be derived from organic plant or microbial 

 sources if the energy status of the soil is such that mineralization can 

 occur. Because much of the organic phosphorus in microbial tissues is 

 in the form of nucleic acids and phosphoproteins, the immobilization 

 of phosphorus in this form is governed by similar considerations as 

 apply in the case of nitrogen, and there is in fact a quantitative rela- 

 tionship between the nitrogen and phosphorus requirements of micro- 

 organisms for protein synthesis. In some plant tissues and notably in 

 cereal crop residues organic phosphorus occurs also in phytin, a com- 

 pound not believed to be synthesized by microorganisms. Like organic 

 nitrogen, organic phosphorus in soil is less available than might be ex- 

 pected, having in mind the chemical nature of the groupings con- 

 cerned. Phytin appears to be precipitated in acid or alkaline soils as 

 insoluble phytates of low availability (/), though phytin itself when 



