180 Mineral Nutrition of Plants 



added to soil may be readily and rapidly decomposed. Nucleotides or 

 nucleic acid fragments may enter into some form of combination with 

 the clay colloids so that availability is much reduced. 



It has not as yet been found possible to indicate the relationships 

 between carbon and phosphorus that determine whether mineralization 

 or immobilization of phosphorus may occur during the decomposition 

 of crop residues. The phosphorus content of many plant materials is in 

 excess of microbial requirements, but, because inorganic phosphate is 

 normally present in both, no clear picture of the transformations which 

 take place can be obtained. Indeed it is conceivable that the mineraliza- 

 tion of nucleic acid phosphorus is controlled primarily by the nitrogen 

 status of the system, and that this phase of the phosphorus cycle may 

 often be subservient to the nitrogen cycle. 



Although sulfur is quantitatively far less important nutritionally 

 than phosphorus, there are certain similarities between the phosphorus 

 and sulfur cycles in soil. Like phosphorus, sulfur is involved in organic 

 combination in the proteins of plant and microbial tissues. It also 

 occurs in a variety of thio and sulfonic compounds. Again like phos- 

 phorus, sulfur is present in soil also in various inorganic forms, not all 

 immediately available. Sulfur differs from phosphorus in that through 

 the agency of certain autotrophic organisms it may undergo a series 

 of oxidative reactions which terminate in sulfate. In the case of this 

 element, as with nitrogen, the fully oxidized form is available to the 

 plant so that the normal oxidative biological reactions that occur in 

 soil are advantageous in so far as the availability of these elements is 

 concerned. That this is not always the case will be seen later. 



OTHER MAJOR AND MINOR NUTRIENT ELEMENTS 



Much more is known of the role of soil microorganisms in the nitro- 

 gen and phosphorus cycles than in those of some other elements es- 

 sential for plant growth. Because of the relatively high requirements 

 of crops for these two elements, the transformations accomplished by 

 microorganisms may more frequently determine the available supply 

 and, therefore, the total growth than in the case of elements which 

 do not enter into organic combinations or which are required in lesser 

 amounts. 



