176 Mineral Nutrition of Plants 



ties of the soil organisms are of great significance in determining the 

 availability or supply of certain plant nutrients. The oxidation of any 

 organic compound resolves itself into two downgrade phases, first, the 

 oxidation of the material itself and, second, the oxidation of the mi- 

 crobial cell substance synthesized by the organism. The chemistry of 

 soil organic matter may, therefore, relate more closely to the chemistry 

 of microorganisms than to that of plant residues. If highly available 

 energy material is added to soil, the primary oxidative phase is very 

 rapid with concurrent synthesis of microbial cells; the second phase 

 involving the decomposition of the cell material is slower. Because 

 nitrogen is required for protein synthesis, the nitrogen transformations 

 that accompany synthesis and decomposition can be used within limits 

 as a rough measure of the cell substance present. 



The amount of cell tissue synthesized per unit of carbon oxidized is 

 not constant with all types of organism. In general, fungi assimilate 

 and utilize for structural purposes a higher percentage of the substrate 

 carbon than do bacteria, and this group has in consequence a higher 

 demand for nitrogen for protein synthesis. When relatively mature 

 but reasonably available plant residues are decomposed by a mixed 

 flora containing fungi, actinomycetes, and bacteria, there appears to be 

 a demand for about 1 .2-1.3 P er cent °f nitrogen to supply the protein 

 needs of the population that develops. If the nitrogen content of the 

 plant material is less than this figure, the decomposition rate will be 

 reduced because the shortage of nitrogen restricts the size of the mi- 

 crobial population that can develop. In this case additional nitrogen, if 

 supplied in the inorganic form, will be immobilized and converted to 

 microbial protein to the extent of the deficit. On the other hand, if the 

 plant residues have a nitrogen content in excess of the requirement, 

 nitrogen will soon be liberated in the form of ammonia as the amino 

 acids in the proteins are oxidized. The nitrogen requirements in organic 

 matter transformations have often been described in terms of carbon- 

 nitrogen ratios. Plant residues with a carbon-nitrogen ratio initially 

 in the neighborhood of 35:1 contain adequate nitrogen for decomposi- 

 tion. As the available carbohydrates are utilized, the population that can 

 be supported becomes smaller with the result that some of the nitrogen 

 initially incorporated into microbial tissue is no longer required for 



