182 Mineral Nutrition of Plants 



and silicon. The available boron has been shown to be correlated with 

 the organic matter content of soils but this is hardly sufficient evidence 

 on which to base a claim that the boron cycle must include steps 

 brought about by microorganisms. 



SUMMARY 



In attempting to summarize the relationships between the micro- 

 organisms of the soil and the availability of the nutrient elements, 

 major and minor, required by the plant, one immediately encounters 

 the difficulty that there is no simple consistent pattern and, therefore, 

 the relationships cannot be neatly defined in a single polysyllable. The 

 soil population is extremely complex. Its activities are not directed 

 towards meeting the needs of the plant. The tempo of its activities is 

 in part controlled by some of the physical factors that determine plant 

 growth but the dominant factor is the energy supply. Although certain 

 changes in the solubility of some plant nutrients may be the resultant 

 of the production of carbon dioxide and other acids, direct effects are 

 of much greater importance. Many plant nutrient elements are required 

 for microbial growth. All oxidation of organic matter is accompanied 

 by the synthesis of microbial tissues with a concurrent demand for 

 essential elements which are utilized and retained, some in organic 

 combination and some only in inorganic forms. The subsequent re- 

 lease of these elements, particularly those such as nitrogen, phosphorus, 

 and sulfur from proteins and nucleic acids, may largely determine the 

 status of the soil with respect to the availability of these elements. 

 Cationic elements having variable valences, though not involved in 

 organic combination and not required in more than trace amounts, 

 may undergo oxidative or reductive transformation by heterotrophic 

 or autotrophic organisms with the result that the availability of the 

 element to plants may be greatly changed. The role of microorganisms 

 in relation to micronutrient supply in soils has not yet been given 

 sufficient consideration. 



REFERENCES 



Since this paper is of a general review character, no attempt has been made to 

 support it by citing a comprehensive list of references. Those that follow may 



