170 Mineral Nutrition of Plants 



THE NUTRITIONAL REQUIREMENTS OF SOIL ORGANISMS 



In examining the question as to the effects which soil organisms may 

 have on the availability and supply of plant nutrients, some considera- 

 tion must first be given to the comparative nutritional requirements of 

 microorganisms and plants in order to determine whether they over- 

 lap and, if so, to what extent and in what circumstances they may be 

 competitive. Plants in general and crop plants in particular, despite all 

 their diversities of form and structure, probably have a greater similarity 

 of requirements than do the heterogenous forms of life that constitute 

 the microbial population of the soil. Plants and microorganisms proba- 

 bly require the same major nutrient elements, though no doubt wide 

 quantitative differences occur in the levels of poverty adjustment and 

 luxury consumption. The qualitative and quantitative range of es- 

 sential minor elements is less well established, particularly for micro- 

 organisms. Plants, in the language of the microbiologist, are photoau- 

 totrophic organisms; the soil population on the other hand is predomi- 

 nantly heterotrophic, though there are some important groups of 

 chemoautotrophs, and some photoautotrophs that probably live hetero- 

 trophically. The plant can and normally does satisfy its nutrient re- 

 quirements from inorganic sources. Many heterotrophic organisms also 

 can satisfy their nutrient requirements exclusively from inorganic 

 sources provided that an organic source of carbon and energy is availa- 

 ble, so, in this respect, these organisms may be said to be capable of 

 competing with plants for the available supply of mineral nutrients. 

 Many heterotrophic organisms, however, are able to utilize organic 

 sources of nitrogen, phosphorus, sulfur, and, no doubt, other elements. 

 In general these are not utilized directly but only after hydrolysis or 

 other degradative transformation that may result in the liberation of 

 the nutrient element in an inorganic form. Some soil organisms re- 

 quire an organic nitrogen source, usually some simple amino acid, and, 

 indeed, attempts have been made to use nitrogen nutrition as an aid in 

 characterizing different soil populations (16). There is evidence that 

 certain legumes under sterile conditions can directly utilize some soluble 

 organic sources of nitrogen, such as aspartic acid (75) ; apparent utiliza- 

 tion of organic nitrogen sources by nonlegumes has also been reported, 



