248 IMPORTANCE OF MICROBES IN SOIL FERTILITY 



fixing bacteria. Some, as the non-symbiotic forms, live in a free 

 state in the soil and require sources of energy (in the form of various 

 carbohydrates) to enable them to fix the nitrogen. Others live in 

 the roots of certain plants (largely legumes), or even in the leaves, 

 the bacteria fixing the nitrogen and making it available to the 

 plants, and the latter manufacturing sugars and other carbohy- 

 drates which they supply to the bacteria as sources of energy. 

 This mutually beneficial growth of green plants and bacteria is 

 known as symbiosis, and the bacteria are known as symbiotic 

 nitrogen-fixing bacteria. 



Subsequent to the elimination of nitrogenous compounds or the 

 death of the bacteria, the complex proteins produced by fixation 

 processes are sooner or later minerahzed by other bacteria, fungi, 

 or actinomyces, and the nitrogen made available to free-growing 

 green plants. These nitrogen-fixing bacteria may be consumed by 

 protozoa, but no loss of nitrogen is involved in this process. The 

 protozoa use the organic nitrogen in their nutrition and in turn 

 die and themselves pass through the mineralization process. 

 Certain nitrogen-fixing bacteria may five symbiotically not only 

 with higher plants such as the Leguminosae, but also with certain 

 microscopic green plants as the algae. This associative develop- 

 ment appears to favor both the algae, which synthesize carbo- 

 hydrates from the carbon dioxide of the air, and the bacteria, that 

 build up complex organic proteins and other nitrogen compounds 

 from the gaseous nitrogen of the atmosphere; each of the two 

 groups of organisms is able to use the products synthesized by the 

 other, thus bringing about processes which lead both to an increase 

 of soil organic matter and of combined soil nitrogen. 



Role of Microbial Metabolic Products in Soil Trans- 

 formations. — The metabolic processes of the microorganisms 

 become very involved because of the fact that the medium, the 

 soil, is so complex, containing an almost limitless variety of 

 organic and inorganic substances. The environment may be 

 very different in one locality from that in another, on account of 

 the inherent soil characteristics and the influence of climate and 

 vegetation. The conditions favor the development of a micro- 

 scopic population composed of an abundance of difi"erent forms 

 which further complicate the mechanism of the transformations 

 by their varied food requirements and metabolic products. Many 

 of the transformations of the soil constituents are affected by the 



