FREE-LIVING SOIL BACTERIA 275 



up. The bacteria derive their energy to do this and to 

 carry on all their other metabolism from the oxidation of 

 carbon compounds such as sugar which occur in the soil. 

 These are derived chiefly from the remains of plants con- 

 sisting largely of cellulose and similar materials. 



Through the activities of these bacteria actual additions 

 of available nitrogen compounds are being made to the 

 soil. It is interesting to note how these nitrogen-fixing 

 organisms supplement the work of the nitrifying bacteria. 

 The latter are engaged in changing already existing 

 compounds of nitrogen to simple nitrates and can- 

 not do their work in the presence of soluble carbohydrates. 

 The former also form nitrates, but they do it by fixing 

 free nitrogen and must have suitable organic food to do so. 

 And so they are most efficient just when the nitrifying 

 bacteria are least efficient. 



A considerable variety of soil organisms can fix nitro- 

 gen: (1) Free-living soil bacteria, (2) Soil bacteria living 

 symbiotically with algse, (3) Bacteria living symbioti- 

 cally with certain leguminous plants in root tubercles, 

 (4) Certain fungi called Mycorrhiza, living symbiotically 

 on the roots of a number of species of trees and shrubs. 



Free-living Soil Bacteria able to fix nitrogen are 

 numerous and of several different kinds (Azotobacter, 

 Granulobacter, Amylobacter, Clostridium). Although 

 they are not equally numerous in all soils, they seem to be 

 almost universal in their distribution. Provided by culti- 

 vation with suitable conditions of growth, they will mul- 

 tiply and enrich the soil in almost any part of the world. 

 The bodies of these bacteria are said to be richer in 

 nitrogen than any other living things so far investigated. 



Bacteria Symbiotic with Alg^. — In the surface 

 layers of the soil there is enough light for some lowly 

 algae. They, of course, fix carbon by photosynthesis and 

 need nitrates. It is said that some sorts of nitrogen- 

 fixing bacteria form a symbiotic arrangement with them 

 in which the algae receive nitrates in return for the sugars 



