SOIL INOCULATION 829 



the nitrogen-fixing bacteria in the soil are (1) the degree of activity of 

 the specific organisms; (2) amount of available carbohydrates and other 

 carbon compounds in the soil, which could serve as source of energy for 

 nitrogen-fixation; (3) presence of mineral substances, especially calcium, 

 phosphates, potassium, iron, etc. ; (4) proper soil aeration favoring the 

 activity of aerobic forms and not injuring the development of anaerobes 

 (together with the aerobes); (5) presence of sufficient water; (6) proper 

 soil temperature; and (7) favorable soil reaction, etc. These factors 

 have been considered in detail elsewhere. It is sufficient to emphasize 

 here that, in addition to the presence of vigorous nitrogen fixing bacteria, 

 the soil must be in a proper physical and chemical condition, before 

 nitrogen-fixation will take place. If soil conditions are made favor- 

 able, the nitrogen fixing bacteria will develop rapidly, since they are 

 present in sufficient abundance in all soils. In acid soils, no infection 

 with Azotobacter is apt to occur soon after the reaction is cor- 

 rected. 41 It is also important to emphasize that a sufficient amount 

 of energy is required for nitrogen fixation to take place, i.e., for every 

 pound of nitrogen fixed by the non-symbiotic bacteria, about 100 

 pounds of available carbohydrate or other carbon compounds are 

 consumed. 



However, when natural organic substances, such as straw and 

 various plant residues, green manure, etc. are added to the soil, they are 

 first of all attacked by the numerous saprophytic soil microorganisms, 

 especially the fungi, which break them down rapidly with the libera- 

 tion of carbon dioxide as the final product. In assimilating the available 

 carbon compounds, the fungi and other microorganisms require available 

 nitrogen to build up their body proteins, about 2 to 5 parts of nitrogen 

 for every 100 parts of carbon available. This nitrogen is obtained 

 from the available nitrogen in the soil, to the detriment of the growing 

 plants, unless it is also introduced into the soil, as in stable manure. 

 The fungi and other saprophytic organisms will stop growing when the 

 available nitrogen is exhausted. It is then when the nitrogen-fixing 

 bacteria become active. In the presence of some available carbohydrate 

 or its derivatives, such as the various organic acids, and in the absence of 

 available nitrogen, they may fix the nitrogen of the atmosphere. After 

 all the available carbohydrate has been used up or transformed into 

 unavailable forms, the fungus mycelium and bacterial cells, including 

 those of the nitrogen-fixing bacteria, begin to be decomposed by the 



41 Remy and Rosing, 1911 (p. 578); Gainey, 1925 (p. 832). 



