152 BACTERIA AND SOIL FERTILITY 



gaps in our knowledge and much work must yet be done before 

 we can state definitely the part which they play in the economy 

 of nature and before we can say which are the very best methods 

 for increasing their usefulness. Nevertheless, it is interesting to 

 consider the results obtained by a few workers. 



Berthelot's early laboratory experiments led him to believe that 

 sands and clays may fix in a year from seventy-five to loo 

 pounds of nitrogen to the acre. In two exceptional instances he 

 noted that nitrogen was fixed by sands at the rate of 525 pounds 

 and 980 pounds an acre, but soils which contained fairly large 

 quantities of nitrogen never made marked rapid gains. 



Thiele, on the other hand, maintained that while there is no 

 doubt that Azotobacter possessed the power of fixing free nitro- 

 gen under laboratory conditions, yet it is not certain that condi- 

 tions would be such in soils for any gain of nitrogen due to the 

 activity of these organisms. We have already seen, however, that 

 the Azotobacter do not require as high a temperature for nitrogen 

 fixation in soil as Thiele thought necessary. It is also certain that 

 in most arable soils the temperature is sufficient during a large part 

 of the year for a fairly rapid nitrogen fixation by bacteria. 



Krainskii thinks that even better results should be obtained in 

 soils than in pure solutions, for there the nitrogen fixers grow in 

 symbiosis with _ auto-trophic organisms which produce organic 

 compounds available to the Azotobacter. In soils the nitrogen 

 fixed is rapidly removed by other plants, because of which the 

 slowing-up process which becomes perceptible so early in labora- 

 tory experiments should not occur. 



In addition to an optimum temperature and moisture content 

 of the soil, the Azotobacter are dependent upon a supply of car- 

 bon for energy and inorganic nutrients for the building of cell 

 protoplasm. Unfortunately, it is too often the case that under 

 natural conditions those soils which are deficient in nitrogen are 

 also lacking in available carbon, and especially in phosphorus, 

 which afe so essential for rapid nitrogen fixation. Then there are 

 the technical difficulties which the chemist encounters in determin- 

 ing the gain or loss of nitrogen which occurs in soils under nat- 



