288 AZOFICATION 



available to the Azotobacter. In soils the nitrogen fixed is rapidly 

 removed by other plants, because of which the slowing-up process 

 that becomes perceptible so early in laboratory experiments should 

 not occur. 



In addition to an optimum temperature and moisture content of 

 the soil, the Azotobacter are dependent upon a supply of carbon 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 are so essential for rapid 

 nitrogen-fixation. Then there are the technical difficulties which 

 the chemist encounters in determining the gain or loss of nitrogen 

 which occurs in soils under natural conditions and which may be 

 attributed to non-symbiotic nitrogen-fixation. 



There are, however, several cases in which the gain has been 

 measured with a fair degree of accuracy. 



Lipman, in pot experiments carried on with a soil containing about 

 5000 pounds of nitrogen per acre-foot of soil, found again of more 

 than one-third this amount in two short seasons^ Much of this 

 must be attributed to non-symbiotic nitrogen-fixation. To these 

 soils had been applied solid and liquid manure; which furnished to 

 the organisms readily-available supplies of energy and various 

 necessary inorganic constituents. This fixation was not nearly so 

 rapid where legumes were turned under as green manures. 



Koch found a gain of from 0.019 to 0.093 per cent, in soil nitrogen 

 during two seasons which must be attributed to non-symbiotic 

 nitrogen-fixation. In addition to this there was a threefold gain in 

 the nitrogen content of the crops oats, buckwheat, and sugar-beets 

 which must also be attributed to the action of Azotobacter. 



Hall noted an annual gain of 100 pounds of nitrogen on Broadbalk 

 field at Rothamsted and 25 pounds on Grescroft field. He feels that 

 much of this gain must be due to the action of non-symbiotic 

 bacteria. Lipman points out that the actual gains of nitrogen are 

 even greater, for this does not take into consideration the various 

 losses which are sure to occur even under the best of conditions. 

 Hopkins takes the stand that the apparent gain is due in a large 

 measure to drifting dust and plant residues coupled with the diffi- 

 culty of obtaining representative samples of soil at the two different 

 periods. Even when all of these factors are considered the evidence 

 points to a gain of nitrogen through bacterial activity. 



The analysis of a great number of soils in Utah showed that the 

 average nitrogen content of the soil which had grown wheat and 

 other non-leguminous plants for from twenty to fifty years was 

 0.2009 per cent., whereas adjoining virgin soil on the average showed 

 only 0.1984 per cent, of total nitrogen. *The evidence is very strong 

 that considerable nitrogen, fes been added to these soils by micro- 

 scopic organisms, for: 



