284 



The bacterial flora of a soil, therefore, is dependent upon climatic 

 conditions which affect many of the other properties of a soil. 



Relationship of Azotobacter to Nitrate Accumulations. The fact 

 that certain spots in western cultivated soils were very 

 nitrates was first observed by Hilgard This he attributed 

 rapid nitrification of the organic matter of the soil in the warm arid 

 climate of the West when the moisture limit was removed by 

 irrigation. 



A number of years later Headden noted these "nitre spots" in a 

 number of Colorado soils, but he attributed it to the fixation of 

 atmospheric nitrogen by the non-symbiotic bacteria which find in 

 the western soils ideal conditions for growth and rapid nitrogen 

 fixation. This conception has been further amplified by Headden 

 and also Sackett. In the early work by Headden it is assumed 

 that the Azotobacter not only fix the nitrogen but also produce the 

 nitrates. It is known, however, that these organisms do not produce 

 nitrates. 



Moreover, there are a number of other vital objections to this 

 theory. (1) Lipman has shown that for the fixation of the quantity 

 of nitrogen which Headden maintains to have occurred, it would 

 require from 1000 to 2000 tons of carbohydrates. There is no such 

 visible supply of energy in these soils. True, many of these soils 

 have a rich algse flora, but it has not been proved that this will 

 furnish a sufficient supply of available energy. (2) The average 

 amount of nitrogen fixed in thirty-two samples collected in the 

 nitrate region was 7.4 mgms. and the average nitrogen fixed in 

 thirty-one samples of (iry-farm alkali-free soil in Utah was 12.2 

 mgms. Yet there is no accumulation of nitrates in these latter 

 soils. (3) The quantity of soluble salts occurring is often sufficient 

 to stop the activity of all nitrogen-fixing organisms, if not to kill 

 them. (4) The quantity of nitric nitrogen and of chlorin in any 

 given " nitre spot" varies in the same spot from year to year or from 

 period to period within a year. (5) The country rock adjacent to 

 the nitrate accumulations and which has contributed to the soil 

 formation contains abundance of nitrates to account for the accumu- 

 lations noted. (6) Soils having a similar physical appearance may 

 be produced in the laboratory in the absence of bacteria. Because 

 of this, we must conclude that the accumulation of nitrates in spots 

 in western soils have their origin as do other accumulations of 

 soluble salts found in the soil and not" in the fixation in place by 

 bacterial activity. 



Soil Inoculation. High hope was entertained that the nitrogen 

 problem in agriculture had been solved, when Caron announced that 

 he had prepared a culture of bacteria which would enable non- 

 leguminous plants to utilize free atmospheric nitrogen, provided 

 certain precautions were observed. Many of the results which he 



