SOIL INOCULATION 289 



(a) In nearly every case the cultivated soil fixed much more 

 nitrogen in the laboratory than did the virgin soil. This was the 

 case when the soil was incubated with or without the addition of 

 carbonaceous material. 



(b) There is a richer nitrogen-fixing bacterial flora in the cultivated 

 than in the virgin soil. 



(c) The conditions of moisture, alkalinity and food constituents 

 in the soil were ideal for rapid nitrogen-fixation, and the temperature 

 of the soil was high enough during a considerable part of the year 

 for the growth of Azotobacter. 



(d) The cultivation of the soil would increase aeration and avail- 

 able phosphorus in the soil. 



(e) The large quantity of plant residues would act as a supply of 

 carbon which is readily rendered available by the soil's rich flora of 

 cellulose ferments. If these soils had produced a wheat crop every 

 alternate year and all of the nitrogen which had been added to the 

 soil without loss from leaching or bacterial activity taken by the 

 crop, it would have necessitated the addition of 25 pounds an acre 

 yearly, which is evidently the very minimum which can be attributed 

 in these soils to non-symbiotic nitrogen-fixation. 



Eighty different samples of these soils were incubated in the 

 laboratory for twenty-one days and the gains in nitrogen determined 

 by comparing with sterile checks. The soils were incubated without 

 the addition of anything except sterile distilled water. At the end 

 of the period the average gain per acre for the cultivated soils was 

 202 pounds and that for the virgin soil was 92. 



True, fixation would not continue long at this rate, for when the 

 nitrogen content of the soil passed beyond a certain limit decay 

 bacteria would increase rapidly, and in the struggle for existence 

 the\ r are able, with the advantage at their disposal, to suppress the 

 more slowly growing Azotobacter, which would gain the ascendency 

 again only when the nitrogen of the soil became low. 



Thus, there is an upper as well as a lower limit to the nitrogen 

 content of the soil as far as bacterial activity is concerned, but by 

 making the conditions for nitrogen-fixation as nearly ideal as possible 

 we may maintain in a soil the upper and not the lower nitrogen 

 content. 



In conclusion, it may be stated that although the part played by 

 Azotobacter in maintaining the nitrogen of the soil has not been 

 definitely measured, it is nevertheless an important factor. Hall 

 found it to be at least 25 pounds, Lohnis 35.7 pounds, and the author 

 25 pounds per acre annually. It is, therefore, conservative to state, 

 as has Lipman, that these organisms, under favorable conditions, 

 add from 15 to 40 pounds of available nitrogen to each acre of soil 

 yearly. 



REFERENCE. 



Greaves, J. E.: Azofication, Soil Science, 1918, vi, 163-217. 

 19 



