154 BACTERIA AND SOIL FERTILITY 



3. The conditions of moisture, alkalinity, and food constitu- 

 ents 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, 



4. The cultivation of the soil would increase aeration and 

 available phosphorus in the soil. 



5. 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 

 twenty-five pounds an acre yearly, which is evidently the very 

 minimum which can be attributed in these soils to non-symbiotic 

 nitrogen fixation. 



Eighty diiferent samples of these soils were incubated in the 

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

 mined by comparing with sterile checks. The soils were incu- 

 bated 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 ninety-two. 



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

 the nitrogen content of the soil passed a certain limit decay bac- 

 teria would increase rapidly, and in the struggle for existence 

 they are able with the advantages 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 nitro- 

 gen 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 altho the part played by 

 A.otoiacUr in maintaining the nitrogen of the so.l has not been 

 definitely measured, it is nevertheless an important factor. Hall 



