FIXATION OF ATMOSPHERIC NITROGEN 



587 



experiments. 120 Among the most important conditions required for 

 non-symbiotic nitrogen fixation are: (1) a proper supply of energy 

 material, (2) sufficient CaC0 3 to neutralize soil acidity and improve the 

 physical condition of the soil, (3) available phosphorus, (4) proper 

 temperature, and (5) aeration of the soil. According to statements 

 usually made in the texts and found at Rothamsted and other stations, 

 non-symbiotic nitrogen-fixing bacteria add, under favorable conditions, 

 15 to 40 pounds of available nitrogen to each acre of soil yearly and 



TABLE 58 

 Influence of sugar upon crop yield and nitrogen content of crop 



usually not more than 10 pounds. In view of the fact that the energy 

 added to the soil is not directly available to the nitrogen-fixing bacteria, 

 that small amounts of available nitrogen are always present in the soil 

 and that the error in the laboratory determination of total nitrogen by 

 the Kjeldahl method is greater than the possible amount of nitrogen 

 fixed by non-symbiotic bacteria, we are still unable to decide the ques- 



120 Hutchinson, H. B. The influence of plant residues on nitrogen-fixation 

 and on losses of nitrate in the soil. Jour. Agr. Sci., 9: 92-111. 1918. 



