226 Journal of Agricultural Research voi. xii. no. 4 



Some interesting results were obtained where the nodules were not 

 removed. In the control pots an increase in nodule formation took place. 

 It can not be stated definitely whether the new nodules appeared as 

 a result of inoculation from the soil or whether the organisms had already 

 gained entrance to the roots before the plants were removed from the 

 field soil previous to transplanting. Nevertheless, it is shown that the 

 number of nodules increased as compared with the number present at 

 the time of transplanting. But where nitrates were added a reduction 

 in number occurred rather regularly throughout all the pots. In two 

 instances the number remained constant, in 10 it was reduced, and in none 

 was it increased. The calcium salt appeared to effect the least reduction 

 in number of nodules. Conclusions concerning the comparative in- 

 fluence of the three salts in this regard can not be drawn because of the 

 small number of determinations made. It is sufficient to note that 

 nitrates present in amounts equal to 25 and 50 mgm. of nitrate in 100 

 gm. of soil did not permit an increase in number of nodules, but rather 

 caused a decrease. 



The conclusions drawn from the experiments relative to the influence 

 of nitrates on nodule formation are: (a) the presence of nitrates is 

 detrimental to the formation of nodules by alfalfa; (b) the nonformation 

 of nodules is not due to a weakening of B. radicicola yvhen grov/n in the 

 presence of nitrates; (c) some reaction takes place between the nitrates 

 and the plant root, thus preventing nodule formation; (d) nitrates in the 

 soil prevent the re-formation of nodules once removed and also cause 

 a decrease in the number of those already present. 



SUMMARY 



(i) Small quantities of potassium, sodium, and calcium nitrates 

 caused a great increase in the number of Azotobacter in sterilized soil. 

 Ammonium nitrate in the same quantities caused a less marked in- 

 crease. Higher concentrations were not so favorable to the growth 

 of the organisms. 



(2) Potassium and sodium nitrates in the concentrationj studied 

 caused an increase in the amount of nitrogen assimilated by Azoto- 

 bacter on agar films. Calcium nitrate in the same amounts brought 

 about a decrease in the amount of nitrogen fixed to a point even below 



' that representing the amount assimilated in the absence of nitrates. In 

 soil cultures nitrates of sodium and calcium caused an increase in total ni- 

 trogen, which was more marked in the unsterilized cultures than in those 

 cultures sterilized and inoculated with a pure culture of Azotobacter. 

 However, the increase in total nitrogen is not commensurate with the 

 increase in the number of Azotobacter noted under the same conditions. 



(3) Under aerobic conditions Azotobacter in liquid cultures reduced 

 nitrate to nitrite, but not to ammonia. More atmospheric nitrogen was 

 assimilated in the presence of nitrate than in the absence of this salt. 



