2IO 



Journal of Agricultural Research 



Vol. XII, No. 4 



lOO gm. of soil the legume organisms were still alive, although present 

 in numbers far below those of the control cultures. In all Azotobacter 

 cultures no organisms survived this concentration. 



No one nitrate produced an excessive stimulation in comparison with 

 the others. The calcium salt present as 150 mgm. of nitrate in 100 gm. 

 of soil at the end of the first week gave the greatest stimulation for con- 

 centrations of that amount. However, at the end of the second week 

 this concentration had caused a marked decrease in the number of or- 

 ganisms. In the case of all three nitrates the concentration representing 

 25 mgm. of nitrate in 100 gm. of soil produced the greatest stimulation. 

 This resulting stimulation also held true throughout the second vv^eek. 

 The decrease in number below those of the control cultures, due to in- 

 creasing concentrations of nitrate, began first in the presence of potassium 

 nitrate at 100 mgm. of nitrate per 100 gm. of soil, then with sodium 

 nitrate at 150 mgm., and lastly with calcium nitrate at 200 mgm. But 

 the number of organisms present in the soil cultures containing sodium 

 nitrate in amounts equivalent to 100 mgm. and calcium nitrate at 100 

 mgm. at the end of the second week was below those of the control 

 cultures. 



It therefore appears from these results that small amounts of potassium, 

 sodium, and calcium nitrate stimulate the reproductive activity of B. 

 radicicola. Concentrations of nitrates greater than those amounts which 

 produced maximum stimulation cause a decrease in the number of or- 

 ganisms. The highest concentration of nitrate studied did not entirely 

 prevent the growth of the bacteria, but it reduced the number of organ- 

 isms far below those contained in control cultures where no nitrates 

 were added. 



Ammonium nitrate was also employed. The soil cultures were pre- 

 pared as already described and inoculated with B. radicicola. The cul- 

 tures were incubated at 28° to 30° C. and counts were made at the end 

 of one and two weeks' time. The results of the study with ammonium 

 nitrate are given in Table XXIII. 



Table XXIII. — Influence of ammonium nitrate on Bacillus radicicola in sterilized soil 



