EXAMINATION OF SOIL 177 



SECTION 7 



THE REDUCTION OF NITRATES TO NITRITES AND 

 ISOLATION OF THE BACTERIA 



1. Prepare a flask of pepton broth and add 0.5 per 

 cent potassium nitrate. 



2. Inoculate with a small quantity of soil. 



3. Incubate for several days at room temperature. 



4. Test by the starch iodin method as described in 

 Section 4, Exercise i. 



5. Obtain pure cultures by plating on nutrient 

 agar. 



SECTION 8 

 THE REDUCTION OF NITRATES TO FREE NITROGEN 



1. Prepare several large fermentation tubes with 

 the medium described on p. 40. 



2. Inoculate the closed arm of the fermentation tubes 

 with varying quantities of soil. 



3. Incubate at 37 C. 



4. When gas evolution has ceased determine the 

 percentage of gas. 



5. Determine the composition of the gas. 



6. These bacteria may be isolated by plating in 

 agar, or better in a medium prepared by adding agar 

 to the original solution used in the fermentation tubes. 



7. Study the morphology of the bacteria in pure 

 culture. 



