DENITRIFICATION IN SOIL 59 



2. Inoculate two bottles with 10 to 20 grains of garden 

 soil; two with a pure culture of a denitrifier. 



3. Incubate at 37° C. 



4. Place bottles in plates, so that the overflow is collected. 



5. After forty-eight to seventy-two hours remove the 

 stopper and insert a glowing splinter. The nitrous oxid 

 should behave much like pure oxygen. 



Exercise 24 

 Denitrification in Soil 



1. Prepare eight loo-gram samples of field soil in tumblers. 



2. Add to each 100 grams of soil 60 milligrams of nitrogen 

 in the form of potassium nitrate. 



3. Treat the series as follows: 



(a) I and 2, control untreated, 



(6) 3 and 4, add 2 s grams of dextrose. 



(c) 5 and 6, control untreated. 



(d) 7 and 8, add 2.5 grams of dextrose. 



4. Mix these materials thoroughly by means of a spatula. 



5. To soil portions i to 4 add sterile water to bring the 

 moisture content to about one-half saturation. 



6. To soil portions 5 to 8 add sterile water to bring 

 moisture up to total saturation. 



7. Incubate for two weeks at 28^ C. 



8. At the end of this time remove a sample for nitrate 

 determination and dry the remainder for total nitrogen 

 analysis. Use the modified Kjeldahl method to include 

 nitrates (see page 147). 



9. From these results calculate the percentage of the 

 nitrogen denitrified, and note the effect of excessive 

 moisture and excessive organic matter on the loss of ni- 

 trogen. 



