38 liABORATOEY BACTERIOLOGY. 



EXEKCISB XXVIII. 



DENITRIFIOATIONS IN SOILS. 



1. Weigh off six 100 gram portions of air-dry, sieved soil, 

 in tumblers. 



2. Add 0.5 grams of NaNOj to each. 



3. Arrange thus : 



1 & 2 — Infusion Soil 1. 

 3 & 4— Infusion Soil 2. 

 5 & 6— Infusion Soil 3. 



4. Add 10 cc. {= 5 grams soil) of 5-minute infusions 

 of fresh samples of the three soils to be tested. 



5. Add sterile water to bring the moisture conditions up 

 to the optimum. 



6. Cover with Petri dish covers and incubate for 10-14 

 days at 20° C. 



7. At the end of that time add water and a little lime; 

 shake, filter, wash, make up to a convenient volume and de- 

 termine the nitrates in aliquots of the leachings. 



8. Dry the residue, grind, and analyze for total nitrogen 

 by the regular Kjeldahl method. 



9. Grind a sample of the air-dry soil used as the medium 

 and determine the nitrogen present. 



10. The difference between the total amount of nitrogen 

 present at the beginning of the experiment (the nitrogen in 

 the soil + the nitrate added) and at the end represents the 

 amount which has been lost by denitrification, and the rela- 

 tive denitrifying powers of the soils may thus be determined. 



