50 SOIL BACTERIOLOGY 



TABLE 19. Nitrification in Solution (Quantitative). 



Exercise 14 

 Isolation of Nitrifying Organisms 



1. Prepare eight tubes of acid sodium potassium silicate 

 (m. 30). 



2. Dilute the second enrichment cultures, nitrite, and 

 nitrate organisms until i c.c. represents i : 1000, i : 10,000, 

 i : 100,000, and i : 1,000,000 of the original culture. 



3. Pour "the acid silicate into a sterile Petri dish with 

 the culture dilutions; add the nutrient salts and enough 

 sodium carbonate to harden the silicate. 



4. When hard, invert the plates and incubate under a. 

 moist bell jar for three to six weeks. 



5. Examine at weekly intervals, using the low-power 

 objective. As soon as small colonies appear, make trans- 

 fers to sterile nitrite or nitrate solution. 



6. The nitrifying organisms may be grown on washed 

 agar (m. 31). 



Exercise 15 



Nitrification of Various Substances 



i. Prepare six portions of field soil, 100 grams each, in 

 tumblers or flasks. Mix and sieve the soil well before using. 



