\ 

 THE STUDY OF MICROORGANISMS IN THE SOIL^ 125 



Exercise 39 



Isolation of Nitrifying Organisms 



Prepare eight tubes of acid sodium-potassium silicate. 



Dilute the second enrichment cultures of the nitrite and 

 nitrate organisms until 1 cubic centimeter represents 1:1,000, 

 1:10,000, 1:100,000, and 1:1,000,000 of the original culture. 



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. 



Fig. 14. — Nitrobacter in liquid culture. 



When hard, invert the plates and incubate under a moist 

 bell jar for 3 to 6 weeks. The cultures may also be streaked on 

 the surface of silica gel. 



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

 As soon as small colonies appear, make transfers to sterile nitrite 

 or nitrate solution. 



The nitrifying organisms may be also grown on washed agar. 



Exercise 40 



Nitrification of Various Substances 



Prepare two portions of soil, 500 to 700 grams each, in small 

 jars. Mix and sieve the soil well before using, 

 (a) Control — no treatment. 



