74 SOIL BACTERIOLOGY 



Follow the same procedure with the solid culture, adding 

 200 ex. of sterile water to the agar. Shake thoroughly, 

 dilute, and plate as given in the preceding directions. 



B, I. Place about ten bacteria-free seeds in each culture 

 bottle. Shake and pour off the liquid. 



2. With sterile forceps remove about five seeds to a 

 sterile Petri dish. 



3. Allow seeds to dry in the Petri dish. 



4. After twenty-four to forty-eight hours count the 

 number of legume bacteria on each seed. 



5. Place the seeds in a loo-c.c. water blank, shake, 

 and plate i-c.c. portions. 



Exercise 41 

 Nitrogen Content of Bacteria (Optional) 



1. Grow a mass culture of bacteria in a large Petri dish 

 or pan. Any vigorous growing organism may be used, 

 e. g.j Azotobacter. 



Note. — ^Use 2 to 2.5 per cent. agar. 



2. When cool, inoculate the surface of the agar with 

 5 ex. of a rich suspension of Azotobacter in sterile water. 



3. Incubate ten to fifteen days. 



4. At the end of this time remove growth by carefully 

 scraping off with a clean glass slide. 



5. Dry the mass culture at 100*^ C. and pulverize by 

 grinding in a mortar. 



6. Analyze for total nitrogen. If desirable, a portion 

 of the material may be saved for further analysis— e. g., 

 potassium and phosphorus. 



