62 SOIL BACTERIOLOGY 



6. Shake vigorously, and transfer i c.c. to a- second 

 loo-c.c. blank, and so on to a third. 



7. From the third dilution pour plates, using i c.c. for 

 each. It is frequently difficult to separate Azotobacter 

 from a small organism known as Bacillus radiobacter. 



Fig. 3. Azotobacter stained with methylene-blue; X 1200. 



Exercise 28 

 Nitrogen Fixation in Soil 



i. Weigh two 5oo-gram portions of field soil into soup 



plates. 



