THE STUDY OF MICROORGANISMS IN THE SOIL 115 



Inoculate with 1 or 2 grams of soil and incubate at 28°C. and 

 note changes occurring in cultures, film formation, and color of 

 film. 



After 2 days, examine the films in hanging-drop or wet mount 

 and note the predominating type of organism. Also examine 

 some of the surface film in a drop of water mixed with a drop of 

 Meissner's or Gram's iodin solution. Prepare stained mounts 



Fig. 10. — Colonies of Azotobacter on mannitol agar, natural size. 



with thionin and carbol erythrosin. The Erythrosin stain is 

 especially good for young cultures. 



Dilute two loops of surface film in a 100-cubic centimeter 

 sterile water blank containing 50 grams of clean sand. 



Shake vigorously, and transfer 1 cubic centimeter to a second 

 100-cubic centimeter blank, and so on to a third. 



From the second and third dilution pour plates, using 1 cubic 

 centimeter for each. Because of the aerobic nature of Azotobacter 



