THE STUDY OF MICROORGANISMS IN THE SOIL 119 



Incubate the plates under anaerobic conditions (potato jar) 

 and pick colonies into Winogradsky's liquid medium or corn 

 mash. 



Fig. 13. — Dorner spore stain of anaerobic nitrogen-fixing bacteria. 



Exercise 31 

 Nitrogen-fixing Capacity of Soil^ 



Large Petri dishes (20 centimeters in diameter) are filled with 

 the selective silica gel medium (see Exercise 26) containing 2 

 grams of mannitol. 



The surface of the gel in the plate is inoculated with 1 gram of 

 soil (on dry basis). 



Plates are incubated at 30°C. for 7 days. 



The gel in the plates is now dried and contents transferred to 

 Kjeldahl flasks and total nitrogen determined. 



1 WiNOGRADSKY, S., and J. ZiEMiECKA, Ann. Inst. Past., 42: 35, 1928. 



