In order to approximate the number of bacteria within the 

 soil, the seventy samples were plated both on nutrient agar, 

 and Ashby's medium to which was added fifteen grams of agar- 

 agar per liter. 



Nutrient medium recommended by Ashby for fixation of 

 nitrogen by azotobacter: 



Mannite 20.00 grams 



Di-potassium phosphate 0.20 gram 



Magnesium sulphate 0.20 gram 



Sodium chloride 0.20 gram 



Calcium sulphate 0.10 gram 



Calcium carbonate 5.00 grams 



Distilled water 1000.00 grams 



A Kjeldahl determination disclosed the fact that good 

 agar-agar contained 0.16% of nitrogen, or a little below the 

 average of good soil. Therefore, each plate of the Ashby 

 medium contained approximately .25 mg of nitrogen, or 

 about .0025%, which is in the neighborhood of one one- 

 hundredth the nitrogen content of a good loam. 



A little nitrogen must inevitably be carried over in the 

 process of dilution, etc. 



The plates were counted after five days incubation at 

 room temperature, which prevailed at about 33C. 



The figures in the adjoining table represent the count per 

 gram of soil dried at 100-1 10C, to constant weight. 



NUMBER OF BACTERIA PER GRAM OF SOIL DRIED 



TO CONSTANT WEIGHT AT 



100-110C 



TABLE I 



