Jan. 28, i9i8 Nitrogen-Assimiloting BacteHa 187 



Plate counts of all soil cultures were made by weighing 20 gm. (dry 

 weight) of the soil into a 200-c. c. water blank. From this suspension 

 all subsequent dilutions were made. Mannit agar ^ was used for the 

 plate counts in the cultures of Azotobacter and B. radicicola. Duplicate 

 plates were made for each dilution poured. 



SOIL USED 



Only one type of soil was employed, Miami silt loam obtained from 

 the Experiment Station farm. No chemical analyses of the soil were 

 made other than an estimation of its organic matter content, which was 

 approximately 2.75 per cent. The soil was neutral in reaction and its 

 nitrate content was approximately 1.5 mgm. of nitrogen as nitrate in 

 100 gm. of the dry soil. 



ISOLATION OF AZOTOBACTER AND BACILLUS RADICICOLA 



Azotobacter. — (i) Strain A was isolated from a silt loam soil. This 

 strain grew well on mannit agar, but produced no pigment after three 

 weeks' growth. (2) Strain B was isolated from a sandy loam soil. 

 This strain grew equally well on mannit agar and produced a brownish 

 black pigment within one week's growth. Both strains assimilated 

 practically the same amount of atmospheric nitrogen under laboratory 

 conditions. 



BACiiyLUS RADICICOLA. — A stock laboratory culture of B. radicicola 

 was replated twice before taking the final culture. The nodule produc- 

 ing power of the organism was determined by inoculating bacteria-free 

 alfalfa seedlings (in soft agar). After sufficient incubation nodules were 

 produced in abundance. 



influence; of nitrates on azotobacter 



INFLUENCE OF NITRATES ON THE GROWTH AND REPRODUCTION OF AZOTOBACTER IN 



STERILIZED SOIL 



What effect do nitrates have on pure cultures of Azotobacter in ster- 

 ilized soil ? Do these salts cause a decrease in the numbers of the organ- 

 isms? Do they cause an increase in numbers? Or do they exert no 

 particular influence one way or the other ? It is difficult to believe that 

 the latter could be true, inasmuch as nitrates have such a profound 

 effect on higher forms of plant life. Such readily soluble and assimilable 

 substances as nitrates could hardly remain without affecting either an 

 increase or a decrease in the number of organisms existing in their 

 presence. 



With the idea of determining what effect nitrates might have on 

 Azotobacter when grown in sterilized soil, the following experiments were 

 planned. In this work both strains of the Azotobacter (described on 



* Fred, E. B. a i^aboratory manual of son, bacteriology, p. io8. Philadelphia and London, 1916. 



