42 
Colorado Experiment Station. 
state and newly cultivated, possess little, if any, nitrogen fixing power. 
Excessive nitrates either destroy or greatly attenuate the nitrogen 
fixing flora of a soil. 
A limited amount of soil nitrate does not seriously affect the 
nitrogen fixing power of a soil. 
Azotobacter chroococcum appears to be the dominant nitrogen 
fixing organism in the soils studied. 
The dark brown color of the nitre soils is due, in a large part, to 
the pigment produced by Azotobacter chroococcum. 
Given a source of energy, the nitrate is the limiting factor in the 
production of the brown color. 
In the presence of nitrates, Azotobacter chroococcum develops a 
chocolate brown to black pigment; nitrites, in certain amounts, produce 
similar results, but to a less degree; nitrogen as NIT Cl,( NIT4) - SO4, 
asparagin, and peptone has no effect upon this function. 
The highly colored extracts obtained from certain nitre soils sug¬ 
gest that the pigment of Azotobacter chroococcum may be soluble in 
the alkaline soil waters. 
Excessive soil moisture, by interfering with the growth of Azoto¬ 
bacter chroococcum, prevents the formation of the brown color on the 
soil, and makes the fixation of atmospheric nitrogen impossible. 
