1. Increasing concentrations of nitrate, amino acid and peptone decreased proportionally the amount of atmospheric nitrogen fixed in culture solutions of Azotobacter.
2. Increasing concentrations of sterile, unheated, plant extracts increased the amount of atmospheric nitrogen fixed up to a maximum limit, after which the fixation gradually decreased with further additions.
3. The addition of sterile, unheated plant extracts to pure solution cultures greatly stimulated the multiplication of Azotobacter.
4. Very heavy applications of plant material to soil effectively checked the assimilation of nitrogen, and at the same time greatly increased the concentration of nitrogen in the soil solution.
5. It is suggested that Azotobacter always prefers to derive its nitrogen from a combined source but that plant tissues contain certain unknown "essential food substances" which stimulate the growth of the organism to such an extent that the supply of available nitrogen derived from moderate applications of vegetable material is soon exhausted, and the organism then assimilates nitrogen from the air.