The foregoing paper reports a study on the effect of various organic compounds on the growth of Azotobacter. The study was induced by the theory that the soil contains organic substances which are deleterious to plant growth and which are important factors in influencing soil fertility. It is, therefore, interesting to determine if this toxicity extends to the lower plants. The Azotobacter was chosen as a representative of the soil flora since it is of recognized importance in the maintenance of the soil fertility and its growth may be accurately measured by analytical means. The compounds used were those likely to be constituents of the soil.

The results of the study indicate that fixation of nitrogen by Azotobacter is only slightly influenced by most of the compounds investigated. A depression is noted in many cases but it is usually the result of a relatively high concentration of the compound used.

Hydroquinone and Salicylic Aldehyde revealed the most toxic properties of any compounds studied.

Esculin, Quinic Acid, and Borneal afforded marked stimulation to the growth of the organism.

The effects of the compounds on Azotobacter are not, as a rule, in accord with what has been reported of their action on the higher plants. In concentrations which are fatal to certain higher plants, many of the compounds only slightly depressed fixation.

A number of nitrogenous bodies were investigated. Such compounds as Nicotine, Picoline, and Skatol exhibited toxic properties commensurate to those usually ascribed to these substances. Caffeine appeared to stimulate the growth of the organism.

Many of the nitrogenous compounds used which have been reported as beneficial to higher plants exercised a marked depression on fixation. It appears that the simpler compounds were more pronounced in this respect than were the more complex ones. It is suggested that this condition is not one of toxicity but that the nitrogen of the compounds was utilized by the Azotobacter in preference to that of the atmosphere. Urea, Glycocoll, Formamide, and Allantoin were especially active in depressing fixation.