1. Two hundred eighty-seven Iowa soils representing 52 soil types and 37 soil series and distributed in the five soil areas of the state -the Wisconsin drift, the Iowan drift, the Missouri loess, the Southern Iowa loess and the Mississippi loess - were examined for the presence of Azotobacter.

2. The presence of the Azotobacter was determined in these soils by the Winogradsky soil plaque and the selective culture agar-plate methods using mannitol as the energy source.

3. Of the 287 samples collected, 101 or 35.2 percent contained Azotobacter.

4. The virgin, high-lime Knox soils of the Missouri loess soil area, the Webster soils of the Wisconsin drift soil area, the Clyde-Floyd soils of the Iowan drift soil area and the bottomland soils were generally positive with 100, 91.7, 71.4 and 76.4 per.cent, respectively, of the samples of these soils containing the bacteria.

5. The Clarion samples of the Wisconsin drift soil area and the terrace soils contained Azotobacter in only about one-third of the samples; the loessial soils, with the exception of the Knox, and the remaining drift samples only occasionally showed any growth of the organisms.

6. The samples were classified according to their topographic position which showed that the more depressed the position, the greater the percentage of samples which contained Azotobacter.

7. An analysis of the samples for pH, content of available phosphate, total nitrogen and calcium carbonate and a calculation of the ratio of carbonate to phosphate was made in order to determine whether or not any relation existed between the presence of the Azotobacter and anyone or all of these soil constituents.

8. A multiple correlation of the data arbitrarily summarized on the basis of pH showed that the presence of the Azotobacter in Iowa soils was closely associated with the pH and available phosphate content of the samples and associated very little with the total nitrogen content, the calcium carbonate content or the carbonate-phosphate ratio. The association with pH was closer than that with available phosphate.

9. A study of the limiting pH and available phosphate content values for Azotobacter in Iowa soils led to the conclusion that soils with pH values from about pH 5.42, the limiting value obtained by an extrapolation of the simple regression equation, to pH 6.0, below which only one sample contained the bacteria, and available phosphate contents less than 35 pounds per acre would probably not contain the bacteria.

10. A method recently developed by Fisher (12) was used to determine whether or not the pH, available phosphate content and total nitrogen content of the soils would serve to differentiate between the samples which contained Azotobacter and those which did not contain the organisms. It was found that these variables did significantly differentiate the two groups. The pH had the greatest influence in this respect, the available phosphate content the next greatest and the total nitrogen content had the least influence as a differentiating factor. These conclusions are in agreement with those obtained by the multiple correlation method.

11. The presence or absence of the Azotobacter in the principal soil series of Iowa may be largely explained on the basis of the characteristic pH and the content of available phosphate of the samples in each series.

12. In addition to the determination of factors which limited the occurrence of the Azotobacter in Iowa soils, a study was made of the factors which influenced the amount of growth which the Azotobacter would make in the soil. For this study only those samples were considered which contained the bacteria.

13. The available phosphate content of the samples which contained Azotobacter did not appear to be associated with the amount of Azotobacter growth.

14. The results for pH showed that an increase in the pH of the sample did improve conditions so that the Azotobacter could make a better growth.

15. The results for total nitrogen indicated that this variable exerted the greatest influence upon the amount of growth which the Azotobacter would make in the soil, when other factors were not unfavorable.

16. Some experimental plots were sampled to determine whether or not soils which did not originally contain the organisms could be treated with lime and rock phosphate to correct the acidity and deficiency in available phosphate and thus be put into condition to support a flora of Azotobacter. The results showed that the addition of lime to these soils had improved conditions so that the organisms were detected. Where rock phosphate was added in addition to the lime, a better growth of the organisms was not obtained.

17. A group of experimental plots on high-lime soils which contained large amounts of organic matter, as indicated by total nitrogen determinations, contained a vigorous growth of the Azotobacter as expected.