578 PRINCIPLES OF SOIL MICROBIOLOGY 



so far as its availability to crops is concerned. Stoklasa 78 found that 

 5.0 to 5.7 mgm. of atmospheric nitrogen are fixed for every mgm. of 

 phosphorus used. The minimum need of phosphorus is 2.43 mgm. of 

 P (or 5.46 mgm. P 2 5 ) for every gram of glucose. Sulfur in the form 

 of sulfates is essential for the growth of Azotobacter. 79 



Iron, in the form of its salts, has a definitely favorable influence upon 

 the development of Azotobacter, by playing a part in its metabolism 

 and exerting a favorable influence through its colloidal condition. When 

 iron sulfate was added to the medium the amount of nitrogen fixed per 

 gram of mannite was increased from 2.23 to 10.3 mgm. 80 It has been 

 suggested that the colloidal iron absorbs the nitrogen and oxygen of the 

 air and thus brings them into more intimate contact with the cells of 

 Azotobacter. In the presence of organic colloids, only small quantities 

 of iron are effective. The favorable influence of soil infusion upon nitro- 

 gen fixation by Azotobacter has been found 81 to be due chiefly to their 

 content of iron and partly to silicic acid. Inorganic colloids like the 

 oxides of aluminum, iron and colloidal silicic acid greatly stimulate 

 nitrogen-fixation by Az. chroococcum 82 After inserting a strip of paper 

 into the medium, Azotobacter grows exclusively in contact with the 

 paper. This led Sohngen to conclude that microbial life in the soil takes 

 place chiefly upon the colloids. The colloid is believed to absorb the 

 nitrogen and oxygen thus making them more readily available to the 

 organism. Iron has the same stimulating effect upon nitrogen-fixation 

 as "humus" substances do, but much larger quantities of the inorganic 

 colloid are required when it is present alone as when it is used together 

 with the organic colloid. The iron is most active in the form of an or- 

 ganic compound. 81 



78 Stoklasa, J. Biochemischer Kreislauf des Phosphat Ions im Boden. 

 Centrbl. Bakt. II, 29: 385-419. 1911; Christensen, H. R. Studien iiber den 

 Einflusz der Bodenbeschaffenheit auf das Bakterienleben und dem Stoffumsatz 

 im Erdboden. Centrbl. Bakt. II, 43: 1-166. 1915; also Soil Sci., 15: 329-360. 

 1923; Greaves, 1918 (p. 563). 



79 See also Koch, A. Ernahrung der Pflanzen durch frei im Boden lebende 

 stickstoffsammelnde Bakterien. Ber. deut. landw. Gesell., 22: 117-121. 1907; 

 Jour. Landw., 55: 355-416. 1907. 



80 Rosing, G. Zusammenfassung der Ergebnisse von Untersuchungen uber 

 die Stickstoffsammlung von Azotobacter chroococcum. Centrbl. Bakt. II, 33: 

 618-623. 1912. 



81 Remy, Th., and Rosing, G. Uber die biologische Reizwirkung natiirlicher 

 Humusstoffe. Centrbl. Bakt. II, 30: 349-384. 1911. 



82 Sohngen, N. L. Einflusz von Kolloiden auf mikrobiologische Prozesse. 

 Centrbl. Bakt. II, 38: 621-647. 1913. 



