68 



Fixation by the virgin samples was noted also with regard to soil type, 

 since a number of samples of widely different character were examined. 

 The results from a number of virgin soils of the same type were averaged 

 and compared with averages from other soil types. The average from the 

 clay soils was highest, showing quite a perceptible increase over the lighter 

 soils. But on account of results in fixation tests with cultivated soils, the 

 writers hesitate to ascribe any marked influence to soil texture and regard 

 it as a rather immaterial factor. Many cultivated soils, regardless of type, 

 showed notable nitrogen fixing power. In Tables II and IV are to be seen 

 summaries of these points. 



In connection with results obtained in the present work, the reports of 

 previous investigators along this line are of interest. A reference to the 

 work of Greaves, in which he found higher nitrogen fixing power of virgin 

 than of cultivated soils, has been previously made in this article. A notable 

 amount of work on factors which influence fixation has concerned itself 

 with the alkalinity 1 and phosphate 2 content of soils. The concensus of these 

 reports emphasize the importance of lime in fixation. That phosphates are 

 essential to the proper development of Azotobacter has been amply demon- 

 strated by laboratory tests, but it is questionable whether they are ever a 

 limiting factor under field conditions. Some fixation tests at this station on 

 soils which had received various combinations of mineral fertilizer for five 

 years fail to disclose any results that might be attributed to inorganic causes. 



The general indications from fixation studies which the authors have 

 carried on are that with the possible exception of lime the humus con- 

 tent of a soil and its cultivation are the only factors which materially affect 

 fixation. It would seem that with these properly adjusted the process could 

 always reach its maximum efficiency cultivation to promote Deration and 

 removal of nitrogen, and humus to supply the energy essential to the 

 activities of the nitrogen fixing flora. 



(3) Studies on Nitrification in Various Soil Types. 3 



The accumulation of so great a number and variety of soils afforded an 

 unusual opportunity for observing the process of nitrification. To study it 

 contemporaneously with fixation was of no little interest for it is probable 

 that each is affected by many of the same factors. 



, S. F. Jour. Agr. Sci. 2. 1907. No. 1, pp. 31-51. 



Christensen, H. R. Cent. f. Bakt. II Abt. 17, 1906, Nos. 3, 4., pp. 109-119. Figs. 2. 5, 

 7, pp. 161-165; Figs. 2, 11-13. pp. 378-383. 



Fisher, H. Journal f. Landw, Bd. 53, 1905. p. 290. 



Gerlach und Vogel. Cent. f. Bakt. Abt. II. Bd. 9, 1902. p. 891. 



Hoffman and Hammer. Cent. f. Bakt. Abt. II, Bd. 28, 1910., pp. 127-139. 



Lipman, J. G. Ann. Kept., N. J. Exp. Sta. Bui. No. 206, 1907. p. 4. 



Mockeridge, F. A. Ann. Bot., London, Vol. 26, 1912. No. 113., pp. 871-897. 

 2 Gerlach und Vogel. Cent. f. Bakt. Abt. II., Bd. 10, 1903. p. 638. 



Hoffman, C. Cent. f.. Bakt. Abt. II., Bd. 36. pp. 474-477. 



Lipman, J. G. Ann. Kept., N. J. Exp. Sta., Bui. 29, 1908. p. 139. 

 "Paper 38 from the Laboratories of Plant Pathology and Bacteriology, Virginia Agr. Exp. Sta. 



