32 



hydrate food available, the species present, their number, and 

 the degree of favourableness of the other factors mentioned. In 

 ordinary cultivated soils the supply of available carbohydrate 

 materials is the factor that usually limits free-nitrogen fixation. 

 The almost complete removal of crops leaves very little carbo- 

 naceous food for these bacteria. To stimulate the development 

 of these bacteria, assuming that they are present, it is essential 

 that considerable carbonaceous matter be incorporated into the 

 soil. 



In grass lands and in wild lands generally, where much of the 

 carbonaceous matter produced finally becomes incorporated with 

 the soil, the fixation of atmospheric nitrogen by the Azotobacter 

 group* and the other independent nitrogen-fixing bacteria is very 

 great. An examination of two fields at Rothamsted which had 

 run wild for more than twenty years showed an accumulation of 

 nitrogen of approximately 45 pounds per acre per annum in a 

 field poorly supplied with carbonate of lime, and 98 pounds per 

 acre per annum in a field well supplied with carbonate of lime. 

 Bacteriological tests of these fields showed that Azotobacter was 

 present in much larger numbers and had greater powers of fix- 

 ation in the field containing an abundance of carbonate of lime.f 



This has been going on in connection with, but entirely inde- 

 pendent of, nitrification for ages, wherever the conditions are 

 favourable and the proper bacteria are present. This is true 

 especially of prairie soils in all parts of the world. The work of 

 these organisms represents an absolute gain in available nitrogen. 

 It is highly essential that we should learn more about them and 

 get better control of their activities for the benefit of agriculture. 

 An attempt has been made to do this in the preparation of 

 "Alinit," which is a culture of Bacillus elleribachiensis. This is, 

 however, a species of very small, if any, nitrogen-fixing power. 

 It is of more value as a peptonizer and ammonifier. When it has 

 given good results it has probably been where it was needed for 

 such work rather than nitrogen fixation. Success in the use of 

 cultures can come only when bacteria of high efficiency are 

 selected for a particular kind of work, and are used under con- 

 ditions favourable to their development and where examination 

 indicates that they are needed. A few failures due to imperfect 

 knowledge must not discourage workers in this important field. 

 Excellent work is being accomplished in the study of these 

 forms in this country, especially by Voorhees and Lipman, of the 

 New Jersey station, and Chester, of the Delaware station.} 



ROOT-NODULE BACTERIA. 



The bacteria of this class, like the Azotobacter group, are able 



* Beijerinck, who described these forms, holds that Azotobacter can fix atmos- 

 pheric nitrogen only in association with other forms, such as Granulobacter and 

 Radiobacter. This point, however, is not yet definitely settled. It is settled, though, 

 that these forms working together fix nitrogen much more actively than when they 

 work independently. 



t Science, new series, Vol. XXII, p. 455. 



X See Bui. 180. N.J. Agr. Exp. Sta., and the station reports for 1903 and 1904 ; also 

 the Delaware station bulletins previously referred to. 



