VII.] NITROGEN FIXATION IN VIRGIN SOILS 143 



soil in nature — is to put some of the flasks back in the 

 incubator and leave them there for one or two months. 

 The other groups of bacteria introduced in the soil 

 sample resume their activity as soon as they can obtain 

 some of the combined nitrogen that has been introduced 

 by the Azotobacter, and a portion of the compounds 

 that have been so elaborated is broken down and 

 successively oxidised into ammonia and nitrates. On 

 eventually testing the contents of the flask no glucose 

 will be found, but the presence of nitrates can be shown 

 by diphenylamine in the usual way. Finally, if the 

 flasks are put in the light, green algae will begin to make 

 their appearance, the spores having been introduced 

 with the soil, and thus a growth of green vegetable 

 matter is seen which has derived the whole of the 

 nitrogen it contains from the atmosphere through the 

 agency of the Azotobacter. 



It will easily be seen that so widely distributed and 

 so active an organism as Azotobacter may have played 

 a considerable part in maintaining and even in creating 

 the stock of combined nitrogen possessed by the 

 world. The one factor necessary for fixation is that 

 the soil shall be kept supplied with the purely car- 

 bonaceous material formed by plants from the atmos- 

 phere by the assimilation process. In the Rotham- 

 sted wheat field it has been shown that little fixa- 

 tion has taken place in the soil of the unmanured plot 

 — only enough to replace the small losses by drainage, 

 weeds, etc., the amount removed in the crop being 

 balanced exactly by the reduction in the stock of nitrogen 

 in the soil during the period under investigation. The 

 crop being wheat after wheat every year, a very small 

 residue of roots and stubble are left as material 

 which the Azotobacter can oxidise. When more carbon- 

 aceous residues are left behind, as on grass land, 



