88 THE SPIRIT OF THE SOIL 



gained, it was necessary to include two groups the 

 Bacillus radicicola group, which acts only in associa- 

 tion with leguminous plants, and the other group 

 or groups that were able to fix Nitrogen while they 

 remained free -living in the soil. As regards the 

 radicicola group enough has already been said, 

 except for the statement that in the specially 

 treated peat the bacillus found a material better 

 suited for its needs than the old form of culture. 

 There was no longer any need for the farmer to 

 dissolve up the materials supplied him, and grow 

 the organisms for himself in home-made apparatus 

 with temperature and other conditions quite un- 

 satisfactory for the process. He had only to spread 

 the material on the land or to mix it with his seed, 

 or if he desired it in liquid form he could extract the 

 peat easily with water, and use the resulting liquid 

 to water the roots of his crop. 



Of the other organisms connected with Nitrogen- 

 fixation it is necessary now to write in rather greater 

 detail. Experiment has shown that just as land on 

 which leguminous crops are grown becomes richer 

 in Nitrogen, so, too, there may be an increase of the 

 Nitrogen content of the soil in cases where the ground 

 has been left bare of vegetation. In such circum- 

 stances the increase will be comparatively slow, but 

 at Rothamsted it has been observed to increase in a 

 year by as much as 25 pounds an acre. There are 

 two groups of bacteria by the aid of which the 

 Nitrogen circulating in the soil air can be changed 

 into forms of nitrogenous plant food. The first is 

 anaerobic, working only in the absence of Oxygen ; 



