BACTERIA AND SOIL FERTILITY 291 



under favorable conditions, appear to have the power 

 of liberating food from the insoluble soil grains. 

 Others have the power when settled on the roots 

 of leguminous or pod-bearing plants to fix nitrogen 

 from the air and convert it into a form suitable for 

 the need of plants. In recent years it has been found 

 that other forms of bacteria, the best known of which 

 is azotobader, have the power of gathering nitrogen 

 from the air and combining it for the plant needs 

 without the presence of leguminous plants. These 

 nitrogen-gathering bacteria utilize for their life pro- 

 cesses the organic matter in the soil, such as the 

 decaying header stubble, and at the same time 

 enrich the soil by the adcUtion of combined nitrogen. 

 Now, it so happens that these important bacteria 

 require a soil somewhat rich in lime, well aerated and 

 fairly dry and warm. These conditions are all 

 met on the vast majority of our dry-farm soils, under- 

 the system of culture outlined in this volume. Hall 

 maintains that to the activity of these bacteria 

 must be ascribed the large Cjuantities of nitrogen 

 found in many virgin soils and probably the final 

 explanation of the steady nitrogen supply for dry 

 ■farms is to be found in the work of the azotobader 

 and related forms of low life. The potash and phos- 

 phoric acid supply can jjrobably be maintained for 

 ages by proper methods of cultivation, though the 

 phosphoric acid will become exhausted long before 

 the potash. The nitrogen supply, however, must 



