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 azotobacter, 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 addition 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 quantities 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 azotobacter 
and related forms of low life. The potash and phos- 
phoric acid supply can probably 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 
