BACTERIA OF OLD SOILS 



115 



4. Though Azotobacter forms are entirely absent in all except 

 soil no. 8, the nitrogen fixation power of the soils is well main- 

 tained by other organisms. 



5. There is no relation between numbers and physiological 

 efficiency. 



6. The persistency of these organisms under dry conditions 

 and their renewed activity in the presence of sufficient moisture 

 is agriculturally important. 



7. The soil exhibits a protective function towards lower forms 

 under adverse conditions. 



I take great pleasure in expressing my thanks to Dr. Chas. 

 B. Lipman for many valuable suggestions, helpful criticisms, and 

 kindly interest in the preparation of this paper. The work which 

 it embodies was carried out at the Soil Research Laboratory of 

 the University of California. 



