200 



Bacteria in Relation to Country Life 



with a given quantity of humus, two or three times 

 as much nitrogen as can be fixed by Clostridium Pas- 

 torianum. The differences in the nitrogen-fixing power 

 of the anaerobic and aerobic bacteria are due to the 

 differences in their mode of action. The chemical pro- 

 cesses which they employ are not the same, hence the 

 results cannot be expected to be the same. However, 

 the power of fixing atmospheric nitrogen is quite vari- 

 able in either class of or- 

 ganisms. It is modified 

 essentially by soil aijd 

 climatic conditions, and 

 may be increased by favor- 

 able soil conditions, or de- 

 creased by unfavorable 

 conditions. 



The quantity of lime in 

 the soil bears a striking re- 

 lation to the development 

 of azotobacter. Being more 

 susceptible to acid condi- 

 tions than Clostridium Pastorianum they will not develop 

 in soils that are more or less sour. They grow best, there- 

 fore, in soils containing an abundance of lime. It has 

 been found that they can be most readily secured by 

 inoculating culture solutions of the proper composition 

 with small pieces of undecomposed lime carbonate 

 found in the soil. Investigations carried on in Germany 

 and in Sweden show the importance of lime for the 

 development of these organisms. The unlimed soils in 

 these experiments failed, for the most part, to yield a 



Fig. 30. Non-symbiotic nitrogen-fix- 

 ing bacteria. — 1. Azotobacter agilis; 

 X 2,000. 2. Azotobacter chroococ- 

 cum; degeneration forms; X 1,600. 

 3. Azotobacter agilis; X 2,000. 

 (Beyerinck.) 



