Fixation of Nitrogen. 205 



kinds of bacteria that are able to fix the free nitrogen of 

 the air and build it up into their own protoplasm. They 

 are able to grow in media that contain no nitrogen and 

 must then satisfy their needs from the nitrogen supply 

 of the air. It is supposed that a large part of the nitro- 

 gen of the soil has been brought into combination by the 

 action of these bacteria. It has been shown that a soil 

 on which no plants of any kind are growing will increase 

 in its content of nitrogen, and it is believed that it is. 

 largely due to these germs. Unlike the nodule-forming 

 bacteria, the fixation of nitrogen by this second class, 

 known as the azotobacter group, can go on independent 

 of the kind of plants that may happen to be growing in 

 the field. 



It is important to know the conditions that favor the 

 growth of these nitrogen-fixing bacteria. They are 

 aerobic ; . hence, grow best in a soil that is thoroughly 

 aerated, stirred and drained. They demand a supply of 

 available potash and phosphorus as well as organic mat- 

 ter from which they derive the energy necessary for the 

 fixation of the nitrogen. In the laboratory the largest 

 amount of nitrogen is fixed in a food medium that con- 

 tains a sugar such as cane sugar or mannit. In the soil, 

 it is believed that they use the different carbohydrates 

 contained in the plant residues that are added in the 

 roots of crops, in the green crop plowed under, or in the 

 manure. 



In the case of uncultivated land, the greater part jf 

 the annual crop falls onto the ground, and undergoes de- 

 composition. The nitrogen-fixing bacteria are thus fur- 

 nished with suitable food to accomplish the fixation of 

 nitrogen, and the content of the soil in this important 



