FIXATION OF NITROGEN 119 



tected from the rain. He found that a constant increase in 

 nitrogen took place. He estimated that the nitrogen thus 

 added to the soil in the fields amounted to from 50 to 75 

 pounds an acre each year. When 1 lie soil was heated no 

 increase in nitrogen occurred. This indicated a biological 

 process. 



It is now known thai tin-re are at least two groups of 

 bacteria in the soil that are able to fix nitrogen. One of 

 these is an anaerobic group. The first organism studied 

 was given the name of Cloxtrid'uim 1'uxlrnritinnx. The or- 

 ganism will grow in a nutrient solution containing inor- 

 ganic salts, which supply the essential mineral elements, and 

 sugar, which supplies the energy. No nitrogen need be 

 present in the solution. In the process of growth, nitroge- 

 nous organic matter is formed, and since the only source of 

 nitrogen is the free nitrogen of the air, the organism must 

 in some way bring it into combination. It was found that 

 for every gram of sugar fermented about two milligrams of 

 nitrogen were combined. 



Another group of nitrogen-fixing bacteria in the soil be- 

 longing to the aerobic type is known as the Azotobactcr 

 group. Most of its members have the ability to form a 

 black pigment ; the name Azotobactcr chroococcum has been 

 given to this type. This type is more efficient in fixing 

 nitrogen than the previous group, in that for every gram 

 of sugar fermented from 10 to 20 milligrams of nitrogen 

 are fixed. Members of this group are found in nearly all 

 soils; they are more abundant in the more fertile soils. 

 They are also found in water, frequently in combination 

 with green alga*. It is supposed that they live in a symbi- 

 otic relationship, the alga? furnishing the carbohydrate to 

 the bacteria, and the latter nitrogen in an available form to 

 the cells of the alga?. Not. only can sugar be used as a 

 source of energy, but also organic acids. It is thought that 



