94 



RECLAIMING LOST NITROGEN. 



grow in the presence of oxygen. In the soil, however, it is often 

 associated with a second bacterium that is aerobic, the latter 

 absorbing the oxygen so that the anaerobic form can grow. Or- 

 dinarily the nitrogen fixation in the soil is due to these two growing 

 together, but the Clostridium alone is able to assimilate nitrogen if 

 kept in an oxygen-free atmosphere. In its growth the bacterium 

 consumes some of the organic material in the humus, and from this 

 source obtains the necessary energy for its action. The organism 

 is widely distributed, having been isolated by several bacteriologists 

 from different soils. Practically nothing is known as to its activity in 

 soil under ordinary conditions. 2. In 1901 it was proved that the 

 soil also contains bacteria of the aerobic type that can fix nitrogen. 



FIG. 21. Clos- 

 terium pasteuri- 

 anum; an anaero- 

 bic nitrogen fixer 

 (Winogradski). 



FIG. 22. 

 Azotobacter 

 agili s, an 

 aerobic n i - 

 trogen fixer 

 (Beyerinck.} 



FIG. 23. B. 

 danicus, an 

 aerobic nitro- 

 gen fixer (Loh. 

 and West}. 



Two different varieties of these were first isolated, and to them was 

 given the general name of Azotobacter (Fig. 22). Several other 

 varieties have been found later (Fig. 23). They are considerably 

 more vigorous than the aerobic type, and fix a considerably larger 

 amount of nitrogen two or three times as much. In order to de- 

 velop efficiently they must be supplied with a considerable quantity 

 of organic food, and in ordinary soil the humus furnishes this food. 

 By the energy they obtain from this source they gather from the air 

 an extra quantity of nitrogen. These nitrogen fixers are very 

 susceptible to the presence of the smallest amount of acid, and fail to 

 fix nitrogen entirely if the soil is even slightly acid. The use of a 

 little lime to neutralize the acidity may thus frequently start an 

 active nitrogen fixation in a soil in which it did not previously occur, 

 arid hence greatly increase its productiveness. It has been shown 

 also that this class of nitrogen fixers, though able to grow alone, will 



