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, 
dan leu s, 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, 
and hence greatly increase its productiveness. It has been shown 
also that this class of nitrogen fixers, though able to grow alone, will 
