I02 RECLAIMING LOST NITROGEN 
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 sgor it was proved that the soil also contains bacteria of the 
aerobic type that can fix nitrogen. Two different varieties of 
these were first isolated, and to them was given the name of 
Azotobacter (Fig. 21). They are considerably more vigorous than 
the anaerobic type, and fix a considerably larger amount of 
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Fic. 20.—-Clos- Fic. 21.-—~ Fic. 22.-—-B, 
terium pasteurt- Azotobacter danicus, an aero- 
anum; an anaero- agilts, an bic nitrogen fixer 
bic nitrogen fixer aerobic ni- (Loh. and West). 
(Winogradski). trogen fixer 
(Beyerinck). 
nitrogen—two or three times as much. In order to develop effi- 
ciently they must be supplied with a considerable quantity of 
carbonaceous 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 fre- 
quently start an active nitrogen fixation in a soil in which it did 
not previously occur, and hence greatly increase its productive- 
ness. It has been shown also that this class of nitrogen fixers, 
though able to grow alone, will perform their functions best when 
growing with certain other soil organisms. They grow well with 
other fungi and with some alge, organisms generally found asso- 
ciated with them in the soil. 
