TOO RECLAIMING LOST NITROGEN 
the compounds in the soil. Free nitrogen is of no use to them, only 
nitrogen compounds Some other source of soil nitrates must be 
sought. 
The Ammonia Theory.—For a time it was held that the am- 
monia in the air was the source from which plants obtained nitro- 
gen, and that it was carried into the soil by the rains. When this 
supply was found to be insufficient to account for soil nitrates, it 
was claimed that plants could absorb ammonia directly from the 
air through their leaves. But this theory failed to stand the test 
of experiment, and was finally abandoned. 
Fixation of Nitrates in Soil—It was next shown that, under 
proper conditions, ordinary soil will increase its stock of nitrates, 
independently of visible vegetation. A lot of earth placed in a 
proper vessel and kept free from vegetation will, in time, be found 
to contain more nitrates than at the outset. Part of these nitrates 
may be due to the process of nitrification already mentioned, by 
which the nitrogen compounds, which were in the soil, but not in 
the form of nitrates, are converted into nitric acid by the nitrifying 
bacteria. But this is not the whole explanation, because analysis 
of such soil shows that at the end of several weeks there may 
actually be a larger amount of total nitrogen in the soil than there 
was at the start. If, then, this total nitrogen has been increased, 
it must have been derived in some way from the atmosphere. 
NITROGEN-GATHERING OR NITROGEN-FIXING 
BACTERIA 
It was soon demonstrated that nitrogen fixation in the soil is 
not due to purely chemical processes, but rather to the growth of 
microorganisms. ‘That it is due to the action of living organisms 
is proved by the effect of sterilizing such soil. Two vessels may be 
filled with similar soil, one of which is sterilized by heating, while 
the other serves as a control. The former fails to gain nitrogen, 
no matter how long it is kept in contact with the air; the latter 
slowly but surely increases its store of fixed nitrogen in the form of 
