Q2 RECLAIMING LOST NITROGEN. 
Where did the nitrates come from that are now in the soil ? Soil is 
made of crumbled rock which did not originally contain nitrates; it 
certainly must have obtained them from some source. The various 
bacteria we have been studying only transform nitrogen compounds; 
they do not make a new supply. The nitrogen in the air would be an 
inexhaustible source if it were only available; but the bacteria we 
have considered have no power of obtaining this nitrogen. They 
can transform nitrogen compounds, but they cannot fix or gather 
nitrogen from the air. It might naturally be supposed that ordinary 
plants could obtain nitrogen from the air as they do CO 2 . But the 
most careful testing has shown that when such plants are growing 
under ordinary conditions they cannot assimilate any nitrogen from 
the air, but must depend upon 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 nitrogen, 
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 Jot 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. 
