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 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. 



