THE LOSS OF NITROGEN 99 
Clearly enough, the supplying of the lost nitrogen will become 
more and more expensive as the great nitrogen stores are used up. 
The seriousness of this problem of a constant draining of nitrogen 
from the soil has been quite prominent in the minds of chemists 
and agriculturists, as they have learned in the last few years the 
significance of nitrogen for agriculture. 
The continuation of agriculture depends upon the existence 
of some means of reclaiming the nitrogen from the atmosphere 
for the use of plants. If there is no such means it is evident that 
the nitrogen store of the soil will be used up and vegetation will 
eventually, and, in highly cultivated lands, speedily die of nitrogen 
starvation. If, on the other hand, there is a possibility of re- 
claiming such lost nitrogen there is no need of nitrogen starvation, 
since there is an absolutely unlimited store of this element in the 
form of the free nitrogen of the air. It is quite evident that there 
is some means within the reach of organic nature for making use 
of this atmospheric nitrogen. Vegetation has continued on the 
earth for an unknown number of centuries without any apparent 
diminution of the nitrogen supply. This would not have been 
possible unless the soil could have obtained from the air a stock of 
nitrogen to replace that lost by the processes already indicated. 
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 ivansform nitro- 
gen compounds; they do not makeanewsupply. Thenitrogen 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 COs. But the most careful testing has shown that 
when such plants are growing under ordinary conditions they can- 
not assimilate any nitrogen from the air, but must depend upon 
