UTILIZATION OF NITROGEN-FIXING POWERS OF LEGUMES II17 
used by the next crop of plants sown on the same field, thus in- 
creasing its yleld by means of the nitrogen which has been fixed 
by the legume and the bacteria together, and has been converted 
into an available form by the soil bacteria. 
A second method of utilizing the nitrogen is by converting it 
into manure. ‘The crop of legumes is reaped and fed to animals, 
the roots and stubble only being plowed into the soil. The por- 
tion fed to the animals is later returned to the soil as manure. 
Part of the nitrogenous material is thus metabolized by the 
animal body to urea, and part passes into the feces unassimilated, 
while part remains in the roots and soil. But it is all eventually 
decomposed by the putrefying bacteria, and goes through the 
same series of metamorphoses which we have already described 
in sufficient detail. The result is that, in the end, most of it is 
returned to the soil in a form available for plant life. This method 
of utilizing the nitrogen is certainly good economy, since it has 
a double advantage: The nitrogen is used twice, once as a food 
for the stock and a second time as a food for the crops in the form 
of manure. 
It is of course manifest that under either of these methods of 
treatment not all of the nitrogen fixed by the legume and the 
bacteria is rendered available for the next series of crops. At 
the very best, part of it will be lost to the soil by the process of 
putrefaction which liberates free ammonia, and by denitrification 
which liberates free nitrogen. It is impossible, by any means 
now at our disposal, to prevent this loss, and thus a portion of 
the fixed nitrogen is, even with the best treatment, dissipated 
again into the air. But by proper treatment this loss can be 
reduced to a2 minimum and there may always be a surplus of 
gain. Even taking into account all the nitrogen loss that comes 
from these processes the use of a leguminous crop upon a soil 
poor in nitrogen furnishes to that soil for the next crop a store of 
nitrogen considerably in excess of that which it possessed before. 
