MAINTAINING PERMANENT FERTILITY 135 
that it will not properly decay, either because the density of the 
soil prevents sufficient aeration, or because too much acid forms, 
checking bacterial activities. No value accrues from green manur- 
ing unless thorough decomposition occurs. For this reason it is 
generally best to plow in the leguminous crop before it has fully 
matured, for then it has assimilated most of its nitrogen but has 
not become too bulky for proper decay in the soil. Frequently 
it is best to reap the crop and feed it to cattle, plowing in only 
the roots and stubble, this giving all the organic matter that can 
be readily decomposed in the soil. If, subsequently, the manure 
from the cattle that eat the crop is added as a dressing, the greatest 
possible use will have been made of the leguminous crop. 
The actual value of such green manuring has been demon- 
strated many times. Sandy soils have been brought under fair 
cultivation, and depleted farms have been reclaimed to cultivation 
by the skillful use of this nitrogen-fixing power of legumes, aided 
by the tubercle bacteria. A constant increase in nitrogen can be 
brought about thus till the quantity is sufficient for large crops. 
In one extended series of experiments and observations it was 
found possible to increase the amount of nitrogen in a soil from 
0.02 per cent. at the start, to 0.17 per cent. at the end of about 
twenty-five years, equivalent to 5,000 pounds of nitrogen per 
acre. In another test the plowing in of a crop of the velvet beans 
stubble upon a soil subsequently planted with oats, increased 
the yield of oats from seven to thirty-eight bushels per acre; 
and in this case the velvet bean crop was reaped and utilized, 
only the roots and stubble being necessary for the increased 
yield. 
MAINTAINING PERMANENT FERTILITY 
The great lesson to be learned from this subject is that by 
means of the nitrogen-fixing power of the legumes, aided by bac- 
teria in their roots, the farmer has a practical means of maintaining 
a high nitrogen content in his soil. To do this is of great impor- 
