126 SOME PRACTICAL LESSONS FROM SOIL BACTERIOLOGY. 
and utilized, only the roots and stubble being necessary for the in- 
creased yield. 
The great lesson to be drawn from this subject is that, by means 
of the nitrogen-fixing power of the legumes, aided by bacteria in 
their roots, the farmer has a practical means of maintaining the 
nitrogen content of his soil at a proper degree for high fertility. 
There is no need of purchasing nitrate fertilizers. The money may be 
better spent on phosphorus and potash. The cultivation of legumes 
seems to be the secret of the continuance of agriculture, and if the 
farmer will only learn the principles and acquire the habit of alter- 
nating legumes with his other crops, he may maintain indefinitely 
a high fertility in his soil, in spite of long-continued cultivation. 
When in addition to this we remember the fact that the soil minerals 
are being constantly dissolved in the soil waters, it becomes evident 
that the farmer is by no means as dependent upon artificial fertilizers 
as has been supposed. 
