Nitrogen in Soils. 87 



These amounts, it will be seen, are far too small to be of 

 great importance to plant life. 



3. The process of symbiosis is a third method by which 

 the nitrogen supply of the soil is maintained and next to 

 the decay of organic matter is the most important of any 

 yet well understood. It was in 1888 that Hellriegel pub- 

 lished the results of his studies, which thoroughly estab- 

 lished the fact that great numbers of microscopic forms of 

 life inhabit the roots of leguminous plants, forming upon 





FIG 26. Showing the growth of rye, oats, peas, wheat, flax and buckwheat in 

 soils fertile in all elements rf plant food except nitrogen, and illustrating the 

 power of the pea, through its root tubercles, to procure nitrogen from the 

 air. After P. Wagner. 



them tubercles in which these organisms live and withdraw 

 free nitrogen from the soil-air for their needs. It had long 

 been known to farmers that in some way clover in rotation 

 with other crops left the soil richer in nitrogen, and it is 

 now known that the bacterium which lives on the clover 

 roots, deriving a part of its food from the clover plant, at 

 the same time increases the nitrogen supply available to the 

 clover crop and so we have two forms of life living together 



