110 AGRICULTURE. 



y^.— LEGUMINOUS PLANTS AS NITROGEN 



GATHERERS. 



I. Nitrogen -fixing Bacteria. 



In recent years it has been discovered (see 

 foot-note, p. 32) that certain plants, through 

 their intimate relation with other low plant 

 forms, bacteria, are able to obtain nitrogen from 

 the inexhaustible supply of the air. The exact 

 relation existing between these soil bacteria and 

 the roots of leguminous plants is not fully under- 

 stood. But it has been proven by many experi- 

 ments that wherever the bacteria which work 

 upon a particular species of plant are present — 

 which is shown by the nodules upon the roots 

 (Fig. 31) — the plant is able to make a luxu- 

 riant growth without the addition of nitrogen- 

 ous fertilizers, providing, of course, that other 

 necessary conditions are present. 



II. Inoculation of the Soil. 



It sometimes happens that the particular spe- 

 cies of bacteria which works upon a certain 

 species of leguminous plant is not present in 

 the soil. In this case the plant — vetch, for ex- 

 ample — has no nodules upon its roots (Fig. 

 30), is weak and sickly, and a profitable crop 

 cannot be obtained unless heavy applications of 

 nitrogenous fertilizers are made, which would 

 entail considerable expense, or the soil of this 

 field be inoculated with the bacteria which work 



