[Reprinted from The Populae Scihncb Monthly, March, 1903.] 



THE SOUKCE OF NITEOGEN m FOREST SOIL. 



By EAPHAEL G. ZON, 



BUBEA0 OF FOBKSTBY, U. S, DEPT. OF AGEIOUlinKB. 



"VrOEMAL development and growth of forest plants is possible 

 -'-^ only when the plants are able to obtain a sufficient amount 

 of nutritive substances from their medium. 



Among the substances indispensable for the nutrition of plants 

 nitrogen occupies a conspicuous place. On an average, sixteen per 

 cent, of this element is included ia the composition of albuminous 

 matter. Nitrogen is necessary for the formation of protoplasm of 

 vegetative cells, and when it is absent no formation of protoplasm 

 can take place; hence, no development of organic life in general is 

 possible. Plants derive nitrogen from the soil, where it is found in 

 a free state (air), or combined in the form of nitrogenous compounds. 



If plants had the capacity of absorbing atmospheric nitrogen and 

 assimilating it like carbon, the question as to the presence of nitrogen 

 in the soil would have no interest, since the air would be a sufficient 

 source of nitrogen. While some plants have the capacity of absorbing 

 atmospheric nitrogen, the majority draw their supply from the salts 

 of nitrogen found in the soil. 



The consumption of atmospheric nitrogen has been fully deter- 

 mined for the wild acacia (black locust) and for the white and black 

 alders. Thus in quartz sand completely free from nitrogen, seeds 

 of black locust, while developing into seedlings, increased their con- 

 tents of nitrogen from 0.0084 gr. to 0.093 gr., or more than 38 times, 

 between May 1 and September 10. The locust in this respect resembles 

 all the other leguminosse, which have long been known to agriculturists 

 as accumulators of nitrogen. On examining plants which absorb the 

 free nitrogen contained in the soil, it was found that the roots of these 

 plants have tubercles inhabited by organisms. These organisms are 

 bacteria, which, attracted by the excretions of the roots, immigrate 

 from the soil and cause the formation of root-tubercles which are 

 invariably present in the papilionacese. The bacteria absorb the atmos- 

 pheric nitrogen and transmit it to the plant. A great many bacteria 

 cause tubercles on the roots; they can be classified according to the 

 species which they choose as their host, more conveniently than by 

 their external appearance. Each species of the leguminosse seems 

 to possess its own race of bacteria, which can be made serviceable to 



