14 



Historical 



processes. This method contributed greatly to the solution of sev- 

 eral important problems in soil microbiology. 



Among the numerous soil microbiological processes, none is of 

 greater importance than the fixation of nitrogen by leguminous 

 plants. This has an important historical background, which can be 

 separated into the following stages : ( 1 ) Our knowledge that legumes 

 enrich the soil, which dates back to the time of the Romans. (2) 



Fig. 9. J. J. T. Schloesing demonstrated the biological nature of the process of 



nitrification. 



Boussingault's emphasis in 1838 that the favorable action of legumes 

 upon the soil is due to their power to fix atmospheric nitrogen. 

 (3) Lachmann's demonstration in 1858 and Woronin's in 1866, to 

 be followed by those of various other botanists, that nodules are 

 formed on the roots of leguminous plants. (4) Frank's demonstra- 

 tion in 1879 that the nodules on the roots of the plants are formed 

 as a result of inoculation with microorganisms. Hellriegel and Wil- 

 farth found in 1885 that, whereas the growth of nonleguminous 

 plants is proportional to the amount of nitrate added to the soil, 

 there is no such relationship in the case of leguminous plants. There 

 was no gain in nitrogen when the nitrogen content of the plant was 

 added to that of the sand in which nonleguminous plants were 

 growing, but there was a considerable gain in combined nitrogen 



