176 SOIL ORGANISMS IN RELATION TO SOIL FERTILITY 



supplied to it; (6) why legumes, that is, such crops as clover, 

 peas, beans, etc., should enrich the soil considerably in nitrogen, 

 and (c) why a crop like wheat or turnips, for example, should pro- 



Fig. 116. — Nodules on the roots of soybean. (Wisconsin Station.) 



duce much larger yields on^clover sod than when grown on such a 

 sod as timothy. 



It was known centuries ago that certain plants had bunches or 

 nodules on their roots ; and it was known for forty years that 

 these nodules contained bacteria. Yet it was not until 1886 that 

 two German investigators proved conclusively that these bacteria 

 within the nodules actually have the power of taking the free 



