406 MICROBIOLOGY OF SOIL 



qualities of certain crops of the legume family. Columella mentions 

 the fact that many Roman farmers regarded beans as possessing these 

 qualities, but does not accept this belief for himself. On the other 

 hand, he points out that luzerne (alfalfa), lupins and vetches improve 

 the land and act as manure. He points out, also, that it was the 

 practice of Roman farmers to plow under lupines in order to enrich the 

 soil. In the centuries following the fall of Rome the use of legumes for 

 soil improvement persisted to some extent in Italy, France and other 

 countries; yet the practice was not followed consistently and the fer- 

 tility of European soils was declining for lack of available nitrogen, 

 and, to a large extent, also of phosphoric acid. The more general intro- 

 duction of clover into Germany and England in the eighteenth century 

 helped to restore the fertility of many farms, and led, ultimately, to the 

 recognition of the peculiar place held by legumes in the maintenance 

 of soil fertility. But while practical farmers knew of the soil-enriching 

 power of legumes, and while they retained their belief in it even when 

 it seemed contrary to scientific authority, they did not know the secret 

 of this power. It remained for Hellriegel and Wilfarth to demonstrate 

 in 1886, and more fully in 1888, that this power, already hinted at by 

 the investigations of others, is the resultant of the combined activities 

 of the plants and of bacteria that enter their roots, and produce there 

 the well-known nodules or tubercles. They showed in no uncertain 

 manner that legumes can improve the soil only in so far as they add 

 nitrogen to it with the aid of the bacteria in the tubercles; in other 

 words, legumes were shown to enter into a symbiotic relationship with 

 certain bacteria and to acquire, thereby, the ability to fix atmospheric 

 nitrogen. 



The presence of tubercles on the roots of leguminous plants was first 

 recorded by Malpighi in 1687. He regarded them as root galls. The 

 botanists who studied them in the first half of the nineteenth century 

 classified them as modifications of normal roots or as pathological 

 processes. In 1866 the Russian botanist Woronin found that the 

 tubercles were filled with minute bodies resembling bacteria and con- 

 cluded that they were pathological outgrowths. Some years later 

 Frank, in 1879, not on ^y showed that tubercles are almost invariably 

 present on the roots of legumes, but that their formation may be pre- 

 vented by sterilizing the soil. Frank was thus in possession of facts 

 that might have revealed to him the true nature of the root-tubercles. 



