LEGUMES IN GENERAL 245 



the machines seems to indicate that they are of little 

 service in the pollination of clover. 



244. Relation to soil fertility. The great favor now 

 accorded the legumes as field crops is not due to a 

 recent recognition of their importance in maintaining 

 soil fertility, for many centuries ago Roman writers on 

 agricultural topics attested to their soil-enriching value. 

 In more recent times, in the first half of the eighteenth 

 century, Jethro Tull, an Englishman, wrote a treatise on 

 agriculture, in which he called attention to the increased 

 yields obtained from grain crops on soils that had pre- 

 viously grown legumes. While it was early known that 

 the legumes possessed some soil enriching virtue not 

 possessed by the non-leguminous crops, the reason for 

 it was a matter of no little speculation. 



245. It was thought by many that the peculiar value of 

 the legumes was due to the fact that they possessed roots 

 that penetrated deeply into the subsoil and thus were 

 able to secure much of their nourishment from a depth 

 beyond the reach of other crops. A part of this plant 

 food was thought to be stored in the roots and stubble 

 near the surface, later to become available to succeeding 

 crops. Liebig, a German chemist, held the opinion that 

 plants received their nitrogen and carbon from the air, 

 and that the clovers, on account of their broad leaves, were 

 able to take up more nitrogen than the other crops. This 

 theory, however, was not long accepted as explaining the 

 matter, for Boussingault, in France, in 1851, and Laws and 

 Gilbert, in England, in 1857, demonstrated, by a series 

 of experiments, that the free nitrogen of the air was not 

 available to the legumes. In 1883 Atwater, at the Con- 

 necticut Experiment Station, grew a number of kinds of 

 plants in pots, analyzing the soil before planting, and then 



