12 THE FERTILITY OF THE SOIL 



is absolutely indispensable to plants, and, in their turn, 

 to animals ; yet though we live in contact with a vast 

 reservoir of free nitrogen gas in the atmosphere, 

 until comparatively recent times we knew of no natural 

 process except the lightning flash which would bring 

 such nitrogen into combination. Plants take combined 

 nitrogen from the soil and either give it back again 

 or pass it on to animals. The process, however, is only 

 a cyclic one, and neither plants nor animals are able to 

 bring in fresh material into the account. As the world 

 must have started with all its nitrogen in the form of 

 gas, it was difficult to see how the initial stock of com- 

 bined nitrogen could have arisen ; for that reason many 

 of the earlier investigators laboured to demonstrate that 

 plants themselves were capable of fixing and bringing 

 into combination the free gas in the atmosphere. In 

 this demonstration they failed, though they brought 

 to light a number of facts which were impossible to 

 explain, and which only became cleared up when in 

 1886 HELREIGEL and WILFARTH showed that certain 

 bacteria, which exist upon the roots of leguminous 

 plants like clover and beans, are capable of drawing 

 nitrogen from the atmosphere. Thus they not only 

 feed the plant on which they live, but they actually 

 enrich the soil for future crops by the nitrogen they 

 leave behind in the roots and stubble of the leguminous 

 crop. Long before this discovery experience had taught 

 farmers the very special value of these leguminous crops; 

 the Roman farmer was well aware of their enriching 

 action, which is mentioned in the well-known words in 

 the Georgics, beginning Aut ibiflava seres, where Virgil 

 says that the wheat grows best where the bean, the 

 slender vetch, or the bitter lupin had been most luxuriant. 



