9 



Nitrogen Fixation — Symbiotic 



Early Observations 



Many centuries before the discovery was made that bacteria exist 

 in the root nodules of leguminous plants and that these bacteria live 

 in symbiosis with the plants, thus enriching the soil with combined 

 nitrogen, the practical agriculturist came to consider the growth of 

 legumes on his land as equivalent to manuring or fertilizing the soil 

 for the succeeding crop. The use of leguminous plants for green 

 manuring was described in great detail by Greek and Roman writers, 

 notably Virgil, Varro, and Columella. Directions were given for 

 preparing the soil and for sowing, cultivating, and harvesting the 

 crop. Lupines, vetches, and alfalfa were frequently mentioned in 

 these books as specific crops to be turned over when the plants were 

 young. 



With the beginning of the nineteenth century, when the basis was 

 laid for modern agricultural science, more accurate information 

 gradually began to accumulate. Sir Humphry Davy, in his book 

 Agricultural Chemistry, published in 1813, observed: "Peas and 

 beans in all instances seem well adapted to prepare the ground 

 for wheat . . . they contain a small quantity of a matter analogous 

 to albumen; but it seems that the azote which forms a constituent 

 part of this matter is derived from the atmosphere." 



These observations were fully borne out in the classical studies of 

 Boussingault, published in 1837-1838. This French agronomist and 

 chemist was the first to develop systematically the idea of nitrogen 

 nutrition of leguminous and cereal plants. A typical field experiment 

 on crop rotation is shown in Table 46. Boussingault established the 

 fact that, when clover is grown in unmanured soils, there is a consid- 

 erable gain of nitrogen; wheat, on the other hand, showed no gain 

 or loss of nitrogen. He suggested that leguminous plants assimilate 

 nitrogen from the atmosphere, whereas cereal plants cannot do so. 



208 



