SYMBIOTIC NITROGEN FIXATION 157 



About 1834 that adventurous South American traveler, Bous- 

 singault began a series of field experiments on his farm in Alsace. 

 In these he studied no fewer than five rotations keeping accurate 

 record of the loss and gain of the various elements. Interesting 

 enough, he found that when the rotations contained legumes 

 there were gains in nitrogen. 



Experiments conducted in closed vessels on heated soil, how- 

 ever, soon demonstrated that the free nitrogen of the air was not 

 available to the higher plants. Could it be that they were feed- 

 ing on the ammonia of the air? No, experiments showed that 

 this was not sufficient. Hence, for a number of years it was ac- 

 cepted as decisive that plants obtain their nitrogen from the soil. 



This was contrary to the agricultural experience of all ages 

 and the carefully conducted experiments of Rothamsted con- 

 tinued to point to the conclusion that legumes behave abnormally. 

 Could it be that the deep-rooting legumes were bringing nitrogen 

 to the surface? Later work explained the apparent contradiction 

 in showing that Boussingault had used sterilized soil, whereas 

 plants normally grow in unsterilized soil. 



To Hellriegel and Wilfarth (1888) is due the credit of solv- 

 ing the mystery. They found that legumes grew quite normally 

 in soils that lacked nitrogen, provided these soils were not previ- 

 ously sterilized. Growth was checked, however, in sterilized 

 nitrogen-free soil because of lack of nitrogen. Addition to the 

 soil of a small quantity of an infusion from unsterilized soil pro- 

 duced normal growth of the plants and resulted in a crop rich in 

 nitrogen. If the added infusion was previously boiled, then its 

 addition produced no effect at allj the plants were retarded in 

 their development and the harvest showed no increase in nitrogen. 

 The soil used in preparing the infusion must be taken from a 

 £eld upon which the kind of plants used in the experiment had 

 been cultivated. For example, if peas are taken the soil used for 

 the watery extract must come from a field where peas have been 

 previously grown. 



The legumes grown in unsterilized soil and those watered with 

 infusions of soil have upon their roots small knobs or tubercles. 



