ABSORPTION OF NITROGEN FROM SOIL AND AIR 59 



The detailed chemistry of nitrogen fixation by these organisms 

 is not clearly understood. There are many reasons to suppose, 

 however, that nitrogen is combined with the hydrogen of the water 

 molecule and that ammonia is the primary product of this syn- 

 thesis. This then is utilized for the formation of the more com- 

 plex substances, such as the amino acids and proteins. 



Since the nitrogen-fixing bacteria require non-nitrogenous 

 organic substances in order to obtain the necessary energy for their 

 work, the incorporation of plant material into soil even if it is 

 poor in nitrogen, as for instance straw, green manure, etc., con- 

 siderably increases the activity of these bacteria and thus con- 

 tributes to the accumulation of fixed nitrogen. A particularly 

 intense activity is displayed by these bacteria in tropical countries, 

 where good harvests are often secured from soils low in nitrogen 

 and not enriched by nitrogenous fertilization. In higher latitudes, 

 however, the activity of microorganisms is not sufficient to restore 

 to the soil the quantity of nitrogen that is removed with the har- 

 vesting of crops. In order to enrich the soil in nitrogen, it is 

 necessary, therefore, either to introduce nitrogenous fertilizers or to 

 resort to the cultivation of leguminous plants. 



20. Assimilation of Molecular Nitrogen by Leguminous Plants. 

 Agriculturists have noted for a long time that the growing of le- 

 gumes, particularly of clover, makes the soil more fertile, and 

 subsequently increases the yield of cereals. It has been estab- 

 lished also that this increased crop production results from 

 an increased nitrogen content of the soil. Naturally, then, 

 leguminous plants have acquired the name of nitrogen collectors. 

 These observations, however, proved to be contradictory to 

 Boussingault's careful experiments, which showed that under 

 conditions of artificial culture no plants, legumes included, were 

 able to assimilate free nitrogen of the atmosphere and that their 

 yield was wholly determined by the amount of combined nitrogen 

 present in the nutritive solution. 



Hellriegel (1886) succeeded in solving this apparent contradic- 

 tion. His attention was drawn to the fact that in soil certain 

 nodules which were absent in water and sand cultures were formed 

 on the roots of leguminous plants (Fig. 25). And as these nodules 

 were found to be filled with bacteria, it was assumed that they 

 assimilate atmospheric nitrogen, while the plant, in turn, supplies 

 these bacteria with the non-nitrogenous nutritive materials and 



