CHAPTER III 

 FIXATION OF ATMOSPHERIC NITROGEN 



THE SOURCE OF NITROGEN IN SOILS 



EARLY THEORIES. When chemistry had made sufficient progress 

 to allow the analysis of soils and plants it was recognized that nitrogen 

 is always present in both. It was also recognized that the soil nitrogen 

 is almost wholly confined to the surface portion and is evidently of 

 atmospheric origin, since the unweathered, underlying rock is devoid 

 of this constituent. The vast accumulations of nitrogen, known to 

 exist in all arable soils, were ascribed, therefore, to the residues of many 

 generations of plants; and the assumption seemed to be justified that 

 the atmosphere, 79 per cent of whose bulk consists of nitrogen gas, is 

 the direct source of this element to plants. It was not long, however, 

 before plant physiologists demonstrated experimentally that nitrogen 

 gas as such could not directly serve as food for plants. There thus 

 arose one of the most interesting and, for a long time, one of the most 

 puzzling problems in agricultural research. Among the earlier in- 

 vestigators de Saussure believed, at the beginning of the nineteenth 

 century, that nitrogen is taken up from the soil in combined form. 

 Liebig in 1840 advanced his well-known "mineral theory" according 

 to which plants secured their nitrogen from the air, in the form of 

 ammonia. He assumed, thus, that plants cannot use elementary 

 nitrogen, and that the supply of atmospheric nitrogen in the form of 

 ammonia was great enough to meet the needs of growing vegetation. 

 The latter view was not accepted by Lawes and Gilbert of the Roth- 

 amsted Station in England. By a series of elaborate and carefully 

 controlled experiments they demonstrated in 1858 that nitrogen in the 

 elementary form cannot be used by plants. They further demonstrated 

 that the amount of combined nitrogen brought down in the form of 

 ammonia, nitrites and nitrates, by atmospheric precipitation was but 

 slight when compared with the quantities annually removed by crops. 

 Hence the problem as to the source and maintenance of combined 

 nitrogen in the soil seemed to be more puzzling than ever. 



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