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 atmos- 

 pheric 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 physi- 

 ologists 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 investigators 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 Rothamsted 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 com- 

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



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