ASSIMILATION OF FREE NITROGEN 393 



made good by manuring. Boussingault calculated the annual amount removed 

 in Alsace by a single harvest to be approximately equal to 51 kilogrammes of 

 nitrogen per hectare (2^ acres), whereas only 2-30 kilos, are annually brought to 

 the soil by rain, dew, and in the form of volatile ammonia 1 . It appears, moreover, 

 that the activity of the nitrogen-fixing bacteria of the soil is insufficient to 

 compensate for this deficit. Lupins, peas, and other Leguminosae, are, how- 

 ever, able by means of their symbiotic union with root-tubercle bacteria to 

 assimilate atmospheric nitrogen in such abundance as to satisfy all their nitro- 

 genous requirements (Sect. 69), and hence by ploughing in a crop of such plants 

 (green manuring) the percentage of combined nitrogen present in the soil may be 

 increased. These plants attain the upper hand in soils deficient in nitrogen, 

 and thus prepare the way for other plants which require a supply of combined 

 nitrogen. In their absence, whether the soil loses or gains nitrogen depends 

 upon circumstances, for the loss by drainage and by the exhalation of ammonia, 

 &c. may or may not be counterbalanced by the gain from rain, dew, and dust. 

 A sterilized and protected soil loses a little nitrogen by evolving ammonia, whereas 

 when suitable bacteria are present the amount of fixed nitrogen may increase. 



SECTION 69. Assimilation of free Nitrogen. 



The fixation of free nitrogen is due to the activity of certain micro- 

 organisms, some of which grow chiefly in symbiotic union with the roots 

 of Leguminosae, while others are capable of independent existence. One 

 of the latter forms is the Clostridium Pasteur ianum discovered by Wino- 

 gradsky 2 . It is an anaerobic bacterium, which when fed with sugar forms 

 butyric and acetic acids as well as hydrogen and carbon dioxide, while at 

 the same time it assimilates nitrogen in such abundance that no addition of 

 nitrogen compounds is necessary. Clostridium Pasteurianum apparently 

 always occurs in nature in symbiotic association with two other bacteria, from 

 which it can only with difficulty be isolated, for all three forms can develop 

 together in a medium free from nitrogen. When isolated, Clostriditim 

 Pasteurianum is an obligate anaerobe, but in this symbiotic union it can 

 grow in an ordinary culture fluid in the presence of oxygen, and hence no 

 doubt can also fix nitrogen in an ordinary aerated soil, for it appears to 

 have the same power of assimilating free nitrogen when in symbiotic union 

 with these accompanying bacteria, as when it is isolated. It has, however, 

 still to be determined whether these other bacteria simply act as a protection 

 against oxygen, or whether they exert other influences as well. 



1 Cf. A. Meyer, Lehrb. d. Agr.-Chem., 1895, 4. Aufl., p. 193; Sachsse, Agr.-Chem., 1888, 

 pp. 78, 586. 



8 Winogradsky, I, Compt. rend., 1893, T. cxvi, p. 1385; II, ibid., 1894, T. cxvm, p. 353; 

 III, Archiv. d. sci. biol. d. 1'Inst. d. med. exp., St.-Petersbourg, 1895, T. II, p. 297. 



