CARBON AND NITROGEN CYCLES IN THE SOIL 199 



The energy relationships are equally little known : it is 

 concluded, however, that only about i per cent, of the total 

 available energy is utilised in the fixation of nitrogen.^ 



Beijerinck's solution works satisfactorily for crude cultures 

 but not for pure cultures. Various hypotheses have been put 

 forward in explanation ; it was supposed that azotobacter re- 

 quired the presence of some other organisnni, or that it lost its 

 efficiency on cultivation. Krzemieniewski (157) found that 

 neither of these views is correct, and in an important investi- 

 gation showed that the determining factor is the presence of 

 a little soil ; so long as this is added pure cultures retain their 

 effectiveness. The active agent is the humus, but its effect is 

 not to furnish carbon or nitrogen to the organism ; further, 

 the humus loses its power after treatment with hydrochloric 

 acid. Remy and Rosing {2T,'jb) frankly call it a stimulating 

 action and attribute it to the iron invariably present.^ Allen,^ 

 on the other hand, supposes that the colloids of the humus 

 prevent complete precipitation of the phosphate and thus 

 facilitate phosphorus nutrition of the organism. 



The nitrogen is found partly in compounds dissolved in 

 the liquid, but mostly in the bacterial mass. The organism is 

 remarkably active, one grm. weight evolving no less than i -3 

 grms. of CO2 in twenty-four hours (273a). An adequate 

 supply of phosphate, calcium carbonate and other mineral 

 nutrients is essential, any deficiency limiting the amount of 

 fixation. Traces of nitrogen compounds are helpful in the 

 early stages, but larger quantities reduce the amount of fixa- 

 tion, and may themselves suffer some change : thus sodium 

 nitrate is partially reduced to nitrite even under aerobic 

 conditions (Stoklasa (275<5)). Hills (134) finds that the bene- 

 ficial effect of small quantities of nitrate of soda and nitrate of 

 potash is not confined to the growth of the organism, but ex- 

 tends (though to a less extent) to fixation also, and it is 



^ G. A. Linhart, yuurn. Gen. Physiol., 1920, 2, 247-251. 

 But see also Reed and Williams, Centr. Bakt. Par., 1915, 43, 166. 

 E. R. Allen, Ann. Mis. Bot. Garden, 1919, 6, i. 



