ASSIMILATION OF FREE NITROGEN 395 



nitrogen. Bcrthelot l was the first to show that no such enrichment is 

 possible when the soil is sterilized, and consequently that the process is due 

 to the activity of micro-organisms. Soil containing bacteria, or bacteria and 

 algae may gain TO or even 25 milligrammes of nitrogen per 100 grammes 

 in a comparatively short time 2 , so that assuming the process continues 

 with equal energy to a certain depth, as much as 30 kilos of nitrogen 

 may be gained annually by each hectare (2^ acres) 3 . The conditions are 

 more favourable when small quantities of soil are experimented with than 

 when they are performed in the open air. Direct observations show that 

 without the help of Leguminosae no such marked gain of nitrogen is 

 possible, although a certain amount of fixation usually occurs even when 

 these plants are absent. It is in all cases possible, however, that nitrogen- 

 producing bacteria may obtain the upper hand so that a loss instead of 

 a gain of nitrogen occurs (cf. Tacke, 1. c.). 



Winogradsky found that C. Pastcuriannm consumed 1,000 grammes 

 of sugar in fixing 1-5 to 3 mg. of nitrogen, but it is not impossible that 

 other bacteria or these same forms may work more economically in the soil. 

 It appears that a supply of some carbon-compound as food-material is 

 always necessary, and Berthelot 4 and Kossowitsch have shown that the 

 addition of sugar causes an increased fixation of nitrogen in the soil. 

 Under normal conditions this food is supplied by other organisms, which 

 in sterilized sand watered with nutrient salts may consist of algae only. 

 The algae obtain in return supplies of combined nitrogen which the soil 

 lacks, and it is possible that in such cases the algae and bacteria live in 

 intimate contact-symbiosis. It has yet to be determined whether the 

 bacteria enter into similar contact-symbiosis with living roots, or whether 

 they obtain organic food supplies simply by the disintegration of dead 

 roots and root-tissues. 



However this may be, the result is the same, the nitrogenous com- 

 pounds which are produced ultimately becoming available for all plants 

 growing in the soil in question, and in the absence of root-tubercle bacteria 

 this is the only way in which the higher plants can obtain nitrogen when 

 grown in a non-nitrogenous soil. 



Frank observed under such conditions that many plants without root- 

 tubercles gained a certain amount of nitrogen, and erroneously concluded that 

 all plants were able to assimilate free nitrogen. Frank did not employ a sterile 



1 Berthelot, Compt. rend., 1893, T. cxvi, p. 842; 1885, T. ci, p. 175; also Tackc, Landw. 

 Jahrb., 1889, Ed. xvill, p. 453 ; Frank and Kossowitsch, 1. c. 



1 Tacke, Landw. Jahrb., 1889, Bd. xviu, p. 453, and the literature here given; Kossowitsch, 

 Bot. Zeitung, 1894, p. 112. 



3 Cf. Sachsse, Agr.-Chem., 1883, p. 588. 



* Berthelot, Compt. rend., 1893, T. cxvi, p. 842. 



