IV. THE EEACTIONS OCCUKEING IN SOILS 67 



Eemy and Bosing l have shown that the stimulative effect of humus 

 upon the nitrogen- fixing power of Azotobacter is mainly due to the 

 iron and silicates contained therein. They found that ferric oxide, held 

 in solution by sugar, was very effective and that ferric silicate is also 

 good. They attribute the beneficial effect of basic slag largely to its 

 stimulating effect, due to this cause, upon the development of Azoto- 

 bacter in the soil to which it is applied and the consequent increased 

 fixation of nitrogen. 



Another class of micro-organisms, of which various species of 

 CLostridium afford the best type, are able, in the absence of oxygen 

 (anaerobic conditions) to effect nitrogen fixation. Such organisms are 

 abundant in woodland soils and on the leaves of forest trees, according 

 to Haselhoff and Bredemann. 2 



Keutner 3 found that both Azotobacter chroococcum and Clostridium 

 Pasteur ianum were abundant in sea- water and that the former can 

 effect nitrogen-fixation, even in the presence of an eight per cent solu- 

 tion of sodium chloride. 



Pringsheim 4 has recently found that many soils contain bacteria 

 which are able to live at as a high temperature as 61 C. and which 

 can fix nitrogen in the presence of dextrose. 



These nitrogen-fixing organisms elaborate from the free nitrogen of 

 the air, nitrogenous compounds which are readily nitrified in the soil 

 by the nitrifying organisms and thus rendered easily available to plants. 



In laboratory experiments the amount of nitrogen fixed per gram 

 of carbohydrate destroyed is small, usually not exceeding 10 milligrams, 

 but there is evidence that in the field a much larger rate of fixation is 

 attained. This is thought to be due to nitrification keeping pace with 

 nitrogen-fixation in the field, thus preventing the accumulation of ni- 

 trogenous material, which, in laboratory experiments, probably limits 

 the action of Azotobacter. Indeed, Koch and Seydell 5 have shown that 

 at first, even in laboratory experiments, the rate of nitrogen-fixation 

 may reach as high as 50 to 60 milligrams per gram of dextrose 

 oxidised, but that the rate soon falls off. 



Henry proved that fallen leaves of forest trees bring about fixa- 

 tion of nitrogen during their decay. Hall, indeed, attributes the rich- 

 ness in nitrogen of virgin soil, where organic matter of the nature of 

 carbohydrates has accumulated, largely to the activity of Azotobacter 

 under these favourable conditions. Some remarkable examples of the 

 great activity of Azotobacter, accompanied by nitrification, have been 

 investigated by Headden and Sackett 7 who found that in some soils in 

 Colorado, patches of brown colour, produced by this means, were 

 barren owing to the excessive quantities of nitrates produced. 



Nitrogen-fixation through Symbiosis. Another class of micro- 

 organisms is of great importance in agriculture, viz., those which 



1 Jour. Chem. Soc., 1911, Abstracts,. ii. 758. 



2 Landw. Jahrb., 1906, 35, 381. 3 Chem. Zentr., 1905, i. 395. 



4 Jour. Chem. Soc., 1911, Abstracts, ii. 916. 



5 Centr. Bakt. Par., 1911, ii. 31, 570. 6 Bied. Zentr., 1904, 33, 795. 

 7 Bull., 178 and 179, Agric. Exp. Stat. Colorado, 1911. 



