47 



It is difficult to separate these two factors, since 

 they react importantly on each other, "but it is certain 

 that the degree of soil aeration has great importance 

 to both. Under conditions of poor aeration, however 

 caused, various reduction reactions occur in the soil 

 and lead to the formation of ferrous salts, hydrogen 

 sulphide and other compounds 1 which are toxic to most 

 plants. Probably these reductions themselves are 

 due largely to anaerobic bacteria 2 , but many other 

 changes in the micro-organisms of the soil follow ex- 

 treme decreases in soil aeration. Thus the organisms 

 responsible for nitrification in the soil are largely 



IT 



aerobic, while the denitrifyers are in part anaerobic. 



T. Hilgard,- Soils, pp. 18-19, 45-46, 75-76, 



231-233 (1906); Gamble and Slater,- Ontario Dept. Agr., 

 Bull. 178, 39 pp. (1909). 



2. Beijerinck,- Gentbl. Bakt., : (1895); 



van Be Id en,- Gentbl. Bakt. 11: (1903); Kossowicz,- Boden- 

 bakt., pp. 11-15, 54-74 (1912); von Wolzogen-Kuhn,- Arch. 

 Suikerind. Mederland. Indie 23: 501-511 (1915); Lipman 

 in Marshall,- l.iicrobiol. , pp. 358-362 (1917). 



3. See the works on soil bacteria referred to 

 below (page , note ) also: Krliger,- Ein Beitrag 

 zur Untersuchung der Stickstoffumsetzung im Boden, Inaug. 

 Diss., Konigsberg. 1908. 



