522 PRINCIPLES OF SOIL MICROBIOLOGY 



determined colorimetrically by the use of indigo-sulfonate or other 

 appropriate indicators, or by means of an electrode of a noble metal. 

 By immersing two electrodes in two liquids of different oxidation 

 potential connected with a siphon, a current is formed and oxidation 

 will occur in the solution about one electrode and reduction in the 

 solution about the other. The intensity factor can thus be measured. 6 

 There is a relation between the hydrogen-ion concentration and the 

 oxidation-reduction potential of the cell. 7 



Oxidation processes in the soil. According to the older conception, 

 oxidation of inorganic and organic compounds in the soil consists in 

 the intake of oxygen and the transformation of the soil compounds 

 into higher oxidized forms. According to the newer conceptions, it 

 consists in the change of a substance from a higher potential to a sub- 

 stance of a lower potential, with the liberation of energy. Although 

 some of the oxidation processes in the soil may be purely chemical in 

 nature or may be carried on by plant roots and their enzymes, 8 the 

 predominant role of oxidation in the soil is played by microorganisms. 



Liebig 9 recognized that proper oxidation is essential for the decom- 

 position of plant and animal residues added to the soil. Mulder 10 

 called attention to the fact that oxidation as well as reduction proc- 

 esses take place in the soil at the same time, with a certain equilibrium 

 established between the two. Oxidation processes usually lead to the 

 complete decomposition of organic substances (mineralization) in the 

 soil (so-called "decay"), the oxidation of ammonium salts to nitrites 

 and the latter to nitrates, the oxidation of elementary sulfur and sulfur 

 compounds to sulfates, and the oxidation of hydrogen, methane, and 

 other substances produced by processes of incomplete oxidation or 

 reduction. Oxidation processes may be looked upon as beneficial 

 processes in the soil. Reduction processes may often become harmful, 

 since, with incomplete oxidation, substances may be formed which 

 are directly injurious to plant growth. It is sufficient to indicate 

 that the reduced forms of nitrates (nitrites), of sulfates (sulfites) and 

 of phosphates (phosphites) are toxic to plant growth. 



6 Gillespie, K. J. Reduction potentials of bacterial cultures and of water- 

 logged soils. Soil Sci., 9: 199-216. 1920. 



7 Needham, J., and Needham, D. M. Hydrogen-ion concentration and the 

 oxidation-reduction potential of the cell interior; a micro-injection study. 

 Proc. Roy. Soc. B, 98: 259-286. 1925. 



8 Schreiner, O., and Reed, H. S. The role of oxidation in soil fertility. Bur. 

 Soils, U. S. Dept. Agr. Bui. 56. 1909. 



9 Liebig, J. Chemie in ihrer Anwendung auf Agrikultur. 4 Aufi., 1842. 



10 Mulder. Die Chemie der Ackerkrume 1. 1863, Tr. by Midler. Berlin. 



