CHAPTER XX 



Oxidation Processes in the Soil — Nitrate Formation 



Oxidation-reduction phenomena. Oxidation-reduction processes have 

 commonly been interpreted in terms of oxygen; "oxidation" designated 

 a process whereby oxygen was added to a substance or hydrogen was 

 removed; "reduction" was applied to reactions involving the removal 

 of oxygen or addition of hydrogen. However, in certain processes of 

 oxidation-reduction no oxygen or hydrogen participate, as in the 

 following reaction: 



FeCl 2 4- CI <=> Fe CI, 



Clark and associates 1 were, therefore, led to consider these processes 

 in the light of addition or withdrawal of electrons. Taking the oxida- 

 tion and reduction of iron, the following reactions may be given: 



2 FC+++ + H 2 <=± 2 Fe ++ + 2 H+ 



2 Fe++ + + 20^2 Fe++ + O, 



Ye+++ + e <=± ¥&++ 



When the reaction proceeds from left to right, reduction is taking 

 place; when it proceeds from right to left, oxidation is taking place. 

 In the presence of methylene blue, nitrate or any other substance 

 capable of acting as hydrogen acceptors, oxidation may take place even 

 in the absence of atmospheric oxygen; this enables certain bacteria to 

 live anaerobically. Processes of oxidation can thus be considered 

 either as (1) aerobic processes, in which atmospheric oxygen acts as 

 the hydrogen acceptor or the oxidizing agent and (2) anaerobic proc- 

 esses, in which organic or inorganic compounds act as the hydrogen 

 acceptors, both processes resulting in the liberation of energy. 



In addition to the action of unactivated molecular oxygen as a hydro- 

 gen acceptor, there are at least three ways in which activation of oxygen 

 can occur: (1), by means of iron, which acts as oxygen carrier; 2 (2) by 



1 Clark, W. M., et al. Studies on oxidation-reduction. Public Health Re- 

 ports, 38: 443, 666, 933, 1669; 39: 381, 804. 1924. 



2 Warburg, O. Uber Eisen, den sauerstoffiibertragenden Bestandteil des 

 Atmungsferments. Biochem. Ztschr., 152: 479-194. 1924. 



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