Redox classes - why is a differentiation important.

The post was last updated on 2022-02-23.

The determination of the soil’s redox status is important and impacts a variety of processes of environmental concern:

  • greenhouse gas emission from soils,
  • nutrient availability,
  • pollutant dynamics, and
  • pedogenesis are only some examples.

From a practical point of view, it is not sufficient to state that a soil is reducing or not. It is encouraged to classify and determine redox classes with a more distinct differentiation.

Differentiation of redox classes taken from Dorau 2016. The indices refer to the following references: 1Yu and Patrick 2004; 2Shrestha et al. 2014; 3Rennert and Mansfeldt 2005; 4Matern and Mansfeldt 2015; 5Hindersmann and Mansfeldt 2014; 6Dalkmann et al. 2013; 7Schuth et al. 2015; 8Mansfeldt and Overesch 2013; 9Peretyazhko and Sposito 2005; 10Schieber 2011; 11Morse et al. 1999; 12Picek et al. 2000

As outlined before, oxygen, manganese, iron, and sulfate are redox sensitive compounds in the soils environment. The transformation (reduction) is coupled to the oxidation of soil organic matter and occurs in a predictable sequence.

The sequential reduction sequence is synonymously called reddox ladder.

Table 1: Prominent redox classes in soils at pH 7
Reddy and DeLaune (2008)oxidizing700300
moderately reducing300100
highly reducing-100-300
Zhi-Guang (1985)oxidizing700400
weakly reducing400200
moderately reducing200-100
strongly reducing-100-300
Mitsch and Gosselink (2015)Nitrogen250
Liu and Narasimhan (1989)Oxygen-Nitrogen250100
Sposito (1989)Oxic7
James (1989)Oxic137

Some things are evident from the table above:

  1. The authors label their classes differently (e.g. strongly and highly reducing is somewhat synonym between Zhi-Guang and Reddy & DeLaune) and there is also some overlap between the classes.
  2. Mitsch and Gosselink (2015) or Liu and Narasimhan (1989) do no label the class but link it directly to the disappearance of an electron acceptor or the appearance of its reduced counterpart.
  3. The authors to not entirely stick to EH but James (1989) and Sposito (1989) employ pe. The pe is a measure of the electron activity and can be calculated from the redox potential, which simplifies to:
    • \(pe = \frac{E_{H}}{59}\) where EH is given in mV.

A benefit to employ the redox parameter “pe+pH” is the partitioning of H+ ions from a chemical reaction into those associated with the redox component and those with the acid-base component (Lindsay 1979). However, the factor 59 (called Nernst-factor) has only to some extent justification in soil science because the MnO2-Mn2+ systems features a value of 118 and for the FeOOH-Fe2+ system 177 (Bohn et al. 2001; Mansfeldt 2019). Since EH is a mixed potential derived from a multitude of redox pairs a justification for a particular factor is hard to tell. Either way, the pH must be mentioned when EH data is presented.

The EH should not be considered as a treshold ❗️ because there are other factors such as temperature and pH which impact transformation rates in soils.

Kristof Dorau
Kristof Dorau
Research assistant

My research interests include redox dynamics in soils, environmental monitoring and recently data analysis in R.