Status of Soil Aeration and Oxidation-Reduction (Redox) Potentials (Eh): There must be an interchange of gases between plants’ root hairs and the soil atmosphere if the plant is to survive. Similarly, there must be adequate interchanges of gases between the soil habiting the plant roots and the free atmosphere above it.
This interchange of gases is necessary for the respiration of higher plants and the decomposition of incorporated organic materials by soil micro-organisms. In these soil respiratory activities, oxygen, O2, is used up while carbon dioxide, CO2, is given off.
Soil aeration is the process of gas exchange in soil that ensures oxygen sufficiency and prevents carbon dioxide toxicity.
A well-aerated soil is one in which gas exchange between the soil air and the atmosphere is sufficiently rapid to prevent a deficiency of oxygen or toxicity of CO2, thus ensuring the normal functioning of plant roots and of aerobic microorganisms.
In a typical well-aerated soil, the number of micropores (water space) occupied by water should not be more than 25%, macropores (air space) also 25%, mineral matter 45%, and the organic materials usually about 5.0%.
Different variations of the percentage composition of these soil constituents give rise to different soil types such as
(i) Coarse textured soils (loamy sand, sandy loam) which are suited to areas of frequent rainfall;
(ii) Medium textured soils (loam, silt loam) have good aeration and are usually of optimum water conditions; and
(iii) Fine textured soils (clay, silt clay) which are easily water-logged but are good in areas with long drought periods.
Oxidation-Reduction (Redox) Potentials (Eh)
Soil is a given situation that has a given potential. That is, the chemical elements in the soil are, at any point in time, in certain oxidation and reduction states depending on the level of soil aeration.
In well-aerated soils, the chemical elements are usually in oxidized forms; for example Ferric iron (Fe3+), nitrate NO -, and sulphate (SO -). In water-logged and poorly aerated soils, reduced forms of such elements are found.
Oxidation Potential is a measure of the tendency for an oxidation reaction to occur. On the other hand, the reduction potential is a measure of the tendency for reduction reaction to occur.
The oxidation-reduction or redox potential (Eh) indicates the oxidation and reduction states of chemical systems in soils.
Redox provides a measure of the tendency of a system to reduce or oxidize chemicals and is measured in volts or millivolts. Positive and high values indicate strong oxidizing conditions while low and negative values indicate that element would exist in reduced forms
Oxidation is the loss of electrons in an atom or ion
Reduction is the gain of electrons by an atom or ion
From the above reactions, oxidation can also be defined as a gain or increase in positive valency while reduction is a decrease in positive valency. Oxidation-reduction reactions involve the transfer of electrons between oxidized and reduced species.
All reduction reactions must be coupled with oxidation reactions to balance the total number of electrons gained and lost.
An oxidizing agent will accept electrons easily and a reducing agent easily donates electrons. Oxygen gas (O2) is a strong oxidizing agent while hydrogen (H2) is a strong reducing agent in the formation of water molecules.