Oxidation-reduction potential, an essential for managing healthy soil
Oxidation-reduction potential (ORP or “Redox”), is an essential process for living organisms. It is also identified as “Eh” in the literature. In agricultural soil it is defined by the availability of electrons that can be transferred from one entity to another. An entity donating electrons is being oxidized; an entity gaining electrons is being reduced. Changes in Redox through the soil profile can have substantial effects on the soil, soil organic matter, soil microbes and plants.
The Redox state of agricultural soil can be influenced by moisture content, microbial activity, nutrient presence and plant exudates. To an extent Redox levels control/signal other processes in a plant, and Redox can be modified by plants and microbes.
Redox is a measure of oxygen levels
Positive Redox indicates high oxygen levels which favor oxidation reactions, while low or negative Redox indicates low oxygen and favors reduction reactions. Redox is an indication of how much oxygen is in the soil. When oxygen is high, there are a limited number of available electrons in the soil, and subsequently low available energy (Oxidized soil). When oxygen is low, then electron levels and thus energy are high (Reduced soil).
Redox is not similar in all soil layers surrounding roots. When sunlight is plentiful, photosynthesis is active in the plant, producing energy as glucose which can lead to an abundance of electrons and a reduced environment in the soil in layers reached by plant roots.
Redox and nutrient cycling
The balance of low/high Redox is essential for nutrient cycling. In the case of nitrogen, the nitrogen fixation process, a reduction by which microbes convert atmospheric nitrogen (N2) to ammonium (NH4+) can only happen at very low Eh, while Nitrification of ammonium to nitrate is an oxidation process that occurs at high Eh. If there is sufficient microbial activity then a harmonic balance might be achieved in the soil between these two processes. It is essential that neither low/high Eh state becomes extreme; high Redox is very low energy, while at low Redox the plant may asphyxiate.
Nitrate or ammonium uptake
Nitrogen uptake by the plant has an impact on Redox in accordance with whether or not the plant uptakes ammonium or nitrate. If ammonium, then the plant will release hydrogen ions (H+) to balance the charge; if nitrate is the uptake, then Hydroxide ions (OH–) are released. In the former case, Redox is lower, the soil is acidic and will be high energy, for the latter the soil is oxidized, alkalinized and low energy. Plants that uptake nitrate must convert to ammonium in order to process it into amino acids and other plant components, which takes energy that could be used elsewhere. This argues that applying ammonium as fertilizer when necessary is more beneficial, however, an active microbial population can quickly convert ammonium to nitrate through the nitrification process, and while both ammonium and nitrate are soluble in the soil, nitrate is far more mobile and accessible by plant roots.
In oxidized soil, the low energy environment tends to invite pathogens that attack the plant and the roots. The high redox environment can generate Reactive Oxygen Species (ROS) that will attack critical proteins and other entities. Some plants will actively acidify soil by releasing protons (H+) or organic acids in some cases.
Moisture can have an impact on Redox in the soil, especially when a large rainfall or flooding event causes a waterlogged condition. Oxygen levels fall as oxygen is unable to penetrate saturated soil layers, and aerobic microbial populations are reduced. Negative Redox is not uncommon in these situations, and Redox will not begin to rise until moisture evaporates into the atmosphere or percolates into the Vados layer.
Traditionally pH, which measures the availability of protons (H+) has been thought to be responsible for maintaining soil balance and control. In recent years Redox has received more attention as a signalling agent in the soil controlling or instigating a variety of processes. There is an argument that pH receives more attention because it is easier to measure reliably in the soil.
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