REDOX PROCESSES

The oxidation-reduction processes exert a very important control on the natural concentrations of Oxygen, Iron, Methane, etc... in the rhizosphere; they also serve to determine the levels of Nitrogen leaching.

Redox processes take place through the transfer of electrons from one atom to another. The order in which they occur can be predicted from thermodynamic equilibrium dynamics. These processes are usually very slow, unless mediated by microbial catalysis. An example is the reduction of sulphates from organic matter, which would be very slow were it not for microbial action, Desulfovibrio sp, which through enzymatic action catalyses the process and the reaction can occur rapidly in natural environments. 

These redox processes require oxidising agents such as oxygen or nitrates. However, the action of reducing agents such as organic matter dissolved in the pore water of our greens profiles also leads to these processes.

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An example of these processes can occur with Fe+2 and Mn +4, as follows:

2 Fe+2 + MnO2 + 4H+ 2 Fe+3 + Mn+2 + 2H2O

In this reaction, two electrons are transferred from Fe(2) to reduce Mn(4) in MnO2. The ferrous ion acts as a reductant and reduces Mn(4), while MnO2 can be called an oxidant and oxidises to Fe+2. Electrons are exchanged between the atoms and do not exist as "free" in the pore water solution.

The measurement of Eh (V), the electrochemical potential determines the distribution of all redox equilibria, in much the same way as pH expresses the distribution of all acid-base equilibria.

The measurement is made through an inert platinum electrode against a standard electrode with a known potential (Grentheet al., 1992 and Christensen et al., 2000).

Thus, we can see different processes depending on the measurements taken:

Oxidised Species Reduced Species mv mv Dissolved Oxygen (ppm)

O2 H2O +750 +800 12 O2 H2O +750 +800 12 Oversaturation

NO3- NO2- +450 +400 4.0

NO2- NH3 +400 +350 0.4 Denitrification

Fe+3 Fe+2 +300 +200 +200 0.1 Ferrous reduction

SO4- S2- -150 -200 0.0 Sulphate reduction

CO2 CH4 -250 -300 0.0 Methanogenesis

The use of ozone injected into the irrigation system maintains the profile of the greens in aerobic conditions, a well-developed rhizosphere is enhanced, oxygen consumption by biofilms is neutralised and all sports surfaces show greater uniformity.

The redox potential of water can be known with very affordable instruments such as: