1. The function of the optional colloidal Pt (fresh) coating on the Pt electrode is to adsorb O2 from the soil air, thus effectively forming an oxygen electrode (–OH; ½O2), analogous to the H+ electrode. In contrast, bright Pt electrodes have a tendency to exhibit carry-over (‘memory’) from previous exposure to solutions. Effective Pt electrodes must show reproducible values in a poorly poised system such as distilled water. Electrode limitations are described by James and Bartlett (2000).
2. Silver/silver chloride (Ag/AgCl2) reference electrodes are preferred (Fiedler et al. 2007) to calomel electrodes containing Hg, even though the latter are less prone to purity, thermal or mechanical limitations. Hg poses a much greater health hazard, however, than does the metal used in the Ag/AgCl electrode.
3. During measurement of Eh, it is important to minimise electrode polarisation caused by current flowing through the electrode (Aomine 1962).
4. Daily measurements are often affected by weather conditions, growth of vegetation, flooding and the like (Vorenhout et al. 2004). Human error can include damaged electrodes, a mal-functioning millivolt meter, poor contact between electrodes and the soil solution, etc.
5. The generic assumption that the pH factor is –59 mV/pH unit at 25°C is in accord with the Nernst equation. Slopes of Eh/pH interrelationships, however, are influenced by different soil chemical conditions. For example, Charoenchamratcheep et al. (1987) reported slopes of Eh/pH interrelationship for ASS between -206 and -256 mV/pH unit. It follows that use of the generic pH factor, without supportive experimental measurements, is quite risky. Also, when soils become saturated, the pH of both acidic and basic soils naturally shifts towards pH 7 (Fiedler et al. 2007).