Soil EH can be a difficult parameter to interpret. The Pt electrode measurement may not reflect changes in some species involved in redox reactions, such as the partial pressure of O2. The presence of Mn, Fe oxides, and nitrates does not have the expected quantitative effect on the Pt electrode measurement. Methane, bicarbonate, N2 gas, nitrate, and sulfate are not electroactive, i.e., they do not readily take up or give off electrons at the surface of the Pt electrode. Since it is a measurement of potential, the Pt electrode also responds to changes in pH and other potentials. Often two or more redox reactions occur simultaneously, thus measured Eh usually reflects a mixed potential.

Nevertheless, platinum-electrode EH measurements are still useful and can be interpreted as a semiquantitative assessment of a soil's redox status. In studies of paddy soils, for example, EH measurements can be used to monitor progressive development of reducing conditions and can distinguish oxic and anoxic conditions.

Rice fields provide a unique aerobic and anaerobic environment to study the relationship between soil EH and greenhouse gas emissions because of controlled irrigation and drainage practices (Fig. 2.7).

During the flood season the paddy soils are a major source of CH4, but an important source of N2O when they are drained. Strategies designed to mitigate CH4 emissions from submerged rice fields can adversely affect greenhouse warming potential by stimulating higher N2O emissions. The different EH conditions required for N2O and CH4 formation and the trade-off pattern of their emissions as found in rice fields make it a challenge to abate the production of one gas without enhancing the production of the other. Figure 2.7 shows the redox window offering the minimum global warming potential contribution from rice soils.

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