Seasonally saturated soils in the puget lowland ii. measuring and interpreting redox potentials

Redox potential measurements can provide insight into the relationships between saturation and color patterns in seasonally saturated soils. Their value is limited, however, because there are no widely accepted methods to measure and interpret field redox potentials. We conducted this research to determine which measurement and interpretation methods are most appropriate for field use in seasonally saturated soils. Redox electrodes were constructed by soldering Pt wire onto a Cu lead, encasing the junction in an acrylic tube, and sealing with epoxy. Redox potentials were measured at three sites in western Washington using permanently installed soil electrodes and at one site using temporarily installed ground water electrodes. Redox potentials were also measured in laboratory soil cores to compare the effects of microsite environments vs. electrode differences on apparent redox variability. Finally, exchangeable Fe(II) was measured in extracts from soil core samples at different redox potentials to estimate the redox environment needed for the onset of Fe reduction and soil mottling in the sample soils. We found that properly constructed soil electrodes responded to changes in the redox environment and were reliable for at least 1 year. Variability among electrodes was often large in changing or intermediate redox environments and was due primarily to microsite differences in the soil. Ground water redox measurements were of little value, because the shallow ground water in the soils we studied was generally oxidizing, even when the soil was reduced. Ground water dissolved O2 combined with soil redox measurements give the most complete picture of the soil-ground water redox environment. In the soils of this study, exchangeable Fe(II) was present at ≤+200 mV (pH 6–7) indicating conditions suitable for Fe reduction and mottling.