Relationships among soil pH, redox potential, and ferrous iron content data obtained from
various field and lab methods were determined, with the following 4 objectives:
1.) Compare pH values obtained in the field to those obtained in the lab
2.) Evaluate different methods to measure pH in field
3.) Determine the relationship between Eh and field pH
4.) Determine Eh threshold at which iron reduction occurs
Statistical analyses showed there was a significant difference between field and lab pH methods. Further analysis using orthogonal contrasts revealed no significant difference among the field methods.
The values obtained from objectives 3 and 4 were compared to previously accepted
literature values. We determined the Eh-pH relationship value was -136 mV per unit of pH. This value lies in between the empirical value of -59 mV per pH unit and the theoretical value of -177 mV per pH unit. There is currently a debate among soil researchers as to which of the values to utilize when normalizing Eh readings to pH 7. Therefore, a value specific to the soil
being measured, as we calculated, is very beneficial.
The iron reduction threshold was found to be lower than the previously reported accepted
range of +300 to +100 mV before and after being normalized to pH 7. The values were found to be +275 mV before pH normalization and +130 mV after pH normalization.
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