Interpreting manual readings in soil
Manual readings: field situation
The readings the researcher sees when using the manual way of reading the potential will change in time. Sometimes they even drift while the researcher moves a leg, or connects another reference probe. Several authors have written about this drift in time. The variability in space , both horizontal and vertical will also contribute to changing potentials during reading in the field.
The installation of the probes into the soil can also be difficult. When gradients will exist, the exact depth is important. With depths of the gradients of several centimeters, the placement of the electrodes needs to be very precise.
The general solution to these issues is to generalise the results. One can for instance group the values into classes, and focus on differences between classes or treatments. This is however generating some loss of information. And, if one wants to compare to previous research, one will still need to have the single values obtained in the field.
The most obvious classes to use are those related to the chemical transitions under study. For instance, when one is interested in the reduction of NO3, the redox values related to this process can be taken, and then three classes will emerge: to high, correct, and too low. This has the great advantage that there is no true focus on the actual value, but on redox potential measurement as the proxy for the process.
In monitoring for archaeological preservation in-situ, the classes are used to indicate local preservation potential. In many cases, a redox potential class that is low enough should be ok for the preservation. The research that should lead to these classes and their bounderies is still going on, mainly at the Institute for Geo- and Bioarchaeology of the VU University Amsterdam.