Potential measurements between the first and second electrodes validate ion transport between {HEAD ~ the soil} and {TAIL ~ the first reference electrode} 
Potential measurements between the first and second electrodes validate ion transport between { HEAD ~ the soil } and { TAIL ~ the first reference electrode }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL