So I've now taken just one core each at S9 and S15 (the only sites with Qsoil sensors). The results were: EC-5 67%/65% and Gravimetric 90%/89%, which could be interpreted as the EC-5 having a bias of 23%. A bias or gain change is plausible, since we are in saline conditions, though I don't know if the marsh exceeds the EC5 salinity operating range (8 dS/m for mineral soils, but this is far from a mineral soil!). ). (Seawater is supposed to be 54 dS/m, so 8/54 = 15% seawater certainly could be exceeded in the marsh.) I'm actually impressed that the two EC-5 probes read as close as they do, that the two gravimetric samples were close, and that the higher EC-5 reading was associated with the higher gravimetric reading. This is especially true given the difficultly in obtaining a gravimetric core, with lots of sample handling issues.
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