Resistance
Resistance-based sensors contain an embedded electrode in a porous block, such as gypsum or ceramic. The resistance of the electrode is proportional to the moisture content of the block. Resistance of the block decreases as the soil moisture around the block increases. Sensors such as these measure water potential rather than volumetric water content.
Resistance sensors should not be used in soils containing high electrical conductity as the EC changes the resistance of the electrodes in the same way that soil moisture will.
Irrometer resistance-based sensor.