A resistivity method whereby gypsum or other porous blocks that contain inbuilt electrodes are buried within the soil. Soil moisture is absorbed into the block forming an equilibrium with soil moisture potential of the surrounding soil. The resistivity of the block can then be measured.
Continuous monitoring of soil moisture status [1].
They are low cost, easy to use sensors and particularly suited to long to deployment with data logging, wireless telemetry etc [2].
Will eventually dissolve (a number of years)
A small measurement sphere
Commercial
1. Johnston (2000)
2. Sikka et al. (2006)