The non stress and stress limits of soil water tension vary with soil type, root distribution and climate. Gypsum blocks consist of two parallel electrodes embedded in blocks of gypsum . Their shape varies from rectangular to cylindrical and they are about matchbox size. They have a pair of wires connected to the electrodes extruding from the block that are about 2 m long. There are several manufacturers of gypsum blocks.
Gypsum blocks are one type of resistance block.
Gypsum blocks are buried in the soil to achieve good contact with the soil. The moisture content of the block varies with soil moisture tension. As the soil dries out, water is drawn from the block. When the soil becomes wet, water is drawn back into the block. Water is a good conductor of electricity and the more water present in the soil, the lower the resistance to electricity passing through it.