Gypsum Block Soil Moisture Sensor 

A soil moisture block, often referred to as a gypsum block or electrical resistance block, is a device for measuring soil moisture changes. The soil moisture block consists of two electrodes mounted in a small block of porous material. Wires are attached to the electrodes.

The sensor is buried in the soil at the depth to be monitored and the wires extend to the soil surface.

Once installed, the block’s moisture content comes into equilibrium with the surrounding soil moisture. A meter is attached to the wires at the surface and an AC current is passed through the block. The electrical resistance measured by the meter is related to the soil moisture surrounding the soil moisture block. The wetter the soil (and the block), the less resistance to the passage of the current. The soil moisture block therefore indirectly monitors soil moisture by measuring electrical resistance.

Most soil moisture blocks are not as sensitive as tensiometers in wetter soils and may not be appropriate for use with drip irrigation systems where soil moisture is kept high by frequent irrigations. Soil moisture blocks will provide information on a broader range of soil moisture levels than a tensiometer. This may be important for heavier textured soils (loams and clay loams) that are irrigated less frequently.

gypsum block measure soil moisture

The amount of water in the soil is determined by the electrical resistance between the two electrodes within the gypsum block. More water present in the soil will reduce the resistance, while less water will increase it.