by Larry Schwankl
Professor of Hydrologic Science
Irrigation and Drainage Specialist
Professor of Environmental Horticulture
University of California, Davis
Drought Tip 92-39 is a publication series developed as a cooperative effort by the following organizations:
California Department of Water Resources - Water Conservation Office
Department of Land, Air and Water Resources University of California
USDA Drought Response Office
USDA Soil Conservation Service If you have comments or suggestions, please email firstname.lastname@example.org.
Last reviewed December 19, 2002
Drought Tip 92-39
Field Use of Soil Moisture Blocks
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.
The number of stations required depends on soil uniformity and management. For areas up to 40 acres, at least two stations should be established. Stations should be in areas representative of overall soil moisture conditions, with separate stations established for problem areas or for areas with different soil conditions. Areas with different crops should be monitored separately since water use and root growth differ from crop to crop.
At least one soil moisture block should be installed in the zone of greatest root activity at approximately one-quarter to one-third of the root depth. A moisture block at this depth should be monitored for scheduling irrigations. It is also recommended that an additional block be installed in the bottom one-third of the root zone. This block will indicate when deep soil moisture is being depleted. If the block reading at the lower depth remains unchanged following an irrigation or continues to indicate that the soil is getting drier during the season, irrigation applications may be insufficient. Private consultants using gypsum blocks for irrigation scheduling often install blocks at one-foot increments within the root zone.
Soil moisture blocks are installed by augering a hole to a depth slightly greater than the desired monitoring depth. A small quantity of a fine mixture of soil and gypsum is added to the bottom of the hole to provide good contact between the soil and the block. A small amount of water is added to the hole to soften the soil at the bottom. Before being placed, the block is immersed in water and checked with the meter to ensure that a reading can be obtained. The wire lead from the block is run through a length of pipe (1/2" CLASS 125 PVC) with a slight tension maintained on the wire so that the block will be held to the end of the pipe. The block can then be pushed into the moist soil at the bottom of the hole. After the block is inserted, the pipe is carefully removed and the hole is backfilled, first with a mixture of gypsum and soil and then with the soil removed from the hole. Care should be taken to not damage the wire leads and to ensure that the backfilled soil is well packed into hole. (A broom handle works well for this purpose). Block depth should be identified with a surface tag or knots in the wire. Blocks to be placed at shallower depths can be installed in the same hole or in a separate hole, using the same procedure.
What Do the Readings Mean?
The manufacturers of soil moisture blocks and meters furnish guides to the meter readings and soil water status. Since each manufacturer’s block/meter combinations read differently, it is not possible to specify here the meter reading at which irrigation should occur.
Soil moisture blocks indicate when soil moisture is being depleted and the depth to which irrigation water is penetrating, but they do not reveal how much soil moisture needs to be replaced unless they are calibrated to the site.
Soil moisture blocks are relatively maintenance-free. The life of a moisture block is one to three years, depending on soil moisture conditions. Constantly wet soil will cause a gypsum block to dissolve more quickly and thereby shorten its life. Longer life can be expected for moisture blocks made of porous materials that do not dissolve. The wire leads may corrode and require periodic scraping to ensure good contact between the wire and the meter.
Cost and Suppliers
Soil moisture blocks cost between $6 and $12, depending on the manufacturer. The cost of the detachable meter for reading the blocks ranges from $150-$400. Following are manufacturers of soil moisture blocks and meters:
51 Indian Lane East
Towaco, NJ 07082
Irrometer Co., Inc.
P.O. Box 2424
Riverside, CA 92516
Soil Moisture Equipment Corp.
P.O. Box 30025
Santa Barbara, CA 93105
3231 Riverside Blvd
Sacramento, CA 95818