Soil Moisture Sensor Installation
To install electrical resistance type soil water sensors like the Watermark soil moisture sensor or other similar devices follow the steps outlined below along with the instructions given by the respective sensor manufacturer. Sensors should be placed in a field within two weeks after plant emergence.
Soak each sensor in clean water for 1 to 2 hours to remove the air and then allow drying for 4 to 6 hours. Repeat this step two times more. Prior to placing sensors into the soil soak at least five minutes. If the sensors were used in a previous season, evaluate for damage to the wire leads or the sensor surface and discard the sensor if surface looks plugged with soil or damaged.
|Figure 11. Typical locations for soil water sensor sites.|
Select two or more locations for the sensors in the first third of the irrigated field and one location in the last third of the field (Figure 11). All sensors should be located in a representative soil type in the field. Under a center pivot, the sensors should not be installed near pivot wheel tracks or other high traffic areas. The sensors should be placed somewhere between the second tower and the last tower. Each sensor should be positioned within the plant row near a healthy plant in a location with a normal plant population. At each site a sensor should be set 4 to 6 inches below the ground surface and another set at 9 to 12 inches below the surface.
To install a sensor in the soil, first make a hole with a soil probe or auger to a depth a little deeper than desired. To get good sensor contact with the soil, pour a little dry soil and water into the new hole. To position the sensor into the hole, draw the lead wire through the soil probe tube and hold the sensor on the end (Figure 12). Push the probe and sensor into the hole to the desired depth until set firmly. Fill the hole by adding some dry soil and a little water at a time and firm with a tamping stick until the hole is filled.
Mark each sensor site with a colored flag or stake to locate the site easily. Wrap the extra lead wire around the stake. Mark each sensorís wire lead to indicate its depth with a tag or creating one or more knots near the wire end to indicate depth position (for example one knot means shallow and two knots means a deep sensor). At one or two of the locations near the start-up position of the irrigation cycle include a rain gauge with a 2-inch diameter or greater. Also place a marker at the end of each row or along the road to indicate the entrance path to the sensor site.
Wait to start taking readings until 1 to 2 days after installation to allow the added water to equalize with the surrounding soil moisture. Sensors should be read every two to three days and recorded in a notebook or spreadsheet to track the soil water changes in the soil profile throughout the growing season.
|Figure 12. A soil probe is used to install a sensor.|
Remove sensors before harvest using a shovel. Clean soil from sensor surfaces with only water pressure and hang up to dry for use next year.
More information on the Checkbook method and in-field soil moisture monitoring tools can be found in University of Minnesota Extension Service bulletins Irrigation Scheduling Checkbook Method FO-1322 and Irrigation Water Management Considerations for Sandy Soils in Minnesota FO-3875. These are available at local county extension offices.
The research and demonstration studies mentioned in this bulletin were done at or near Staples, Minnesota. They received funding support over the years from the Northarvest Bean Growers Association, Irrigators Association of Minnesota, USDA-NRCS Environmental Quality Incentive Program (EQUP), Agricultural Utilization Research Institute (AURI), Elf Atochem, Dupont, and the University of Minnesota Agricultural Experiment Station.
For further information contact:
Richard Meronuck, (612-625-6290 or
Department of Plant Pathology
Jerry Wright, (320-589-1711 or
Department of Biosystems and Agricultural Engineering
George Rehm, (612-625-6210 or
Department of Soil, Water and Climate
In accordance with the Americans with Disabilities Act, this material is available in alternative formats upon request. Please contact your University of Minnesota Extension office or the Extension Store at (800) 876-8636.