SOIL MOISTURE SENSORS Walk the TIGHTROPE
Using soil moisture sensors correctly helps maintain plant health with optimal water use. By Mike Mills
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s soon as Archimedes built the first screw pump to bring water into the Egyptian irrigation canals, the burning question for the farmer was,
“How much water should go on the field?” To this day, scientists and engineers continue to research the water needs of agriculture plants for optimum yield. Maintaining the necessary water available in the soil is critical to success.
It’s like walking a delicate tightrope when irrigating, applying just enough water to fill the soil to field capacity without causing deep percolation or runoff. While knowing the “sweet spot” has traditionally been more art than science, modern moisture monitoring devices can find that optimum soil moisture and reach it without overwatering.
UNDERSTANDING SENSORS
Soil moisture devices can be divided into three basic types: volumetric, tensiometers and solid state. Volumetric sensors directly measure the amount of moisture in the soil and are primarily used in research applications. These devices are generally the most accurate, but they can be very expensive and require a lot of calibration at installation. Tensiometers measure the amount of tension between the soil and water particles. The higher the tension, the more difficult it is for plants to extract water. These devices are relatively inexpensive and accurate but require a lot of maintenance throughout the season. Solid state devices work by measuring the electrical conductivity of a soil and calculating water tension to determine the amount of water available to the plant. These are typically the
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most affordable sensors and are generally easy to install, but they can lose accuracy in high salinity soils.
Installing a soil moisture sensor is the equivalent of adding a “water gauge” to a field indicating when that field is “full” (field capacity), “empty” (wilting point) and the current water level in between. Knowing this information allows the irrigation operator to accurately calculate exactly how much water is required, if any at all. With this powerful tool, operators can control pumping costs while reducing or eliminating nutrient leaching and also increasing both yield and quality.
The financial benefits from proper soil moisture monitor utilization can be significant. With the high cost of energy and maintenance for pumping plants, reducing water events throughout the season can return thousands of dollars to the operator in terms of saved expenses. In addition, by maintaining a proper soil moisture level, field crops often see an increase in both yield and quality. One study of an actual production operation measured “side-by-side” performance of two fields with similar soil composition, the same seed varieties and plant populations, and identical cultivation practices. One field was watered using the growers’ traditional irrigation scheduling methods, the other field followed the irrigation recommendations of the moisture sensor. At the end of the season, the field scheduled using soil moisture monitoring saw irrigation costs reduced by $19/acre while also increasing revenue by $33/acre for a total positive impact of $52/acre.
While knowing the “sweet spot” has traditionally been more art than science, modern moisture monitoring devices can find that optimum soil moisture and reach it without overwatering.
Winter 2024 | Irrigation TODAY 25
Photo: Mike Mills
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