Using Soil Water Sensors

  • Maintaining adequate soil moisture is needed, especially during critical crop growth stages to help achieve maximum yield potential.    
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  • Soil water content measurements with water sensors can be used as a guide to determine the timing and amount of irrigation.   
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  • Soil moisture measurements can be used in combination with ET-based irrigation scheduling. 
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The goal of irrigation management is to use water in the most profitable way, while maintaining yield potential at sustainable production levels. Irrigation needs to be applied only when soil moisture measurements and crop growth stage warrant water inputs, and with a method that limits any waste or excess.

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Checkbook Method

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Using Soil Moisture Sensors

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Evapotranspiration (ET) soil water estimates should be periodically validated using soil moisture sensors. Comparing predicted soil moisture with actual soil moisture measurements every couple of weeks is a good strategy to confirm the ET-based scheduling method is accurate.

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Soil moisture sensors can be placed at differing depths in the crop root zone to provide a direct measure of changes in soil water content. This can help determine when irrigation might be needed.

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Examples of some common soil moisture sensing instruments include (Table 1):

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  • Tensiometers are most suitable for sandy or loamy soils. They measure soil water tension or suction. As soil dries, water is pulled from the porous cup at the lower end of a tube inserted in the soil, creating a vacuum and causing the vacuum gauge on the top of the tube to move.
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  • Electrical resistance blocks, also known as gypsum blocks or soil moisture blocks, measure the change in electrical resistance during wetting and drying cycles of the soil. Water moves into or out of the soil-buried blocks, depending on the moisture content of the soil. This creates a change in the resistance between the two electrodes that are embedded in the gypsum block.
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  • Capacitance probes and frequency domain reflectometry (FDR) measure the soil dielectric constant using two or more plates or rods, which are embedded into the soil. 
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  • Time domain reflectometry (TDR) and time domain transmissometry (TDT) soil moisture sensors measure a soil’s dielectric constant.   
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Taking Accurate Measurements

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For representative readings, measurements should be taken from representative soil types, within the active crop root zone, and away from high spots, depressions where water may collect, and slope changes. Soil water content may vary greatly within a field, so multiple measurements throughout the field and at appropriate depths will be required to reduce errors. A greater number of readings will be needed with soil moisture sensors that measure smaller volumes of soil, such as with capacitance probes.1

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When choosing locations in the field to measure soil water, it may be most cost effective and efficient to take measurements in areas where soil and plant properties are most representative of the field.

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The depth to which a particular method can measure soil moisture and the resolution at increasing depths should also be considered. Ideally for irrigation scheduling, soil water should be measured to well below the maximum depth of root water extraction, though this is not always attainable with most soil water measurement methods. Conventional TDR is one method that allows the flexibility of deeper measurements.

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Typical installations include one or more sensors for each foot of active rooting depth. The variability amongst soil samples increases as soils dry, indicating that more measurements will be needed for accuracy as soil water content reaches the Management Allowable Depletion (MAD).2 The latter refers to the amount of water allowed to be depleted from the root zone before irrigation is scheduled. The MAD is usually given as a percentage of maximum water-holding capacity of the soil. At the time of irrigation, the soil water deficit should be less than or equal to MAD. 

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Table 1. Advantages and disadvantages of different methods for measuring soil moisture. 
 Method 
 Advantages 
 Disadvantages
 Gravimetric
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(oven drying)
 High accuracy with increased sampling, direct measure
 Very labor intensive,
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delays to obtain data
 Tensiometers
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(soil water tensions)
 Instantaneous, approximates soil moisture content
 High maintenance, tension breaks, freezing temperatures
 Electrical Resistance
 Instantaneous, increased range, approximates soil moisture content
 Slower response, less sensitive at low moisture, affected by soil salinity
 Capacitance and FDR
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(frequency domain)
 High accuracy, volumetric water content and salinity
 Highly influenced by adjacent moisture/voids
 TDR and TDT
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(time domain)
 High accuracy, volumetric water content and salinity, robust calibration  
 High installation cost, highly influenced by adjacent moisture/voids
Source: Table modified from Crookston, M.A. 2011. Utilizing soil moisture readings in irrigation scheduling. Proceedings of the 23rd Annual Central Plains Irrigation Conference: 47-51. 
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Installation and Calibration

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Sensors must be in direct contact with undisturbed soil in order to provide accurate readings. During installation, damage to roots and soil structure should be minimized and air voids, large roots, rocks, and other obstructions should be avoided.

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All soil moisture sensors should be calibrated in the field for the specific soil type, even if the manufacturer suggests otherwise. Laboratory calibrations of the devices are often made on re-packed soils, where tight soil-access tube contact is ensured and variability in the soil is minimized. These laboratory measurements may not be transferrable to your field, particularly for the Capacitance or other FDR sensors. Field calibration can provide more accurate readings because the sensor is placed in the actual soil to be measured.

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Summary

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  • Water management and scheduling are important issues for  maintaining adequate soil moisture, especially during the critical crop growth stages.
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  • Proper installation is important for accuracy and longevity of the soil moisture sensors.
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  • In-field calibration should be conducted on the sensors to help provide accurate readings. 
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  • Soil moisture measurements and ET-based irrigation scheduling should be used for determining an accurate irrigation amount and proper timing.   
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