By Davie Kadyampakeni, Sandra M. Guzmán and Ajia Paolillo
Using soil moisture sensors for irrigation scheduling can reduce water stress in citrus trees. Sensors that are easy to calibrate and maintain, specifically designed for managing irrigation and fertigation on Florida’s sandy soils, are the preferred choice. These tools enable growers to use their water resources wisely and efficiently.
There are at least seven different operating principles used by various brands of electronic soil water sensors: time domain reflectometry (TDR), time domain transmission (TDT), frequency domain reflectometry, amplitude domain reflectometry, phase transmission, tensiometer and resistance granular matrix sensors. The suitability of each sensor depends on the cost, accuracy, response time, installation, need for calibration, management, maintenance, durability and soil type.
For Florida sandy soils, sensors that require minimal soil disturbance are preferred. TDR and TDT sensors are more ideal due to low maintenance requirements, high degree of accuracy and low need for calibration. Sensors are typically operated by battery power, and some also utilize solar panels. When deciding on a location, be sure to find an area with adequate light coverage if using solar-operated sensors.
Some sensors may have a built-in rain gauge. This is beneficial to determine amounts of rainfall in a particular area and if the precipitation is causing the change in soil moisture, especially if you do not have a weather station near your grove.
When installing sensors in the root zone, it is important to place them within a 1- to 1.5-foot radius of the irrigated zone to make sure the sensor is reporting accurate information. When the sensor is installed outside the irrigated zone, one can overestimate the need for irrigation by assuming that soil moisture is low and needs to be replenished. For proper irrigation planning, install sensors in the active root zone, which is typically within the top 12-inch soil depth.
To prevent nutrients from leaching beyond the root zone, it is important to install TDR or TDT sensors at depths of 18 or 20 inches (or even deeper, if possible) to detect any shifts in soil moisture. If using a soil moisture probe, having one at 24-inch depth might suffice. Elevated soil moisture below 18 inches indicates a higher likelihood for nutrient leaching, underscoring the need to fine-tune the irrigation rate. For better accuracy, it is preferred to install the sensors after a rain event or in a well-watered soil to reduce the influence of air gaps.
Get guidelines on how to install soil moisture sensors here.
Davie Kadyampakeni and Sandra M. Guzmán are University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) assistant professors. Ajia Paolillo is a former UF/IFAS multi-county citrus agent.
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