2018 ASHS Annual Conference

Irrigation systems are designed to maximize crop productivity and optimize uniform water application. The amount of water applied is usually determined by empirical methods, which are based on timers instead of the actual crop requirements. Several technologies have recently been developed looking for alternative methods to improve water management efficiency based on weather and soil sensing methods. One of the most relevant advances are the capacitance sensors, offering a great potential to estimate soil volumetric water content (VWC) and electrical conductivity. We conducted a laboratory study to evaluate the accuracy of data collected from several commercial capacitance sensors and establish a calibration equation for different soil types. Tested treatments were five sandy soils (Pineda, Riviera, Astatula, Candler and Immokalee) divided in two depths (0-30 and 30-60 cm) representing the majority of Florida soils used for citrus production. We also tested a soilless substrate (peat:perlite 80%:20% v/v). Each sample was oven dried and placed into 5-gal buckets, with three replications. Readings were taken using thirteen capacitance sensors from different manufacturers [CS650, CS616, CS655 (Campbell Scientific), GS3, 10HS, 5TE, GS1 (Decagon Devices), TDT-ACC-SEN-SDI, TDR315, TDR315S, TDR135L (Acclima) and Hydraprobe (Stevens)] connected to a CR1000X datalogger (Campbell Scientific). The bulk density of each soil was determined by following ASTM D7263 standards and the specific gravity according to ASTM D854 standards, defining the reference for further calculations related to VWC. Known amounts of water were added incrementally to obtain a broad range of VWC values until reaching the saturation point; the soil was dumped into a larger container, thoroughly mixed and put back in the original containers. The measurements were performed by inserting the sensor in the middle of the sampling container. Small 450-cm³ samples were taken and dried in an oven at 75°C for 48 h to determine the gravimetric water content. Gravimetric values were multiplied by the bulk density to determine the VWC used to obtain the soil-specific calibration equations and compare the sensor accuracy. Results indicated that factory-supplied calibration equations performed well for some sensors in sandy soils, especially 5TE, TDT-ACC-SEN-SDI and GS1, with higher correlation between the sensor readings and the determined VWC; that was not the case for 10HS, GS3 and Hydraprobe. The sensor readings were measured successfully in increasing VWC values, establishing soil-specific calibration equations for Florida soils. Those results allow the improvement of sensor reading accuracy, optimizing irrigation scheduling and water management in Florida citrus production areas.