Using continuous soil and plant water status sensors calibrated against stem water potential for irrigation scheduling – nectarine as a test case

Abstract

This study investigated the use of on-site calibrated soil and plant based sensors for irrigation control in a nectarine (Prunus persica), orchard on heavy clay soil. Irrigation was applied according to predetermined mid-day stem water potential (SWP) thresholds for each of the three phenological stages of fruit development. Calibration of sensors relative to SWP was done in a drying and wetting plot in a separate part of the orchard. Irrigation was withheld in that plot for several periods of time during two seasons, allowing SWP of six nectarine trees to reach values of moderate water stress. The readings of continuous tensiometers, soil volumetric water content sensors and dendrometers were regressed on those of SWP at that time. Water stress thresholds were then calculated from the regressions and subsequently used for irrigation scheduling. The irrigation aimed to keep non-limiting conditions in stages Ⅰ and Ⅲ by keeping SWP at ~-0.9 MPa, and moderate water stress in stage Ⅱ by keeping SWP at ~-1.5 MPa. Adjustments were made weekly in five treatment plots for each sensor type, according to the thresholds. Results showed that tensiometers could be used for stages Ⅰ and Ⅲ, as they were highly sensitive to small changes in soil wetness. However, when stress was applied soil water tension exceeded the range of the tensiometer sensors at 30 cm depth.