Comparison of Galvanic and Chemi-Luminescent Sensors for Detecting Soil Air Oxygen in Flood-Irrigated Pecans

Low soil O2 levels have been shown to limit growth in pecan [Carya illinoinensis (Wangenh.) K. Koch] seedlings and may limit yield in mature trees. To assess changes in the gas-phase O2 concentration in a pecan orchard soil in response to flood irrigations throughout a growing season, two types of O2 sensor were field tested: a galvanic O2 sensor and a spectrometer-coupled chemical sensor (FOXY sensor). Galvanic sensors, housed in diffusion chambers, were buried at four depths and a datalogger recorded continuous voltage output. The FOXY O2 sensor was utilized as part of a mobile O2 detection system to field analyze gas samples withdrawn periodically from buried diffusion chambers. The FOXY sensor was found to be unstable, however, and difficult to calibrate under conditions of changing temperature and humidity. Laboratory experiments simulating submersion of the galvanic sensor indicated that voltage outputs were comparable to the range observed in the field, but the absence of diurnal concentration fluctuations, typically found in soil measurements, provided a way to discriminate between normal and aberrant output. The responsiveness of the galvanic sensor and its capability to continuously gather hourly data makes it superior to methods dependent on manual sample collection. Galvanic sensors were adequately suited for long-term in situ use in agricultural soil when housed in appropriate diffusion chambers. Higher costs, limited access to diffusion chambers during flood periods, and high variability associated with manually collected data make the FOXY mobile O2 detection system comparatively less optimal for use in agricultural settings.