Nitrate concentrations in the root zone estimated using time domain reflectometry

Improved ability to monitor soil water and ionic chemical distributions in field soils will contribute to better land management opportunities, We evaluated the potential to use time domain reflectometry (TDR) to simultaneously estimate volumetric soil water content (theta), soil solution electrical conductivity (sigma(w)), and soil nitrate concentrations in an irrigated peppermint (Mintha piperita L.) field using simple models and calibration methods. Two models and three calibration approaches for estimating sigma(w) from TDR were compared to sigma(w) obtained from soil cores and soil solution samplers at three depths, and over a wide range in applied KNO3. The applied N rates were well within those commonly used in peppermint production. Soil nitrate concentrations were estimated based on TDR sigma(w) predictions and a simple linear calibration of sigma(w) vs. nitrate concentration and then compared to independent soil core measurements. Estimates of sigma(w) and nitrate concentrations using TDR exhibited similar pattern, magnitude, and variance to those based on direct soil measurements, The Mualem and Friedman (1991) model (MF), which is based on soil water retention measurements, provided sigma(w) and nitrate estimates in better agreement with soil cores than did the Rhoades et al, (1976) model. Our results suggest that simple physical-conceptual models combined with laboratory or field parameter estimation methods may. be suitable for automated real-time field monitoring.