Calibration of a frequency domain reflectometry sensor for humid tropical soils of volcanic origin

Recently, a frequency domain reflectometry (FDR) was developed for measurin g soil water content. It has a multivibrator that sends electromagnetic wav es along its probes, and it measures the frequency of the reflected wave, w hich varies with water content. This FDR sensor has several advantages over time domain reflectometry (TDR); it is less expensive, has a lower power c onsumption, and continuous monitoring of soil moisture at several remote lo cations is easily automated using dataloggers. Our goal was to derive a cal ibration function for the FDR sensor with the following criteria: it should be applicable to soils with high clay and organic matter contents and with bulk densities between 0.7 and 1.1 g cm(-3). We used undisturbed soil samp les to account for the natural heterogeneity in soils. Our results show tha t the calibration functions derived from a three-phase mixing model perform ed better than the manufacturer's empirically derived function for the soil volumetric content (theta) range of 0.45 to 0.70 m(3) m(-3). Separate valu es of the geometry parameter (alpha) and of the specific output period for soil matrix (Per(s)) were established both for the topsoil (0-0.5 m depth) and for the subsoil (>05 m depth). The manufacturer's calibration function underestimated the soil water content by up to 0.15 m(3) m(-3). The three-p hase mixing model uses a physical basis for the derivation of the calibrati on function in that the soil porosity is used for volumetric partitioning a mong soil components. This physical basis renders the calibration function widely adaptable.