SOIL MOISTURE DETERMINATIONS USING CAPACITANCE PROBE METHODOLOGY

Abstract : Determining soil moisture content by measuring the dielectric constant of the soil is not a new concept. However, determining the dielectric constant by measuring capacitance directly rather than through the use of time domain reflectometry (TDR) systems is a relatively new approach to soil moisture measurements. A unique probe assembly and a readout device that measures voltage drop and phase shift were developed and used for direct capacitance measurements. The capacitance measurement was calibrated using known capacitors and resistors. Soil moisture measurements were calibrated by adding known amounts of distilled water to dry soil enclosed in a known volume. The effect of salinity on the measurement technique was evaluated. Once calibration had been accomplished, actual soil moisture measurements at three test depths through an entire winter's freeze-thaw cycle demonstrated the feasibility of using this capacitance measurement system. The dielectric constants measured using this fixed-frequency capacitance measurement system fall within the same general range as the values obtained using TDR equipment with the Topp or Roth general calibration equations, and they could probably be used directly in these equations after minor corrections. The conclusions drawn from these tests are that this measurement technique could and should be developed as an easier, more economical, and more easily automated and calibrated system for soil moisture measurement.