Advances in determining soil matric potential using an engineered porous ceramic and dielectric permittivity

Background/Question/Methods Soil water potential is a key parameter for determining water availability for plant growth, water flow, and soil stability.  Although an in situ measurement of matric potential has been the focus of considerable research over the years, existing solutions still have many draw backs such as necessary routine maintenance, limited longevity, individual calibration requirements, high cost, and small measurement range.  The objective of this research was to develop a sensor that could be used in the field to accurately measure soil water potential without the limitations noted above.  We developed a sensor consisting of a dielectric sensor sandwiched between porous ceramic.  When porous material (ceramic) is put in contact with the soil, water will flow into or out of that material until it reaches equilibrium.  At equilibrium, the energy state of the water (water potential) in the ceramic and in the soil are equal.  As with any porous material, ceramic has a unique, static relationship between the amount of water in the matrix (water content) and its water potential, called a moisture characteristic.  The sensor, named MPS-1, measures the water potential of the soil by equilibrating a ceramic matrix with the soil, measuring the dielectric permittivity of the ceramic to find its water content, then calculating the water potential through the moisture characteristic relationship.  Instead of converting the sensor output to dielectric and then water content, correlations are made directly between sensor output and water potential. The MPS-1 was tested over a range soil types, electrical conductivities, and temperatures to calibrate and characterize its output.  Sensors were packed into saturated soil on 1 and 5-bar pressure plates and allowed to equilibrate for at least 48 h at a variety of pressures.  Two soil textures (sandy loam and silty clay loam) were tested to ensure sensor calibration was constant in differing soil types.

Results/Conclusions Data show consistent calibration curves between sensor output and actual soil water potential over a variety of soil textures and electrical conductivities. Although temperature showed an effect on sensor output, it was small compared to overall sensor output.  Likewise, salt effects were not visible in saturated matrices up to 10 dS/m. Data suggest the sensor will be an effective and robust tool to determine in situ soil water potential.