WET-SENSOR PORE WATER EC CALIBRATION FOR THREE HORTICULTURAL SOILS

The WET-sensor is a frequency domain dielectric sensor that measures permittivity, conductivity and temperature, which can be used for monitoring soil water content and electrical conductivity in horticulture. By using a specific model it measures pore water conductivity as well. However, under practical circumstances, large errors have been observed using the existing model. The purpose of this study was to obtain a empirical correction for this model and to investigate the nature of the errors, so horticultural growers can use the WET-sensor to measure pore water EC in-situ. Using 36 potted samples of sand, clay and peat, the relation between permittivity, bulk EC and pore water EC was studied in a laboratory over a temperature range from 19-22 oC and a pore water EC range from 4 – 12 mS/cm. This resulted in an adaptation of the constant in the existing model and an extra second order polynomial correction factor. After temperature correction, an overall linear fit with a coefficient of 0.9948 was found at a reasonable correlation (0.79). Errors may occur up to ±40%, but for permittivity values larger than 20, errors have a maximum of ±15%. The overall standard deviation is 15%, which is in accordance with simulated results. For growers using relatively wet soils at water contents above 20%, the WET-sensor can be used to measure pore water EC in situ with a reasonable accuracy. Further study is needed to explore this correction at larger temperature ranges and for more soil types.