Response of the TEROS 12 Soil Moisture Sensor under Different Soils and Variable Electrical Conductivity

In this work, the performance of the TEROS 12 electromagnetic sensor, which measures volumetric soil water content (theta), bulk soil electrical conductivity (sigma(b)), and temperature, is examined for a number of different soils, different theta and different levels of the electrical conductivity of the soil solution (ECW) under laboratory conditions. For the above reason, a prototype device was developed including a low-cost microcontroller and suitable adaptation circuits for the aforementioned sensor. Six characteristic porous media were examined in a theta range from air drying to saturation, while four different solutions of increasing Electrical Conductivity (ECw) from 0.28 dS/m to approximately 10 dS/m were used in four of these porous media. It was found that TEROS 12 apparent dielectric permittivity (epsilon(a)) readings were lower than that of Topp's permittivity-water content relationship, especially at higher soil water content values in the coarse porous bodies. The differences are observed in sand (S), sandy loam (SL) and loam (L), at this order. The results suggested that the relationship between experimentally measured soil water content (theta(m)) and epsilon(0.5)(a) was strongly linear (0.869 < R-2 < 0.989), but the linearity of the relation theta(m)-epsilon(0.5)(a) decreases with the increase in bulk EC (sigma(b)) of the soil. The most accurate results were provided by the multipoint calibration method (CAL), as evaluated with the root mean square error (RMSE). Also, it was found that epsilon(a) degrades substantially at values of sigma(b) less than 2.5 dS/m while epsilon(a) returns to near 80 at higher values. Regarding the relation epsilon(a)-sigma(b), it seems that it is strongly linear and that its slope depends on the pore water electrical conductivity (sigma(p)) and the soil type.