Low-Cost Sensors for Soil Moisture Measurement: Modeling and Characterization

An analysis of the impedance spectra in the [20 Hz, 10 MHz] range of low-cost sensors for soil moisture, with different soil types (i.e., clay, loam, and clay with the addition of fertilizer) and moisture levels (i.e., volumetric water content 0%, 5%, 10%, 20% and 30%) is presented. Two sensing approaches are investigated, using prototype resistive sensors and a commercial capacitive sensor. The obtained results demonstrate that all the measurements are affected by the soil conductivity and thus vary in accordance with the soil type hence, for accurate measurement, the sensors must be appropriately calibrated according to the soil under test. Moreover, the resistive sensor is proved to have satisfactory sensitivity in a greater frequency range with respect to the capacitive device, and this paves the way for possible usage with low-power microcontrollers for the creation of low-cost embedded systems.