Low-Cost and Low-Frequency Impedance Meter for Soil Water Content Measurement in the Precision Agriculture Scenario

The use of low-cost embedded devices with ubiquitous connectivity systems is a possible strategy to monitor the consumption of water used in agriculture even in countries that do not have good economic resources. In this article, we propose a low-cost system based on a measuring approach of capacitive soil water content (SWC) sensors never explored previously. This solution takes advantage of the sensor properties to respond to the need to measure at the same time salinity and soil volumetric water content. For this purpose, a bifunctional system based on a modified commercial capacitive sensor and an AD5933 impedance converter integrated circuit (IC) is described. The considered frequency range for impedance measurements is 10–100 kHz, which is a “low-frequency” range compared with typical operating frequencies of professional systems devoted to water content measurements. Measurements are carried out to check the system accuracy and to characterize the impedance of the sensor both in the air and in water, which represent the two boundaries of possible operating conditions in physical soils in terms of the real part of the electrical permittivity. Variations of the imaginary part of the electrical permittivity are taken into account using water with conductivity between 1 and
$2300~\mu \text{S}$
/cm. Experimental results are compared with the measurements obtained by using a laboratory inductance in honor of Heinrich Lenz, capacitance and resistance (LCR) meter, and a maximum error of +6.12% for the capacitance and +5.6% for the conductance is obtained. The proposed system is also employed to characterize silica sandy soil with different water contents and conductivities showing quite promising results useful to overcome the well-known limits of low-cost SWC sensors.