Response of Capacitance Probes to Soil Solution Nitrate Concentration

Abstract

Dielectric properties of soil are highly correlated with volumetric water content (VWC) of the medium, but at a relatively low frequency soil salinity has an important effect on permittivity measurements. A laboratory experiment was conducted to understand the potential of monitoring nitrate and chloride ions in soil solutions using capacitance-type soil probes EC-5 and EC-10 operating at frequencies of 70 and 5 MHz, respectively. Dielectric response of soil samples wetted with nitrate and chloride solutions of different concentration were compared at each frequency within the volumetric water content ranging from 0.1 to 0.3 m3/m3. Linear regression models were fitted through data to correlate the actual VWC, concentration of solutions, soil temperature and sensor output. At 70 MHz frequency the sensor response was primarily explained by moisture content for both solutions. Dielectric response of the EC-10 probe to change in ionic concentration was different for each wetting solution. Change in chlorine concentration demonstrated no evidence of having effect on the sensor response, while nitrate solution illustrated that the EC-10 probe is sensitive to the change in nitrate-N concentration within the water content and salinity range tested (from 0.05 to 0.51 dS/m). None of the fitted models demonstrated the statistically significant effect of temperature on dielectric measurements due to the little variation of the temperature (+/-1.5°C) during the experiment.

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“…In soil and agricultural sciences, the conductivity of the solvent saturating the soil indicates the saline content of the soil (Larcher 2003). Specifically, EC measures the ease with which electric current flows through a soil suspension in pure water (Chighladze et al 2009;Kurtulus et al 2009;Nuttall and Armstrong 2010). EC is proportional to the amount of dissolved salts; an EC of 2.0 or greater is considered unsuitable for agriculture.…”