Validation of the suitability of electrical impedance spectroscopy for the in-situ monitoring of nitrate in sandy soil

Abstract:
This study presents the design and development of an impedance sensor to detect the nitrate concentration in soils based on the sensitivity of the soil dielectric constant to ion conductivity and on electrical double layer effects at electrodes. The impedance of samples with nitrate-nitrogen concentrations ranging from 0 to 15 mg/L was measured at frequencies between 20 Hz and 5 kHz and noticeable conductance and susceptance effects were observed. Based on the electrical impedance spectra and the known water content, two regression models using long short-term memory (LSTM) recurrent neural networks were implemented to predict the nitrate-nitrogen concentration in artificial soil (quartz sand). A coefficient of determination of R2 = 0.9580 was achieved with a model including the density of the soil as a feature, and R2 = 0.9295 was achieved with a model that did not include the soil density as a feature. This shows that the nitratenitrogen concentration could be determined almost independently of soil density. Hence, the sensing system has the potential to be used in real time and in situ to monitor nitrate concentration of soils.