Determination of water content and characteristic analysis in substrate root zone by electrical impedance spectroscopy

The root zone of a plant is an important system that provides a base for use of nutrients and water and has been the focus of many researchers' work. This paper examines a new electrical impedance spectroscopy (EIS) measurement approach within the 100 Hz-2 MHz range in the substrate root zone that uses an adjacent stimulation and measurement pattern that overcomes the shortcomings of conventional methods and better reflects physiochemical changes in stereoscopic space. We used a highly controllable mixed solid substrate to simulate the root zone. The experimental results show that three impedance features, comprising the mean of the real part of the complex impedance within the 1-10 kHz range, the characteristic frequency and the extremum of the imaginary part of the complex impedance are strongly exponentially correlated with the water content of the cylindrical substrate, in which the minimum coefficient of determination denoted by R-2 is 0.9247. This indicates that these features can serve as indicators of the substrate's water content. In addition, the substrate-plant continuum was investigated and we found that the presence of the plant had a strong impact on the impedance response, and particularly on the data from the equipotential contralateral side, which can be used for root zone plant positioning. Additionally, analysis of the variation of the phase angle shows that the position of the plant underground has little effect on the frequency corresponding to the maximum absolute phase difference, which provides a reference for frequency selection in the impedance measurement and tomography system of the substrate root zone. We conclude that EIS can be used to monitor the plant morphology and also provides a way to determine the optimal frequency for impedance measurement.