Distinction Between Hydration and Fertigation Events in Plant Tetrapolar Bioimpedance Measurements

In this study, we assess the appropriateness of tetrapolar bioimpedance needle plant sensors for interrogating the full plant stem and for differentiating between hydration (watering) and fertigation (feeding) events in herbaceous plants. Our sensor features a tetrapolar configuration of needle electrodes arranged in “paired pairs”. The goal is to develop a bioimpedance-based sensor that automatically monitors watering and fertigation status in tomato plants, offering optimal performance, easy installation, and cost-effectiveness. We detail our design methodology using COMSOL FEM simulations to assess the needle geometry in terms of its ability to effectively interrogate the stem across its diameter. The tomato stem's bioimpedance was modelled as a lumped-impedance equivalent circuit, with R and C values computed from experimental data. The theoretical model indicated similar characteristics to those extracted experimentally. Experiments were carried out over 27 days with impedance magnitude and phase periodically monitored for ten frequencies, with the results clearly indicating watering and feeding events, which were accurately classified using a PCA-based clustering algorithm.