LAUSR

The efficiency of the root capacitance method is still judged contradictorily. It was, therefore, aimed to verify that the electrical capacitance ( C R ) and impedance ( Z R ) measured between ground electrode (pushed into the soil) and plant electrode (clamped on the stem) really represented the root system present in the soil. An ‘electrode separation experiment’ and a ‘root excision experiment’ were performed on potted maize plants cultivated in arenosol. The partial electrical separation of the ground electrode (restriction of the current flow in the topsoil) caused no significant changes in the capacitance and impedance of either the soil or the plant–soil system. This suggested that the current flows through a multitude of soil solution pathways, not predominantly on the wet soil surface. The progressive excision of the main root branches caused a gradual decrease in C R (31–39% of the initial value) and an increase in Z R (4.4- to 5.2-fold that of intact plants). C R showed a much stronger correlation with root dry mass ( R 2  = 0.807) than with stem cross-sectional area ( R 2  = 0.424) in intact plants. The closeness of the latter relationship decreased further ( R 2  = 0.377) after whole root excision. The results clearly demonstrated that the root system dominated the measured capacitance and impedance, though the role of the stem base was not negligible. In conclusion, the capacitance method is worth considering in future studies as an adequate non-intrusive approach to evaluate root size in the soil.