Table 2.

Summarized references using EIS method of in situ plant root research

Method	References	Plant	Growth substrate	Concern	Frequency range (Hz)	No. of frequency	No. of plant
Electrical Impedance Spectroscopy (EIS)	[39]	Willow	Hydroponics	Resistance, capacitance	40 ~ 340 k	488	64
	[24]	Tomato	Potting oxisol	Resistance, capacitance	10 ~ 1 M	51	12
	[73]	Sitka spruce	Field	Resistance, capacitance	40 ~ 340 k	–	18
	[68, 72]	Willow	Hydroponics	Resistance, capacitance	60 ~ 60 k	31	15
	[71]	Maize	Potting Arenosol	Resistance, capacitance, Phase Angle	100-10 k	55	18
	[70, 74]	Scots pine	Potting Perlite	Resistance, capacitance	5 ~ 100 k	44	72
	[69]	Scots pine	Potting Perlite	Resistance, capacitance loss factor δ	80 ~ 1 M	42	630
	[44]	Pea	Hydroponics	Resistance, capacitance, Phase Angle	800 ~ 1 M	100	30
	[53]	Pea	Potting Perlite	Resistance, capacitance, Phase Angle	30 ~ 1 M	100	30–40