Electrical Signature of Roots in Hydroponic Solution and in Soil

Exploration of root systems is challenging, as there is no direct access to the root. We propose the spectral induced polarization (SIP) method to monitor roots and rhizosphere in a noninvasive fashion. The SIP method is sensitive to polarization processes at relatively low-frequency range (mHz to kHz). The source of polarization in the root and in the soil is the electrical double layer at cell membranes and at grain’s surface, respectively. An interpretation of SIP requires correlation between the root properties (e.g. biomass) and the SIP measurements. The establishment of such correlations is the objective of this study.

We measured the SIP of wheat roots in hydroponic solution and in soil. In the first case, we placed a month-old plant in a column. We measured the SIP, removed the plant from the medium, cut part of its root, and returned the plant to the column. We repeated this procedure three times until no roots remained. In the second case, we planted the wheat in a column with a sandy loam soil and measured the SIP at days 1, 5, 11 and 13 after planting.

In the hydroponic solution, the polarization positively correlated with the root mass. The relaxation frequency increased from ∼2.7 Hz to ∼10 Hz in accordance to the increase in the average root diameter. In all measurements, the low-frequency polarization, was lower when root was present. This was a non-intuitive finding, as the root contributes to the polarization. One possible explanation is that organic carbon secreted by the roots changes the physicochemical properties of the mineral surface and decreases the soil polarization.

These preliminary results demonstrate the sensitivity of the SIP method to the presence of roots. Ultimately, this study will lead to the development of a non-invasive method to monitor roots in the subsurface.