Non-Contact Thermoacoustic Sensing and Characterization of Plant Root Traits

Measuring below-ground plant and soil traits such as root biomass, water distribution, and soil compaction is of high interest to plant breeders and agronomists alike. However, there are limited technologies available that can sense these traits. Current methods for sensing roots are either invasive or not readily field deployable. Thus, we propose a novel non-contact thermoacoustic sensing system that can be used to characterize roots in a high-throughput, non-invasive and nondestructive fashion. Upon microwave excitation, the dielectric contrast between roots and soil generates an ultrasound signal via the thermoacoustic effect. Detection of the ultrasound signal in air is achieved using highly sensitive capacitive micromachined ultrasonic transducers with minimum detectable pressures as low as 278 µP a
RMS
, in order to overcome the large interface loss due to the impedance mismatch at the soil-air boundary. In this paper, we demonstrate a system that can detect agarose-based root phantoms in two different soil types. A linear-regression mapping of the received thermoacoustic data to properties like soil water content, root osmotic potential and root size shows excellent correlation, with R
²
greater than 0.9 in all cases.