A smart nail platform for wireless subsoil health monitoring via unmanned aerial vehicle-assisted radio frequency interrogation

Monitoring subsoil conditions across large agricultural areas is essential for optimizing resource use and supporting sustainable crop production. However, most existing sensing systems rely on battery-powered electronics that are costly, require maintenance, and are difficult to scale. To address these limitations, we introduce HARVEST (Hybrid Antenna for Radio frequency-enhanced Volumetric water content and Electrical conductivity-based Soil Tracking), a low-cost, wireless platform that eliminates the need for onboard electronics. HARVEST uses nail-shaped sensing probes physically and electrically coupled to a triple-ring antenna positioned above the ground, reducing signal loss while preserving sensitivity to subsurface changes. Variations in soil moisture and salinity modify the electrical characteristics of the buried probes, shifting the antenna's resonant response, which is wirelessly detected by an aerial reader. The system is optimized through electromagnetic simulations and validated through laboratory experiments and full-season field deployment. HARVEST enables reliable detection of subsoil moisture and conductivity with drone-based readout from altitudes up to 1.8 m. This platform provides a scalable, maintenance-free solution for real-time subsoil monitoring, advancing next-generation precision agriculture technologies. By enabling distributed, low-cost sensors across large fields, HARVEST delivers deeper spatial and temporal insight into subsoil conditions, supporting data-driven agricultural management while reducing environmental impact.