SoilNutri: A Passive Metasurface-based, Low-cost System for Soil Moisture and Nitrogen Monitoring

Smart agriculture requires efficient and scalable sensing of key soil parameters. While recent wireless sensing systems provide low-cost alternatives, active designs depend on powered underground components with potential instability and contamination risks, whereas passive battery-free approaches suffer from limited accuracy and are typically restricted to moisture detection. This work presents SoilNutri, a fully passive and battery-free metasurface capsule that enables simultaneous, high-precision monitoring of soil moisture and nitrogen concentration. The system employs a complementary split-ring (CSR) metasurface integrated within a sandwich-structured capsule that provides electromagnetic isolation and stable coupling with the surrounding soil. A lightweight two-stage soil-meta-VAE framework reconstructs soil-distorted reflection spectra and jointly predicts moisture and nitrogen from a shared latent representation. Extensive in-lab and field experiments demonstrate high-precision moisture estimation with 0.64% MAE and ppm-level nitrogen prediction with 5.14 ppm MAE across diverse soil types, depths, and seasonal conditions. With a per-unit cost below $1 and a reusable encapsulated design, SoilNutri offers a scalable, sustainable, and physically interpretable solution for multi-nutrient, in-situ soil monitoring for next-generation smart agriculture.