Juexing Wang, Binbin Xie, Yimeng Liu, Minhao Cui, Xiao Zhang, Guangjing Wang, Ke Sun, Zhichao Cao, Huacheng Zeng, Qingxu Jin, Younsuk Dong, Hui Li, Jie Xiong, Tianxing Li
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.