Development and Upgradation of an IoT-Based Soil Monitoring System from Capacitive Sensor to TDR Sensor for Enhanced Accuracy and Multi-Parameter Soil Analysis

Geotechnical engineering increasingly requires continuous, high-resolution monitoring of soil state variables to support applications such as slope stability assessment, foundation performance evaluation, earth dam safety, and moisture-induced deformation analysis. In-situ sensing integrated with Internet-of-Things (IoT) platforms provide a promising approach for real-time characterization of soil hydraulic and physicochemical behavior. However, many existing IoT-based soil monitoring systems rely on low-cost capacitive moisture sensors, whose accuracy is strongly affected by soil texture, salinity, and environmental variability, limiting their reliability for geotechnical applications.This study presents the development and upgradation of an IoT-based soil monitoring system by replacing a capacitive moisture sensor with a high-precision Time Domain Reflectometry (TDR) sensor and integrating a 7-in-1 RS-485 soil probe capable of measuring volumetric water content, temperature, electrical conductivity, pH, and NPK nutrients. Sensor data are acquired and processed using an ESP32 microcontroller with Modbus communication, local LCD display, and real-time cloud synchronization. Laboratory and field evaluations demonstrate that the TDR-based system achieves ≤1% RMSE relative to gravimetric measurements across multiple soil types, significantly outperforming capacitive sensors. The comparative analysis indicates that the TDR-based NPK sensor provides more stable and reliable moisture measurements than conventional capacitive sensors while simultaneously enabling multi-parameter soil characterization. The proposed platform provides a robust, scalable, and cost-effective solution for multi-parameter soil monitoring, with direct relevance to geotechnical engineering practice and research.