A Critical Review on Optimization of Water Use in Vegetable Crops Using IoT-Based Low-Cost Sensors

Water scarcity and inefficient irrigation practices remain pressing challenges for sustainable crop production, particularly in regions with black cotton soils (Vertisols). These soils, characterized by high clay content and swelling–shrinkage behaviour, complicate irrigation scheduling and undermine the performance of conventional soil moisture sensors. The integration of the Internet of Things (IoT) with low-cost sensing technologies offers new opportunities for precision irrigation and improved water management. This review synthesizes insights from 86 studies published between 2007 and 2025 on IoT-based soil moisture sensor systems for vegetable crop irrigation. The analysis covers sensor technologies, calibration methods, IoT architectures, wireless communication protocols, and field applications across multiple crops, with emphasis on challenges in black soils. Capacitive sensors integrated with microcontrollers such as NodeMCU ESP8266 and Arduino UNO were the most widely used due to their affordability and ease of deployment. Studies consistently reported 20–45% water savings and significant improvements in Water Use Efficiency (WUE) in crops including tomato, okra, spinach, cauliflower, coriander, and sweet corn. Despite these advances, research on Vertisols remains limited. Sensor performance is often compromised by clay mineralogy, necessitating soil-specific calibration, conductivity compensation, and sensor fusion approaches. Additionally, long-term multi-crop evaluations and economic feasibility studies are scarce. Emerging trends highlight the integration of IoT with AI/ML models, renewable energy systems, and climate-smart agriculture platforms, which could enable predictive irrigation scheduling and scalable adoption in resource-constrained settings. In conclusion, IoT-enabled low-cost soil moisture sensing systems represent a transformative approach to sustainable agriculture. However, their success in black soils will depend on advances in calibration, long-term validation, and farmer-centric design to ensure reliability, affordability, and wide-scale adoption.