The Internet of Things (IoT) has become a foundational technology for modern precision and smart agriculture, connecting sensors, microcontrollers, wireless networks, and cloud or edge computing platforms to enable real-time monitoring and automated decision-making on farms. This narrative literature review synthesizes findings from systematic reviews, bibliometric analyses, and meta-analyses published largely between 2023 and 2026 to examine the major approaches through which IoT is applied in agriculture, the accumulated evidence on its effectiveness, and the barriers that continue to constrain widespread adoption. The review covers four core application areas: IoT-based smart irrigation and water management, environmental and soil sensing, crop and pest monitoring supported by AI-driven analytics, and farm automation and edge/cloud integration. Across the reviewed literature, IoT-enabled irrigation systems are reported to reduce water use by roughly 9 to 50 percent relative to conventional irrigation while maintaining or improving yields, and a large-scale meta-analysis of 85 global studies found that precision agriculture technology adoption increases average return on investment by 22.3 percent and net profit by 18.5 percent, alongside measurable reductions in nitrogen loss, pesticide use, and greenhouse gas emissions. At the same time, the literature consistently identifies high upfront infrastructure costs, unreliable rural connectivity and electricity, limited interoperability between platforms, cybersecurity and data-privacy weaknesses, and a persistent digital divide disadvantaging smallholder farmers as the primary barriers to scaled adoption. The review concludes that realizing IoT's agricultural benefits at scale depends less on further sensor innovation than on addressing these structural, economic, and infrastructural barriers, and it outlines implications for practice, policy, and future research.