Leaf Colour Chart (LCC) a Reliable Tool for Nitrogen Management: A Comprehensive Review

Efficient nitrogen (N) management is central to improving crop productivity while reducing adverse environmental impacts such as nutrient leaching, nitrous oxide emissions, and groundwater contamination. Among available diagnostic tools, the Leaf Colour Chart (LCC) has proven to be a simple, cost-effective, and non-destructive method for assessing crop nitrogen status and guiding site-specific nitrogen management (SSNM). Developed initially for rice cultivation, LCC has since been validated in other cereals and small millets, including sorghum, pearl millet, and finger millet, reflecting its adaptability across diverse production systems. The principle of LCC relies on visual comparison of leaf greenness with standardized colour panels, which correlates strongly with leaf chlorophyll content and nitrogen concentration. This enables farmers to make timely fertilizer applications, thereby enhancing nitrogen use efficiency (NUE), sustaining yields, and minimizing input costs. Field experiments across agro-ecological regions consistently demonstrate that LCC-based N management can match or surpass blanket fertilizer recommendations while lowering losses and improving resource use efficiency. The reliability of the tool, however, may be influenced by varietal differences in leaf colour, environmental conditions, and user subjectivity. Optimal readings are generally obtained from the youngest fully expanded leaves during mid-morning prior to topdressing. Recent advances, including digital LCCs, smartphone-based colour calibration, and integration with precision agriculture platforms, show promise in overcoming these limitations and improving accuracy. This review synthesizes current knowledge on LCC calibration, crop-specific threshold values, and correlations with chlorophyll and leaf N content. It also evaluates practical applications, benefits, and constraints of LCC across different crops and environments. Overall, evidence suggests that LCC is a reliable, adaptable, and sustainable tool for precision nitrogen management, offering significant potential for climate-resilient agriculture and providing valuable guidance to researchers, extension agents, and farmers worldwide.