Abstract
The variable carbon (C) and nitrogen (N) content of organic soil amendments makes it challenging to predict their seasonal nutrient supply. We hypothesized that remote sensing technologies can effectively evaluate N availability from amendments via early-season plant growth differences. This study used UAV-based normalized difference red edge index (NDRE) to quantify in-season crop N status at five time points under different organic amendments and N fertilizer rates at two California sites. At the triticale site, NDRE correlated strongly with leaf chlorophyll (R 2 = 0.9) and grain yield (R 2 = 0.7–0.8) across amendments (digestate solid, digestate liquid, and hydrolysate) and N rates (0–180 kg N ha −1 ). NDRE captured differences in nutrient supply during vegetative growth, impacting forage and grain yields ( p < 0.05). The sufficiency index (SI), a relative measure of NDRE within each experiment, accurately predicted crop productivity and N uptake at harvest, demonstrating potential as a tool to reduce external N fertilizer inputs and improve N recovery efficiency (NRE). Amendments outperformed the urea Control in SI at intermediate N rates, reflecting improved crop N status and productivity. At the wheat site, SI was consistently high (0.87–0.97) with or without digestate solid across a wide range of N rates (0–280 kg N ha −1 ). Without crop N deficiency signal, this remote sensing approach indicates external N inputs could have been avoided while maintaining productivity and improving NRE. These results demonstrate the potential of remote sensing technologies to evaluate N supply from organic sources and support adaptive N management strategies that sustain yields while reducing pollution.