Sustainable Quantification of Urea in Aqueous Solutions and Corn Cultivation Soils Using Raman Spectroscopy: Towards Precision Agriculture and the Reduction of Environmental Impact

The reliable quantification of urea in agricultural systems requires methods that combine metrological rigor with low environmental impact. This work develops and validates a micro-Raman method (λ = 532 nm) for the direct determination of urea in aqueous solutions and soils. The method is formally compared with the reference procedure ISO 19746:2017 (HPLC). Calibration, based on the 1000–1200 and 1460–1670 cm−1 windows, showed near-ideal linearity in the 0.25–25% w/w range (r2 = 0.9999). LOD and LOQ values were 0.178 and 0.735% w/w, respectively. Intra- and inter-day accuracy proved adequate for routine use (RSD ≤ 5%). A one-way ANOVA (p = 0.983) confirmed no statistically significant differences between concentrations obtained by micro-Raman and ISO 19746:2017. In the soil matrix, recoveries ranged between 94 and 101, and the contained biases demonstrate good tolerance to matrix effects. Application to maize plots allowed for monitoring urea disappearance at three depths (0–2 cm, 5–7 cm and 10–15 cm) over 90 days. These differentiated areas of rapid surface hydrolysis from more persistent fractions at depth. The Eco-Scale (96), GAPI (pictogram dominated by green areas), and AGREE (0.88) metrics confirm a significantly lower environmental footprint than that of the chromatographic method. The proposed micro-Raman methodology is emerging as a green, fast, and traceable alternative for monitoring urea in fertilizers and agricultural soils.