Fast and low‐cost method for direct and simultaneous determination of nitrogen and carbon in soybean leaves using benchtop and portable near‐infrared devices

Abstract BACKGROUND The current techniques for determining carbon and nitrogen content to provide information about the nutritional status of plants are time‐consuming and expensive. For this reason, the objective of this study was to develop an analytical method for the direct and simultaneous determination of nitrogen and carbon elemental content in soybean leaves using near‐infrared spectroscopy and compare the performance of conventional (1100–2500 nm spectral range) and portable equipment (1100–1700 nm spectral range). Partial least‐squares regression models were developed using 27 soybean leaf samples collected during the 2021 harvest and applied for the simultaneous determination of carbon and nitrogen in 13 samples collected during the 2022 harvest. RESULTS The root‐mean‐square error of prediction values for nitrogen and carbon were low (2.42 g kg −1 and 4.37 g kg −1 respectively) for the benchtop method yielded low but higher for the portable method (3.82 g kg −1 and 10.7 g kg −1 respectively). The benchtop method did not show significant differences when compared with the reference method for determining nitrogen and carbon. In contrast, the portable methodology showed potential as a screening method for determining nitrogen levels, particularly in fieldwork. CONCLUSION The methodologies evaluated in this study were implemented and evaluated under real crop monitoring conditions, using independent sets of calibration and prediction samples. Their utilization enables the acquisition of cost‐effective, safe analytical data aligning with the principles of green analytical chemistry. © 2023 Society of Chemical Industry.