On-farm evaluation of near-infrared spectroscopy sensors for assessing nutrient variability in liquid organic manures

Liquid organic manures (LOMs) exhibit high variability in dry matter (DM) and nutrient concentrations. Accurate knowledge of these characteristics is essential for farmers to meet plant needs and avoid environmental problems caused by over-fertilization. Near-infrared spectroscopy (NIRS) seems to be a promising tool for the determination of DM and nutrient concentrations directly on farms. In this study, two commercially available NIRS sensors were deployed at 41 farms in northern and southern Germany to assess DM as well as total nitrogen (TN), ammonium nitrogen (AN), phosphorus (P), and potassium (K) under realistic on-farm conditions. A total of 582 samples of cattle and pig slurry as well as biogas digestate were analyzed, and the sensor readings were compared with laboratory values. Both, laboratory measurements and NIRS data revealed high variability in DM and nutrient concentrations across the three LOM types and the two study regions. Both NIRS sensors provided reliable DM data, particularly in pig slurry (R 2 > 0.9), while data of TN, AN, P, and K exhibited considerably lower coefficients of determination depending on the substrate, sensor, and region. Furthermore, substantial temporal fluctuations in nutrient concentration were observed within individual storage tanks. Although the NIRS sensors were generally capable of capturing this variability and temporal trends under farm conditions, the quantification of individual nutrients was not sufficiently reliable. Overall, the results show that NIRS has considerable potential for detecting relative changes in nutrient concentrations under farm conditions. However, reliable on-farm application requires improved calibration strategies and better consideration of substrate variability and measurement conditions.