S18 - Session O2 - Evaluating different multispectral vegetation indices for assessing the N status of white cabbage (Brassica oleracea) under organic farming conditions

Authors: Felix Besand *, Kai-Uwe Katroschan

The horticulture sector still widely lacks applicable remote sensing methods for assessing the nitrogen (N) status during crop growth. The objective of this study was to evaluate different common vegetation indices (NDVI, GLI, NDYI, CCCI, NDRE) as indicator for the N status of organically managed white cabbage. The limited N availability in organic farming can be expected to be reflected in generally lower leaf chlorophyll levels, which affects the reflectance behavior of crops and potentially the performance of individual indices. In each of two field experiments, variation in cabbage N supply derived from 12 preceding clover/grass green manure crops differing in sward management. During cabbage growth, plots were nadir-scanned four times with a 5-channel multispectral camera (MicaSense RedEdge MX) mounted on a quadcopter drone. To obtain the reflectance behavior of the plant cover only, picture areas with bare soil were excluded by image processing. Cabbage N supply was quantified by destructive plant and soil sampling at harvest and by means of associated soil incubation experiments during cabbage crop growth. Shortly prior to cabbage planting, soil material in amounts of approximately 35 kg was retrieved from each plot, incubated aerobically at a constant temperature of 15°C, and regularly tested for NO3-N and NH4-N to assess soil net N mineralization. For comparison of indices, index values were correlated with the respective incubation experimental data. In 2020 at the first aerial survey four weeks after planting, indices were ranked in the order NDRE < NDVI < CCCI < GLI < NDYI (R from 0.87 to 0.62), which is in contrast to other studies on Brassica vegetables. The results of the study indicate that relative index suitability varies with growth stage and overall N supply level.