Nondestructive optical crop sensing to monitor nitrogen content in woody ornamentals

The nutrient and especially nitrogen management in hardy nursery stock plants is challenging due to the large number of species and cultivars. Sub-optimal nitrogen nutrition negatively influences the commercial quality of the plants (height, branching, leaf colour) while supra-optimal N has an environmental impact due to nitrate leaching. Therefore, (a) reliable indicator(s) of plant nitrogen status is/are needed. Sensors based on the optical properties of chlorophyll have already proven their use in various agronomic cropping systems. These sensors have the advantage that they can be used as a quick, nondestructive and relatively inexpensive tool to support decisions concerning N fertilization during the growing season. Research on these optical N sensors in woody ornamentals is scarce. This study investigates the potential of these sensors on woody ornamentals in outdoor production. Four species were chosen, Acer pseudoplatanus, Ligustrum ovalifolium, Prunus laurocerasus ‘Rotundifolia’ and Tilia cordata, and received three different N treatments in three replicates. The median amount was based on fertilizer recommendations after soil analysis. The experiment was first conducted in 2016 and repeated – with adapted N dosages – in 2017. The leaf nitrogen content as well as non-destructive measurements were regularly monitored during the growing season. At the end of the growing season, plant nitrogen uptake was determined. Both at leaf as at canopy level sensors were tested. At leaf level, the SPAD-502 chlorophyll meter was evaluated in 2016 and 2017. In 2017, another portable sensor for leaf measurements (Dualex Scientific, Force-A) was used. At crop level, the species were monitored with a GreenSeeker RT100 (NTech) which calculates the normalized difference vegetation index (NDVI) in both research years. This canopy reflectance index can be correlated with N and biomass. An extra field spectroradiometer (SS-100, Apogee) was used during the second growing season. This instrument measures wavelengths continuously in the range of 340 to 820 nm, which makes it possible to calculate other vegetation indices that may be better related with the nitrogen content of the crops. Early results show significant correlations between foliar leaf nitrogen and SPAD values for Ligustrum, Acer and Tilia, while for Prunus correlations were absent. At canopy level, only for Ligustrum a significant correlation was found between NDVI and %N in 2016. In 2017 this relation was less clear. Although NDVI was an estimator for biomass for Acer, Prunus and Tilia, robustness needs to be investigated with more data. At this moment, more data is analyzed and an overview of the results can be presented during the conference.