Application of Proximal Optical Sensors to Fine-Tune Nitrogen Fertilization: Opportunities for Woody Ornamentals

Abstract

Today, high amounts of residual nitrogen are regularly being reported in the open field production of hardy nursery stock. In some cases, excessive fertilizers or side-dressings are applied when circumstances are not favorable for uptake. Aquatic as well as terrestrial ecosystems are sensitive to enrichment with nutrients, but growers also benefit when losses are avoided. In this study, the potential of proximal optical sensors to optimize nitrogen fertilization was investigated in four woody species: Acer pseudoplatanus L., Ligustrum ovalifolium Hassk., Prunus laurocerasus 'Rotundifolia' L. and Tilia cordata Mill. For three consecutive growing seasons, plants were grown under three different fertilization levels to generate different nitrogen contents. Plant growth and nitrogen uptake were monitored regularly and combined with sensor measurements including Soil Plant Analysis Development (SPAD), Dualex and GreenSeeker. Here, we show that optical sensors at the leaf level have good potential for assisting growers in the sustainable management of their nursery fields, especially if leaf mass per area is included. Nevertheless, care should be taken when plants with different leaf characteristics (e.g., wax-layer, color, and leaf thickness) are measured. When all measuring years were considered, high correlations (R 2 ≥ 0.80) were found between area-based foliar nitrogen content and its non-destructive proxy (i.e., chlorophyll)measured by Dualex or SPAD. Based on our results, we recommend a relative rather than absolute approach at the nursery level, as the number of species and cultivars produced is very diverse. Hence, knowledge of absolute threshold values is scarce. In this relative approach, a saturation index was calculated based on the sensor measurements of plants grown in a reference plot with an ample nitrogen supply.Agronomy 2019, 9, 408 2 of 24 crop and soil type. Farmers are encouraged to apply well-considered nitrogen fertilization. However, high amounts of residual nitrogen are regularly being reported for the open field production of ornamental trees and shrubs. Opposed reasons are an incorrect nitrogen fertilization strategy and a mismatch between timing of N fertilization and N uptake by plants [2].Despite diverse research on fertilizer rates in woody ornamentals, there is a lot of variation in recommended fertilizer use. Some authors report little or no effect of fertilizers on ornamental plant quality (e.g., height and length) [3,4], others do find effects [5,6], and others report mixed results depending on the species [7]. According to Rose, suggested annual N rates vary between 48 and 287 kg N ha −1 [8]. The author emphasized that these rates are based on research from the 1950s to the 1970s when maximizing the fertilizer response was the primary goal and other parameters such as soil N content, yield potential and field history were not taken into account. This is indeed very important for open field production with big differences in soil organic matter content, soil type, water availability,...

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