## 1 Introduction  
As in every crop production system, an optimal supply of nutrients for plant production is essential. In particular, nitrogen (N) is an elementary nutrient component for optimal plant growth. Plant N concentrations below an optimal value can lead to significant yield losses [1; 2; 3]. High doses of applied nitrogen will lead to a loss of N in different forms, i.e., ammonia, nitrous oxide emissions from the soil, and nitrate leaching into the groundwater [4; 5; 6; 7]. This loss leads to negative effects to the environment, as well as to a contribution to climate change [8; 9; 10]. However, this does not only apply to conventional farming systems. Even in organic farming, there are risks of N losses, such as nitrous oxide emissions after ploughing clover grass or green manure [11; 12]. In comparison to conventional farming, the input of nutrients in organic farming is relatively low, especially in terms of nitrogen [13; 14; 15; 16]. In organic farming, a large source of nitrogen input, besides compost, is N\({}_{2}\) fixation because of the symbiotic interaction of legumes and rhizobia. This provides substantial amounts of N supply to the plants and the soil as well. Furthermore, it reduces the need for additional N fertilizers [17]. However,fertilization only based on N2 fixation does not suite the needs of the plants. A possible solution for this lack of nutrients is to close the cycle of nutrients. With regard to climate change and reducing the cost of fertilizer, the recycling of nutrients, especially nitrogen and phosphorus, is becoming more and more important [18; 19]. One way to recycle nutrients is through the use of compost [20; 21]. The use of compost also offers an optimization of nutrient supply for organic plant production. Unlike mineral fertilization, the release of nutrients from organic fertilization is much slower [22]. In particular, N from compost is initially stored in the soil N pool and will be released in a plant-available form often after decades [23]. It is, therefore, much more difficult for the farmer to determine the effect of organic fertilizers on the crop and the soil as well. N mineralization, which produces a plant-available form of N, is essential for plants and depends on the soil as well as the weather conditions [24], but does not coincide with plant requirements [13]. In addition to the above-mentioned N mineralization, the release of soil-borne N shows no homogeneity within a heterogeneous field [25]. As the necessary amount of fertilized N is the difference between the required N of the crops minus the supply of soil-borne N, the challenge for organic farming is to know and decide which and how much organic fertilizer to use (in this study different composts were used as organic fertilizers) to best suit the needs of the crop. A conventional approach for researchers to gather reliable information about the optimal N fertilization is the use of multi-location and multi-annual field trials [3; 26], to gather many soil and plant samples to analyze them based on their nutrient status. For an estimation of the N requirements only based on soil analyses, a high number, as well as a high areal resolution, of soil tests becomes necessary. This approach is time-consuming and not economic [27; 28]. The growth and development of plants are dependent on the actual nutrient status. Therefore, plants themselves are the best indicators for estimating their nutritional status, i.e., N requirement and above-ground biomass [29; 30; 31; 32]. The current N status can be determined easily using spectral sensors, which detect the light of different wavelengths reflected by the plants [33; 34; 35; 36]. In addition, this method is non-destructive, which means that in field use it is not required to cut plant samples to analyze them afterwards in a laboratory to obtain information about their actual nutrient status. In conventional farming, non-destructive spectral sensing has already proved its ability to guide nitrogen fertilization in winter wheat [29; 37].