Improving nitrogen use efficiency is important for the potato crop, because of its relatively low ability to take up available soil mineral nitrogen (N). Splitting of N fertilizer application is a suitable approach to better match N need and supply. In-season crop N monitoring methods are therefore required to support such strategies. This paper deals with the state of the art and potential development of characteristics, use and implementation of well known and more recent methods aimed to assess in-season potato Crop Nitrogen Status (CNS). A short overview of this concept is given for the potato crop. The most important and available methods for CNS assessment are evaluated for their accuracy, precision, sensitivity, sensibility and feasibility. These are: the petiole sap nitrate concentration test; the leaf chlorophyll concentration measurement using a handheld chlorophyll meter; the measurement of crop light reflectance through a hand-held radiometer using passive sensors. More recent methods still under investigation based on near, ground-based, air-borne or space-borne remote sensing are discussed for their scientific and practical interest in the near future. The current and potential use and implementation of these methods into decision support systems for potato N fertilization management aimed at improving the potato crop nitrogen use efficiency are analysed by: comparing relative and raw data; establishing threshold values of CNS; and combining or integrating the CNS values into models dedicated to N recommendation or to crop growth simulation.

This paper presents experimental results from a new field-scale N management strategy for winter wheat and potato crops in Belgium. At the beginning of the season, a global N recommendation is given using the balance-sheet method. The crop development and related N needs are strongly influenced by unexpected factors during the growing season. Therefore, the N predictive recommendation needs to be adapted to the crop N status. For this assessment, the chlorophyll meter was preferred over several plant diagnostic tools. In order to eradicate environmental influences, the measurement procedure was defined. The proposed procedure was based on the establishment of chlorophyll meter relative values. In winter wheat, the procedure allowed the third N dressing amount to be modulated using an over-fertilized plot. The same approach was tested on potato crop to introduce N split-application. The results showed, however, that an over-fertilized plot as reference is not suitable for determining the supplemental N required. nitrogen / fertilization / winter wheat / potato / chlorophyll meter / diagnostic tool Résumé-Gestion de la fertilisation azotée des cultures de froment d'hiver et de pommes de terre à l'aide du chlorophyllomètre, outil rapide de diagnostic du statut azoté des cultures. L'objectif de cet article est de présenter de nouvelles stratégies de fertilisation azotée, à l'échelle de la parcelle, en blé d'hiver et pommes de terre en Belgique. En début de saison, un conseil global de fumure azotée est établi selon la méthode du bilan. Mais le développement de la culture et les besoins azotés correspondants sont fortement influencés par des facteurs imprévisibles. Ainsi le conseil de fumure doit être modulé par le statut azoté de la culture estimé grâce au chlorophyllomètre. Afin d'éliminer l'influence de facteurs extérieurs, un protocole d'utilisation a été défini. Cette procédure est basée sur l'établissement de valeurs relatives. En blé d'hiver, elle permet de moduler la quantité d'azote à apporter en troisième fraction grâce à une référence sur-fertilisée. La même approche a été testée en culture de pommes de terre en vue d'introduire le fractionnement dans la fertilisation azotée, mais les résultats ont montré qu'une placette surfertilisée ne permet pas de déterminer le besoin supplémentaire en azote. azote / fertilisation / blé d'hiver / pomme de terre / chlorophyllomètre / outil de diagnostic

In a precision farming research project financed by the Belgian Ministry of Small Trade and Agriculture, the methods of precision agriculture are tested on grain fields with a view of implementation of precision agriculture methods in Belgian field agriculture. The project encompasses methods for automatic information gathering on soil and crop and analysis of this data for management of within-field variability. Automatic information capturing is combined with traditional data sources of soil sample analysis and crop observations. The measurements and part of the results on one particular field in Sauveni" e ere are presented here. Five nitrogen management strategies were compared, but the resulting differences in nitrogen dose were small and did not lead to significantly different yield results. The yield results were correlated to topography-related variations in soil texture and chemical components and to crop reflectance measurements in May.

A decision support system (DSS) was based on the splitting of total nitrogen (N) fertilizer application combined with in-season assessments of crop N requirements aimed to matching, at field scale, potato (Solanum tuberosum) total crop N requirements and mineral N supply from soil and fertilizers. After the preplanting establishment of the total N recommendation based on the predictive balance-sheet method at a specific field scale, 70% of the recommended amount was applied to the crop at planting. Subsequently, at 20–50 days after emergence (DAE) the need for supplemental N was assessed through noninvasive measurements of leaf chlorophyll concentration directly in the field. A simple conditional relationship was established to support potato growers’ decisions on the usefulness of applying the remaining 30% N. This required a crop N status (CNS) assessment in the fertilized field and within a small, untreated area (zero-N for reference). The strategy developed is economically feasible, easy to operate, and validated for several potato varieties. It also gives the grower the possibility of improving N use efficiency (NUE). Several tools to assess CNS have been investigated, or are currently being investigated, at the Walloon Agricultural Research Center in Gembloux, Belgium (CRA-W) for integration into this strategy. All the tools are evaluated for four main characteristics: measurement accuracy and precision, sensitivity to N, specificity to N, and feasibility. There are invasive or noninvasive tools. The use of a chlorophyll meter (CM) has been currently developed in the DSS. Current CRA-W research is investigating the potential of crop light reflectance as an indicator of CNS (ground-based radiometers for near remote sensing and satellite multispectral imagery for spatial remote sensing).

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