Evaluation of chlorophyll meters as tools for N fertilization in winter wheat under humid Mediterranean conditions

Nitrogen management strategies have been investigated for winter wheat grown under humid Mediterranean conditions, in which a global N recommendation is adapted to the specific requirement of each location and season by mean of a chlorophyll meter. Application of N fertilizers is split in three topdressings, and additional applications are justified mainly, when seeking for grain quality. We evaluated the role of the chlorophyll meter to: (i) identify sites in which application of a third dressing increases yield; (ii) decide if a supplemental N application would increase grain N concentration; (iii) predict wheat yield and grain N concentration at harvest. The meters were tested over a 6 year period on 24 N response experiments located in Navarra (Spain). Experiments were sown with winter wheat (Triticum aestivum L.), cv. Soissons or cv. Marius. Readings with two chlorophyll meters, N-Tester® (Hydro-Agri) or SPAD-502® (Minolta), were carried out at different growth stages. The relationship of relative yield and grain N concentration with chlorophyll meter readings, either absolute or normalized, was studied by mean of a quadratic model and a Cate-Nelson statistical procedure. Readings at GS-37 were able to identify treatments that would show yield response to a third N fertilizer application 91% of the cases. Using normalized readings allowed to compare between different cultivars and chlorophyll meters, and reduced error. For cv. Soissons, when readings at GS > 45 were lower than 600, grain N concentration was below 2.1%, and N fertilization is needed to increase it; when readings were above 700, grain N concentration was over 2.6%. The error in the prediction of the yield and the grain N concentration with readings after GS-55 was low; therefore, chlorophyll meters could be useful tools for planning harvest and storage necessities.