1. Chair of Plant Nutrition, Department of Plant Science, Life Science Center Weihenstephan, Technische Universität München, Am Hochanger 2, 85354 Freising, Germany
2. present address: University of the Fraser Valley, UFV Agriculture, Trades and Technology Centre, 5579 Tyson Rd., Chilliwack, British Columbia V2R 0N9, Canada
3. Department of Agri‐Environmental and Land Sciences, University of Bari, Via Amendola 165/a, 70126 Bari, Italy
4. present address: Consiglio per la ricerca e la sperimentazione in agricoltura – (CRA – Research Unit for Cropping Systems in Dry Environments), Via Celso Ulpiani 5, 70125, Bari, Italy