1. Plant Nutrition Laboratory, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK–1871 Frederiksberg C, Copenhagen, Denmark (S.H., M.M., J.K.S.); and
2. Institute of Agronomy and Plant Breeding, University of Göttingen, von Siebold Strasse 8, D–37075 Göttingen, Germany (C.M., M.W.)