1. Department of Crop Physiology and Soil Science, Danish Institute of Agricultural Sciences, Research Centre Foulum, P.O. Box 50, DK–8830 Tjele, Denmark (M.N.A.);
2. Agricultural Chemistry Institute, Rheinische Friedrich-Wilhelms-Universität Bonn, Karlrobert-Kreiten-Strasse 13, D–53115 Bonn, Germany (F.A.);
3. Horticultural Sciences Department, University of Florida, 1143 Fifield Hall, P.O. Box 110690, Gainesville, Florida 32611–6479 (Y.W., K.E.K.);
4. Department of Agricultural Sciences, The Royal Veterinary and Agricultural University, Agrovej 10, DK–2630 Taastrup, Denmark (C.R.J., V.O.M.); and
5. Department of Plant Physiology, University of Copenhagen, Øster Farimagsgade 2A, DK–1353 Copenhagen K, Denmark (H.N.)