1. Univ. of Hohenheim, Inst. for Crop Prod. and Grassl. Res. (340) D‐70599 Stuttgart Germany
2. Bot. Dep. Univ. of Dublin, Trinity College Dublin 2 Ireland
3. Svalöf Weibull AB‐S 268 81 Svalöv Sweden
4. Departamento de Fitotecnica Universidade de Évora, Herdade da Mitra P‐7001 Evora/Codex Portugal
5. Rothamsted Exp. Stn. Harpenden Hertfordshire AL5 2JQ United Kingdom
6. Danish Inst. of Agric. Sci. Dep. of Soil Sci., Res. Cent. Foulum P.O. Box 50 8830 Tjele Denmark