TheRoyalVeterinaryandAgriculturalUni6ersity,DepartmentofAgriculturalSciences,PlantNutritionandSoilFertilityLaboratory, Thor6aldsens6ej 40,DK-1871 FrederiksbergC ( Copenhagen ),Denmark *Correspondingauthor.Tel.:+4535283499;fax:+4535283460;e-mail:tm@kvl.dk Received1July1997;accepted23April1998 Abstract EcologicalModelling111(1998)1–15