J . W . V a n G r o e n i g e na,b , G . L . V e l t h o fb , O . O e n e m ab , K . J . V a n G r o e n i g e nc & C . V a n K e s s e ld aDepartment of Soil Quality, Wageningen University, PO Box 47, 6700 AA, Wageningen, the Netherlands, bAlterra, Wageningen University and Research Center, PO Box 47, 6700 AA, Wageningen, the Netherlands, cDepartment of Botany, Trinity College, University of D...