Three solution experiments were performed to test the importance of NH 4+ versus NO 3- +NH 4+ to growth of 23 wild-forest and open-land species, using field-relevant soil solution concentrations at pH 4.5. At N concentrations of 1-200 μM growth increased with increasing N supply in Carex pilulifera, Deschampsia flexuosa, Elymus caninus and Bromus benekenii. Geum urbanum was the most N demanding species and had little growth below 200 μM. The preference for NH 4+ or NO 3- +NH 4+ was tested also at pH 4.0; no antagonism was found between NH 4+ and H+, as indicated by similar relative growth in both of the N treatments at both pH levels. Growth in solution with NH 4+ relative to NO 3- +NH 4+ , 200 μM, was negatively related to the mean pH of the field occurrence of the species tested; acid-tolerant species grew equally well with only NH 4+ as with NO 3- +NH 4+ (Oxalis acetosella, Carex pilulifera, Festuca gigantea, Poa nemoralis, Deschampsia flexuosa, Stellaria holostea, Rumex acetosella), while species of less acid soils were favoured by NO 3- +NH 4+ (Urtica dioica, Ficaria verna, Melandrium rubrum, Aegopodium podagraria, Geum urbanum, Bromus benekenii, Sanguisorba minor, Melica ciliata, Silene rupestris, Viscaria vulgaris, Plantago lanceolata). Intermediate species were Convallaria majalis, Elymus caninus, Hordelymus europaeus and Milium effusum. No antagonism between NH 4+ and Ca2+, Mg2+ and K+ was indicated by the total uptake of the elements during the experiment.