Assessing the relative effects of hydrogen and aluminum ions on primary root growth of white clover seedlings

White clover (Trifolium repens L., cultivar Huia), a dominant forage legume in Appalachia, usually grows poorly on acidic soils common to the region. The effects of bulk solution concentrations of calcium (Ca), hydrogen (H), and aluminum (Al) on the relative root growth (RRG) of white clover were determined using one- to three-day-old seedlings to assess the relative toxicity of H+ and Al. The RRG was affected by bulk solution concentrations of Ca, Al, and pH, in a manner indicative of significant interactions among these parameters. The RRG was directly related to the activities of Al3+ or H+ at the surface of the root as calculated by the Gouy-Chapman-Stern model. Fifty percent inhibition of RRG occurred at activities of 5 and 200 mu M Al3+ and H+, respectively. A large part of the interaction between bulk solution concentrations of Ca, Al, and H could be explained by how these parameters affected the activities of these ions at the root surface.