Cation mobility in acidic soils with low organic-matter contents depends not only on sorption intensity but also on the solubility of the species present in soil solution. In general, the following leaching gradient is observed: potassium (K +) > magnesium (Mg 2+) > calcium (Ca 2+) > aluminum (Al 3+). To minimize nutrient losses and ameliorate the subsoil, soil solution must be changed, favoring higher mobility of M 2+ (metal ions) forms. This would be theoretically possible if plant residues were kept on the soil surface. An experiment was conducted in pots containing a Distroferric Red Latosol, with soil solution extractors installed at two depths. Pearl millet, black oat, and oilseed radish residues were laid on the soil surface, and nitrogen (as ammonium nitrate) was applied at rates ranging from 0 to 150 mg kg -1. Corn was grown for 52 days. Except for K + and ammonium (NH 4+), nitrogen rates and plant residues had little effect upon the concentrations and forms of the elements in the soil solution. Presence of cover crop residues on soil surface decreased the effect of nitrogen fertilizer on Ca leaching. More than 90% of the Ca 2+, Mg 2+, and K + were found as free ions. The Al 3+ was almost totally complexed as Al(OH 3) 0. Nitrogen application increased the concentrations of almost all the ions in soil solution, including Al 3+, although there was no modification in the leaching gradient.
