ION DYNAMICS IN ACID FOREST SOILS AS AFFECTED BY ADDITION OF CA FERTILIZERS

Ca fertilizers may be used as tools to improve the chemical status of acidic, base poor forest soils. The downward movement of ions after ad dition of CaCO3, CaCO3 + MgO, and CaSO4 . 2H(2)O was studied by monito ring the leachate composition from three types of columns - A1/B, mull /A1/B and moder/A1/B - reconstructed from an acid brown forest soil pr ofile. The fertilizers were surface-applied at rates of 0, 0.56, 2.80 and 5.60 t ha(-1) equivalent CaO, and the treated columns were leached with local rainfall in an open-air nursery during a 20-month period. For all column types and rates of application, the leaching of Ca incr eased in the order: control < CaCO3 + MgO < CaCO3 much less than CaSO4 . 2H(2)O. Retention of solubilized Ca from gypsum was mainly achieved by simple exchange with resident cations, with no major pH change rel ative to the control. The elution of Ca and desorbed cations with pred ominantly SO42- anions resulted in an important increase in total cati onic mobilization compared to the control. Due to kinetic restraints a nd strong buffering by the soil, the downward movement of Ca (from CaC O3) and Mg (from MgO) ions was greatly limited. The mobility of Mg ion s from CaCO3 + MgO was however much greater than that of Ca, despite c onsiderably larger inputs of the latter. In the leachates from the col umns without humus, total Al concentrations decreased exponentially wi th increase in pH. For the other column types the reduced Al leaching, when observed, was not associated to any major pH increase. Gypsum ap plication depressed nitrification but increased DOC leaching relative to the control. Liming generally had no or little influence on either NO3- or DOC leaching, but increased SO42-, mobilization in some cases. Total cation leaching from the non gypsum-treated columns was chiefly affected by NO3- dynamics. Varying the rate of fertilizer application did not modify the mechanisms involved in the downward movement of th e various fertilizers, but could influence the intensity of the proces ses as well as the rate of soil chemical evolution.