DOWNWARD EFFECTS OF DOLOMITE AND KIESERITE ON 2 ACID SOILS DIFFERING IN THEIR ORGANIC-CARBON CONTENT

Mg application proved to be a valuable tool for the revitilization and restabilization of declining forest ecosystems growing on acid soils. Adequate fertilization however still requires clear understanding of the downward effects associated with Mg-fertilizers. To that purpose, leaching experiments with Mg solutions were performed in laboratory co lumns filled with two types of acid soil materials differing largely b y their organic carbon content : 80.2 g C kg-1 for soil A and 8.1 g C kg-1 for soil B. Magnesium was supplied either as a solution saturated in dolomite or as a pulse of kieserite solution. The columns were dis mantled for analysis after 8, 16, 24, and 32 d. leaching. Dolomite inc reased soil pH and diminished exchangeable Al in both substrates. Subs tantial increase of electrical charge was noticed only in substrate A. Consequently Ca and Mg reloading in substrate B was mainly due to ion exchange with Al, whereas in substrate A the major part of the increa se in exchangeable Ca and Mg was due to enhanced organic charge. Kiese rite induced very little changes in soil pH and all modifications in t he composition of exchange sites were achieved after the passage of th e sulfate peak. In the A substrate, the increase of Mg loading resulte d from an exchange process with Ca, whereas it was achieved at the exp ense of exchangeable Al in the B substrate. Calcium and Mg losses from the soil columns after 32 d were greater with kieserite than with dol omite. These losses were also larger for the hemiorganic A substrate t han for the mineral B substrate. These results imply that both the cha rge characteristics of the substrate (variable or fixed) together with the kind of Mg-fertilizer are essential to consider in fertilization programmes.