Downward Effects of Dolomite and Kieserite On two 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 columns filled with two types of acid soil materials differing largely by 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 dismantled for analysis after 8, 16, 24, and 32 d. leaching. Dolomite increased soil pH and diminished exchangeable Al in both substrates. Substantial 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 increase in exchangeable Ca and Mg was due to enhanced organic charge. Kieserite induced very little changes in soil pH and all modifications in the composition of exchange sites were achieved after the passage of the sulfate peak. In the A substrate, the increase of Mg loading resulted from an exchange process with Ca, whereas it was achieved at the expense of exchangeable Al in the B substrate. Calcium and Mg losses from the soil columns after 32 d were greater with kieserite than with dolomite. These losses were also larger for the hemiorganic A substrate than for the mineral B substrate. These results imply that both the charge characteristics of the substrate (variable or fixed) together with the kind of Mg-fertilizer are essential to consider in fertilization programmes.