LOCALIZATION OF BASE CATIONS IN PARTICLE-SIZE FRACTIONS OF ACID FOREST SOILS (VOSGES MOUNTAINS, N-E FRANCE)

The chemistry and the quantitative mineralogy of clay, silt and sand f ractions of two acid forest soils from the Strengbach catchment (Vosge s Mountains) were investigated. Storage of Ca, Mg, K and Na as well as their potential availability for forest nutrition was determined. Min eral fine earth contains between 0.06 and 0.40% of CaO. Calcium is loc ated in apatite (highest percentages in coarse silt) and albite (highe st percentages in fine sand). Sand fractions store relatively high amo unts of Ca in protected positions. Magnesium storage in mineral fine e arth is also very small, i.e. between 0.37 and 0.81% of MgO. Magnesium is principally located in small-sized minerals, such as smectite and illite, as well as in corresponding transformation products. The poten tial release rate of Mg by mineral weathering is limited by the presen ce of stabilizing hydroxy-Al polymers in interlayers of smectite and t he transformation of illite into interstratified illite-vermiculite, n ot accompanied by a high Mg release. Sodium and K are clearly more abu ndant than Ca and Mg. Similarly to Ca, Na shows higher percentages in rather coarse grained particle size fractions, especially in albite of fine sand. Potassium storage depends on the distribution of white mic a (highest amounts in fine fractions) and of K-feldspar (highest amoun ts in coarse particle size fractions). Altogether, extremely small sto res of Ca and Mg determine potentially low release rates. Additionally , reduced reactive surface areas of apatite, incongruent transformatio n reactions of illite, as well as interlayered hydroxy-Al polymers of smectite, hinder the release of Ca and Mg from mineral fine earth. Whe ther there is a sufficient supply of these elements by mineral weather ing to balance the mineral budget of these forest stands is very uncer tain. (C) 1998 Elsevier Science B.V.