Localization of base cations in particle size fractions of acid forest soils (Vosges Mountains, N-E France)

Abstract

The chemistry and the quantitative mineralogy of clay, silt and sand fractions of two acid forest soils from the Strengbach catchment (Vosges Mountains) were investigated. Storage of Ca, Mg, K and Na as well as their potential availability for forest nutrition was determined. Mineral fine earth contains between 0.06 and 0.40% of CaO. Calcium is located in apatite (highest percentages in coarse silt) and albite (highest percentages in fine sand). Sand fractions store relatively high amounts of Ca in protected positions. Magnesium storage in mineral fine earth 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 potential release rate of Mg by mineral weathering is limited by the presence of stabilizing hydroxy-Al polymers in interlayers of smectite and the transformation of illite into interstratified illite–vermiculite, not accompanied by a high Mg release. Sodium and K are clearly more abundant 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 mica (highest amounts in fine fractions) and of K-feldspar (highest amounts in coarse particle size fractions). Altogether, extremely small stores of Ca and Mg determine potentially low release rates. Additionally, reduced reactive surface areas of apatite, incongruent transformation reactions of illite, as well as interlayered hydroxy-Al polymers of smectite, hinder the release of Ca and Mg from mineral fine earth. Whether there is a sufficient supply of these elements by mineral weathering to balance the mineral budget of these forest stands is very uncertain.