Effects of parent material and weathering on feldspar content in different particle size fractions from forest soils in NW Germany

The feldspar content of three soils developed from Pleistocene calcareous till, glacial sand and loess in NW Germany was investigated in order to characterize the stores of feldspar in detailed particle split as well as changes as a function of soil depth. Feldspars are present in all fractions <2000 μm except the <0.2-μm fraction. In the 100- to 500-μm sand fractions, there is less feldspar content than in coarser or finer fractions. Changes during soil development intensify this distribution mainly because feldspars in the sand fractions have undergone physical weathering and appear in the silt and clay size fractions. The feldspar content of particle size fractions of a soil derived from calcareous till is definitely higher than that of a soil from glacial sand. The feldspars are composed of nearly equal amounts of Or and Ab phases, whereas the amount of An phase is smaller. An and Ab phases decrease most strongly in a soil derived from glacial sand. The change in feldspar contents has its highest intensity in a soil derived from glacial sand. Gains in feldspar in silt fractions result from weathering of feldspar sand grains. Additionally, decomposition of feldspar-containing rock particles of gravel size and aeolian sedimentation may have contributed to these gains as well. On account of the distinct differences in mineralogical composition between different particle size fractions, feldspar contents of the bulk soil <2000 μm are clearly determined by particle size distribution. This is important if there is need for an easy way of predicting the nutrient store from the feldspar content.