Hydroxy-interlayered minerals of forest soils in A-Li Mountain, Taiwan

Two forest soils (Typic Hapludult and Typic Haplorthod), on A-Li Mountain, central Taiwan were selected for this study. Soil physical and chemical properties, clay mineral compositions, and characteristics of layer charge were investigated. The amount and composition of their interlayer materials were studied by sodium citrate extraction in combination with X-ray diffraction (XRD) analysis. High soil acidity and strong leaching processes resulted in low soil pH (3.5–5.2) and low base saturation. Illite and kaolinite were the major clay minerals in surface horizons, no hydroxy-interlayered vermiculite (HIV) was found in O, A, and E horizons. The greatest extent of interlayering phenomena was present in the Bt2 horizon of Ultisol and the Bs2 horizon of Spodosol. Iron (Fe) incorporated with Al is the major component of the interlayer materials because the hydroxy-interlayered materials partially blocked the interlayer space and neutralized layer charge. A possible explanation for the occurrence of hydroxy-interlayered vermiculite (HIV) in forest soils was suggested. The limited interlayering in surface horizons was interpreted to be caused by a low pH, preventing the precipitation of hydroxy-interlayered materials. In addition, interlayer material from 2:1 clay minerals may be removed by abundant organic matter, through its marked chelating properties relative to Fe and Al cations. The downward leaching of Fe and Al were logically necessary for the development of extensive interlayering behavior in the B horizon where soil pH increased and organic matter contents decreased.