SPATIAL VARIABILITY IN EXCHANGEABLE CALCIUM CONCENTRATION IN VERMONT SPODOSOLS

Spodosols formed via weathering of glacial till in the Green Mountains of Vermont display variability in exchangeable calcium concentrations. Soils that formed on till deposited down gradient and within 5 km of the carbonate-rich Champlain Valley are depleted in exchangeable Ca relative to till-derived spodosols > 8 km from the Champlain Valley. A, E and B horizons within 5 km of the valley contain 28 - 310 mg/kg of exchangeable calcium. These soils contain 1 - 6% (wt.) plagioclase but no other apparent primary Ca source— carbonate minerals are absent in all soils to a depth of 130 cm. In A, E and B horizons located > 8 km from valley carbonates, the concentration of exchangeable calcium ranges from 150-1200 mg/kg. These soils contain 15 - 20% plagioclase as well as up to 12% hornblende, indicating that the main controls on exchangeable Ca are the distribution and weathering rate of primary Ca minerals. The relative stability of plagioclase and hornblende (as compared to carbonates) in weathering environments provides a long-term Ca source to the soils derived from till rich in metamorphic rock clasts. Bedrock in the Champlain Valley is chlorite grade sedimentary rocks that are carbonate-rich and relatively lacking in calcium-bearing silicates. Also interesting to note is the spatial variability of exchangeable Al. A, E and B horizons of Ca-depleted, valley margin soils contain exchangeable Al concentrations of 75-500 mg/kg. Conversely, in Ca-enriched soils farther from the valley, exchangeable aluminum is present at levels < 50 mg/kg. Thus, till-derived spodosols on the western flanks of the Green Mountains are Ca-depleted, Al-enriched, and likely are more prone to effects of acid deposition than spodosols higher in the Green Mountains. The spodosols analyzed in this study contain a heterogeneous mix of vermiculite, hydrobiotite, smectite, R1-ordered illite/smectite and rarely kaolinite, yet variability in clay content does not correlate well with variability in exchangeable Ca.