Tunbridge Soil Series

Tunbridge is a Spodosol — a soil order that forms only in cool, humid climates under acid forest. Rainwater, made acidic by decomposing pine needles and maple leaves, dissolves iron, aluminum, and organic matter from the upper soil and carries them downward. The result is a pale, almost white layer just below the surface, and a vivid reddish-brown layer below that where the dissolved material has re-deposited.

The soil is well-drained and moderately deep — typically 20 to 40 inches before striking the schist, gneiss, or granite bedrock of the Green Mountains. It is acidic throughout, with a pH between 4.5 and 5.5, which suits sugar maple, yellow birch, and American beech far better than most farm crops.

Soil scientists at the USDA Natural Resources Conservation Service selected the Tunbridge series to represent Vermont because it is the most widespread upland soil in the state, covering the hillsides and ridges of the Green Mountains and the surrounding foothills that define Vermont's landscape.

The series is named after the town of Tunbridge in Orange County, Vermont, where the soil was first formally described. Tunbridge sits in the White River valley on the eastern flank of the Green Mountains — in the heart of the soil's core distribution.

The Tunbridge series is recognized by the USDA as Vermont's state soil. It was chosen because it underlies Vermont's two defining land uses: the sugar maple forests that produce more maple syrup than any other state, and the rocky stone-walled pastures that anchored New England dairy farming for two centuries.

Dig into Tunbridge soil under a sugar maple forest and the first thing you hit is dark, spongy organic matter. Below that comes a very dark brown sandy loam, then something unexpected: a pale gray layer — almost white — where acid rain has stripped away all the iron and color. Then the soil turns dramatically darker: a reddish-brown spodic horizon stained by iron and humus carried down from above. A few inches further, the color fades to brown, and then you hit a rock.

Tunbridge soil covers the hillsides, ridges, and upland glacial terraces of the Green Mountains and their eastern and western foothills — the core of Vermont's terrain. It forms on slopes of 3 to 60 percent at elevations of 500 to 2,500 feet, in areas that receive 35 to 50 inches of precipitation per year.

Orange County is the type location, but the soil spreads across nearly every county in central and northern Vermont. Washington, Lamoille, Caledonia, and Windsor counties all have large Tunbridge areas on their hillsides and ridges.

Sugar maple is the tree most closely associated with Tunbridge soil. Vermont produces more maple syrup than any other state, and the majority of Vermont's sugar bushes — the maple groves tapped each spring — grow on Tunbridge and related stony, well-drained upland soils. The low fertility and acidity that limit farm crops suit sugar maple perfectly.

Dairy farmers cleared Tunbridge hillsides of their forest and boulders through the eighteenth and nineteenth centuries. The boulders went into the stone walls that still cross Vermont's landscape. The cleared land became pasture for the Holstein and Jersey cows at the heart of Vermont's dairy economy.

Today, many former Tunbridge pastures have returned to sugar maple and yellow birch forest as dairy farms consolidated and fields were abandoned. Hay, oats, and silage corn are grown on the flatter Tunbridge areas near valley floors, with lime applied to correct the naturally low pH.