Tanana Soil Series

The Tanana soil series is Alaska's recognized state soil. It sits on the flat alluvial terraces and floodplains of interior Alaska's major river systems — gently sloping land built from layers of silty loess and river sediment.

What sets Tanana soil apart from every other state soil in the country is permafrost. Beneath the surface layers lies frozen ground that stays frozen year-round. That layer controls drainage, limits root depth, and defines the character of interior Alaska.

Soil scientists at the USDA Natural Resources Conservation Service selected the Tanana series to represent Alaska because it captures the state's most distinctive feature: permafrost. Alaska is the only U.S. state with a permafrost soil as its recognized state soil.

The Tanana series was first established in the Yukon-Tanana Region of Alaska in 1914 — one of the earliest soil series described in the state. It has been studied and mapped across the interior lowlands for over a century.

Unlike most states, Alaska's Legislature has not passed a formal law designating an official state soil. The Tanana series is recognized by the USDA as Alaska's representative state soil, chosen by Alaska soil scientists to represent the interior lowlands that define so much of the state's geography.

If you dug into Tanana soil, the first thing you would see is a dark, spongy mat of moss and plant litter — sometimes several inches thick. Below that, the soil turns to brownish-gray silt. Dig to about two feet down, and the ground is permanently frozen solid.

Tanana soil is found in the interior lowlands of Alaska, on flat floodplains and low terraces alongside major river systems. It is most common in the Greater Fairbanks area and the surrounding interior lowlands.

The soil is estimated to cover 3 to 5 million acres across interior Alaska, though much of that range has not yet been fully mapped. Slopes are gentle — typically 0 to 12 percent — because Tanana soil forms on level to gently sloping river terraces.

In its natural state, Tanana soil supports a boreal forest of aspen, paper birch, white spruce, and black spruce — the same tree species that cover interior Alaska's lowland valleys.

When Tanana soil is cleared and the organic surface layer is removed, the permafrost below begins to thaw over time. As it thaws, the soil can become usable farmland. Cleared Tanana soils support hay and pasture, small grains, and vegetables.

Farming Tanana soil requires careful management. Removing the moss layer that insulates the frozen ground causes the permafrost to thaw and the surface to sink and become waterlogged — a process called thermokarst. Drainage work is essential for any sustained agricultural use.