Tokul Soil Series

Tokul soil is defined by what sits beneath the surface: a cemented iron-and-humus hardpan called ortstein. This layer, roughly a foot or two down, blocks drainage and limits root depth — shaping the character of the conifer forests that cover millions of acres of western Washington.

The soil formed from glacial till left behind when the Vashon Glacier retreated about 15,000 years ago. As rain filtered downward through the sandy surface layers for thousands of years, it carried dissolved iron and organic acids into the subsoil and cemented them into the reddish-brown hardpan that makes Tokul soil unmistakable.

Washington's Legislature designated the Tokul series as the state soil in 2010, following a campaign organized by the Soil Science Society of America. The Washington Association of Conservation Districts and the USDA Natural Resources Conservation Service supported the effort.

Tokul was selected because it covers more land in western Washington than any comparable series and because its iron hardpan is a feature that foresters, farmers, and soil scientists across the Puget Sound region know by sight.

The series is named after Tokul Creek in King County, near the town of Snoqualmie, where USDA soil scientists first formally described and mapped the series. The type location remains in King County.

Dig into Tokul soil and the first thing you find is a thick mat of conifer duff — decaying needles, cones, and bark. Below it is a dark, spongy surface layer rich in humus. A few inches deeper comes a surprise: a pale, almost white layer where rain has stripped every trace of iron and color out of the sand. Then a shovel hits something hard — the ortstein, a reddish-brown layer cemented solid by iron. Below that is dense glacial till left by the Vashon ice sheet.

Tokul soil covers the gently rolling lowlands and hillsides of the Puget Sound region, from Whatcom County near the Canadian border south to Clark County near the Oregon line. It sits at elevations from near sea level to about 2,000 feet, on slopes of 0 to 65 percent, in areas that receive 40 to 80 inches of rain each year.

King County holds the type location, near Tokul Creek in the Snoqualmie Valley. The soil is also widespread in Snohomish, Pierce, Kitsap, and Mason counties — the counties ringing Puget Sound and extending to the Olympic Peninsula.

Douglas fir is the dominant tree on Tokul soil. Western Washington's timber industry was built on this series — Douglas fir tolerates the shallow, seasonally wet conditions created by the ortstein hardpan and grows tall in the cool, rainy climate. Western red cedar and western hemlock are also native to Tokul landscapes.

Christmas tree farms are a major use of Tokul soil in King, Snohomish, and Pierce counties. Noble fir, Douglas fir, and grand fir grow well in the shallow-rooted conditions. Blueberries, raspberries, and strawberries also grow on Tokul soils where moisture is held near the root zone but drainage is adequate.

Pastureland with orchardgrass and tall fescue occupies flatter Tokul areas with better drainage. Most row crops on Tokul soil need drainage management — tile drains or raised beds — to work around the cemented hardpan.