The Soil That Makes Washington Oh So Evergreen

21 Feb

Can you name the Washington state bird? How about the flower? Okay, now what about our official state soil?

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Many residents are completely unaware of the existence of an official state soil. But it exists! What a righteous way to honor the state you live in. The NRCS (Natural Resources Conservation Service) reports that “each state in the United States has selected a state soil, twenty of which have been legislatively established”. Washington is one of those righteous states, that has taken the time and energy to officially declare a state soil. Better know it! It’s the Tokul series.

But what is a ‘soil series’. How can a soil have an identity? Soil is after all, a living body that is constantly changing and developing. A series is the last classifying branch on the soil taxonomy tree. Soils have a taxonomic system, just like living organisms do; it is how soils are named. A soil series is a class of soils made up of soil individuals (like individual trees within a species) that have a common suite of soil profile properties.

The Tokul series is certainly one to take pride in. It is one of the most productive soils in the world! The name ‘Tokul’ comes from a small community and creek in King County. You will find over one million acres of the Tokul series along the Puget trough, from south of Seattle to the Canadian border. This soil fosters the growth of Douglas-firs, Western Hemlocks (our state tree) and other conifers. Therefore, the Tokul soil series is ultimately responsible for Washington’s well-known nickname, the Evergreen State.

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The Tokul soil series is found west of the Cascades, in conifer forests along the Puget trough. The areas in green depict where you’ll find this extensive soil series.

The Tokul series has a beautiful profile. It has an organic layer at the surface, due to the coniferous and forest debris. Below that, a dark-brown, organic rich, gravelly loam creates a fertile and well-drained environment for roots to penetrate and thrive. This sub-layer is strongly influenced by volcanic ash, which makes this soil a volcanic soil! Below the beautiful brown-reddish layer of soil (extending about 1-2 feet) you’ll uncover a somewhat cemented layer of soil, of a complete different shade, more of a grey color. This is because this soil formed from a different ‘parent’ material, a dense glacial till, which then was overlaid with volcanic ash and loess (wind-blown sediment). The combination of different geological events leads to a beautiful soil profile.

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Tokul soil profile. A volcanic ash and loamy layer overlays a dense, slightly cemented glacial till sub-layer. In this case, the top layer reached a depth of three feet! The very top layer, taking on a dark brown color (from 0-1/2 ft) is due to organic matter accumulation.

Sources:

Official NRCS Soil Series description:

https://soilseries.sc.egov.usda.gov/OSD_Docs/T/TOKUL.html

USDA publication on Tokul:

ftp://ftp-fc.sc.egov.usda.gov/NSSC/StateSoil_Profiles/wa_soil.pdf

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