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Snohomish Series

The Snohomish Series is a two-layer soil with an unusual structure: mineral silt loam on top, buried organic deposits underneath. The organic layer starts at about 30 inches and continues to at least 60 inches deep. Soil scientists call this a thapto-histic feature, which simply means there is a buried organic (peat) layer beneath the mineral surface. Here is what happened: thousands of years ago, a bog or marsh occupied these low-lying floodplain areas, accumulating thick organic deposits just like the Seattle Series. Then river flooding gradually buried those organic deposits under layers of mineral sediment. The result is a soil with two completely different personalities at different depths. The upper 30 inches is silt loam with a volcanic ash component, similar to other floodplain soils. Below that, the profile changes to decomposed organic material (muck and mucky peat) that behaves nothing like mineral soil. The practical consequence is that this soil has two drainage rates. The mineral surface drains moderately slowly, about what you would expect from silt loam. But once water reaches the buried organic layer, drainage changes. The organic material has moderate permeability when saturated, which is actually better than the fine-textured mineral soils above it. This is the opposite of what most people expect: the deeper organic layer moves water more easily than the shallower mineral layer. If you see skunkcabbage growing on a site, you are very likely looking at Snohomish soil. Skunkcabbage is the classic field indicator for this series. The combination of poor drainage, organic subsoil, and floodplain location creates exactly the conditions skunkcabbage prefers. Like other floodplain soils, Snohomish has a high water table (within a foot of the surface from November through May) and is subject to occasional flooding. Many areas mapped as Snohomish have been drained for agriculture, but the limits of that improvement should be understood: this is fundamentally a wetland soil that has been converted, not a well-drained soil.

Quick Facts

Texture Silt loam over buried organic deposits
Drainage Poorly drained
pH Range 5.8-6.2 (moderately acid to slightly acid; most vegetables and landscape plants do fine in this range, though acid-sensitive crops may benefit from light liming)
Parent Material Alluvium with volcanic ash component, underlain by organic deposits
Landform Floodplains and backswamps
Prevalence Small extent — floodplain backswamps in Puget lowlands
Seasonal Water Table Within 1 foot of the surface from November through May unless artificially drained
Taxonomic Class Fine-silty, mixed, superactive, nonacid, mesic Thapto-Histic Fluvaquents

Key Challenges

  • Poorly drained with a high water table within a foot of the surface from November through May. The flat floodplain terrain and fine-textured surface layer keep water in the root zone for most of the wet season.
  • Subject to occasional flooding from the rivers whose sediment created the mineral surface layer. Flood risk should be assessed before investing in permanent plantings or infrastructure.
  • The buried organic layer at 30 inches creates a soil with two different drainage behaviors in one profile. The mineral surface drains moderately slowly, while the organic subsoil has moderate permeability. This means drainage tile or deep French drains may behave differently than expected, because they are cutting through two fundamentally different materials.
  • This is a converted wetland, not a naturally well-drained site. Agricultural drainage (tile systems) has made many Snohomish areas productive for farming, but the underlying conditions have not changed. If drainage systems fail or are not maintained, the site reverts to its natural waterlogged state. Understanding this history helps set realistic expectations for what the soil can support long-term.

Amendment & Management Strategy

  • Raised beds are essential for most vegetable and ornamental gardening. The mineral surface layer is workable silt loam, but it sits on a water table that stays within a foot of the surface for seven months of the year. Raising the planting zone above that water table is the most direct path to successful growing.
  • Organic matter worked into the mineral surface layer improves structure and aeration. The volcanic ash component already gives this soil better nutrient-holding capacity than other poorly drained floodplain soils, so amendments build on a reasonable foundation.
  • The moderately acid pH (5.8-6.2) works for most landscape plants without amendment. If growing vegetables that prefer neutral pH (like brassicas or legumes), a light lime application every few years will bring the surface closer to 6.5.

Drainage Solutions

  • Tile drainage has historically been the primary tool for converting Snohomish soil to agricultural use. The organic subsoil below 30 inches actually has moderate permeability, which means tile drains placed at that depth can be effective at lowering the water table. This is a meaningful advantage over soils like Woodinville, where fine texture persists all the way down.
  • Surface grading and swales help move standing water off the flat terrain during the wet season. Even modest grading changes make a difference on floodplain soils where slopes are measured in fractions of a percent.
  • For restoration projects, understanding the buried organic layer is important. The organic subsoil represents the original wetland ecosystem, and removing drainage infrastructure allows the site to return to conditions closer to its natural state. This makes Snohomish soil particularly valuable for floodplain restoration work.

Plant Suitability

Well Suited

  • Native wetland and riparian species like western redcedar, red alder, and Sitka spruce, which tolerate the seasonal high water table and occasional flooding. These are the species that naturally colonize Snohomish soil when it is left unmanaged.
  • Skunkcabbage, sedges, salmonberry, redosier dogwood, and Douglas spirea are indicator species of this soil and thrive without any modification. If skunkcabbage is already growing on your site, lean into it rather than fighting it.
  • Floodplain restoration plantings. The buried organic layer makes this soil especially valuable for restoration because it preserves a record of the original wetland ecosystem. Restoration projects on Snohomish soil tend to establish quickly because the underlying conditions are already close to what wetland plants need.
  • Raised bed vegetables and ornamentals, taking advantage of the volcanic ash fertility in the surface layer and the slightly acid pH that works for most crops.

Avoid

  • Upland species planted at grade without drainage modification. Seven months of near-surface water table is fatal to most trees and shrubs that evolved in well-drained conditions.
  • Anything described as needing year-round drainage unless planted in raised beds or on mounds well above the seasonal water table.
  • High-value permanent plantings in areas with active flood risk. Fruit trees, ornamental specimens, and expensive landscape investments should only go in after you have assessed the flood risk and accepted it.

Official Source

USDA-NRCS Official Series Description

Native Tree Species

Red alder Western redcedar Western hemlock Sitka spruce

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