Library / Soils / Puyallup Series

Puyallup Series

The Puyallup Series is the best soil for growing things in the Puget Sound region. That is not an exaggeration. It has deep, dark, fertile topsoil over well-drained subsoil, with no hardpan, no seasonal water table, and a near-neutral pH. It is the soil that made the Green River, Puyallup, and Snoqualmie valleys into some of the most productive agricultural land in western Washington. The top 18 inches is what soil scientists call a mollic epipedon, which simply means a thick, dark, organic-rich surface layer. You can see it when you dig: the soil is noticeably darker than surrounding soils because it has accumulated organic matter over centuries of river deposition. This dark topsoil is naturally fertile and holds both water and nutrients well. Below 18 inches, the texture changes abruptly to gravelly sand. This coarser subsoil drains freely, which prevents waterlogging. The combination of a fertile, moisture-holding top layer over a well-drained bottom layer is ideal for plants: roots get both the nutrients and the oxygen they need. The catch is that Puyallup soil sits on floodplains. River flooding deposited the sediment that made this soil so good, and rivers still flood these valleys periodically. If your property is in an active floodplain, the same process that created this excellent soil also creates the risk of periodic inundation.

Quick Facts

Texture Fine sandy loam / loam over gravelly sand
Drainage Well drained
pH Range 6.0-6.8 (slightly acid to neutral, which is ideal for most vegetables and landscape plants)
Parent Material Recent mixed alluvium
Landform Floodplains and low stream terraces
Prevalence Moderate extent — river terraces in Green, Puyallup, Snoqualmie, and other valleys
Seasonal Water Table None under normal conditions
Taxonomic Class Coarse-loamy over sandy or sandy-skeletal, isotic over mixed, mesic Fluventic Haploxerolls

Key Challenges

  • Flood risk is the main concern. These soils formed from river sediment, and the rivers that deposited them still flood periodically. Check whether your property is in an active floodplain before investing in permanent plantings.
  • The change from fine topsoil to coarse gravel at about 18 inches means water behavior shifts abruptly with depth. The topsoil holds moisture well, but once roots reach the gravel layer, water drains away quickly. Summer irrigation may be needed sooner than expected.
  • Flood zone regulations may restrict what you can build or plant in certain areas. It is worth checking local rules before planning major landscape work.
  • The near-neutral pH (6.0-6.8) is ideal for most plants but too high for acid-loving species like blueberries and rhododendrons without amendment.

Amendment & Management Strategy

  • Maintain the organic matter that makes this soil productive. Regular compost additions to the surface and cover crops in vegetable beds over winter keep the topsoil dark, fertile, and well-structured.
  • Cover crops are especially valuable here. The exposed topsoil on flat valley land erodes during heavy winter rain, and a cover crop of crimson clover or cereal rye holds the soil in place and adds organic matter when turned under in spring.
  • Moderate fertilization is usually sufficient. The naturally fertile topsoil provides a strong baseline, and excessive nitrogen on this soil type leaches into the gravelly subsoil and eventually into groundwater.

Drainage Solutions

  • Drainage is not typically a problem. The well-drained subsoil takes care of excess water under normal conditions.
  • For properties in active floodplain areas, raised beds or bermed planting areas provide some protection for permanent plantings during minor flood events.

Plant Suitability

Well Suited

  • Vegetable crops of all kinds. This is the best agricultural soil in the region, and it shows. Anything you want to grow in a vegetable garden will likely do well here.
  • Fruit trees and berry crops, which benefit from the deep, fertile topsoil, good drainage, and unrestricted root depth
  • Deep-rooted ornamental trees thrive because there is no hardpan or restrictive layer to stop root growth
  • Most landscape plants perform well in this near-neutral, well-drained soil without significant amendment

Avoid

  • Acid-loving plants like blueberries and rhododendrons may need sulfur to lower the pH, which runs higher here (6.0-6.8) than on most other regional soils
  • Permanent, high-value plantings in areas with active flood risk, unless you have assessed and accepted that risk

Official Source

USDA-NRCS Official Series Description

Native Tree Species

Douglas-fir Western redcedar Bigleaf maple Black cottonwood Western hemlock Red alder

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