Soils

The Alderwood Series is the single most common soil under residential landscapes in the Seattle, Tacoma, and Everett corridor. If you garden on a hill or ridge anywhere in King, Snohomish, or Pierce County, you are very likely gardening on Alderwood. What makes it distinctive is a two-layer structure. The top two to three feet is loose, gravelly sandy loam left behind by glaciers. Water and roots move through it easily. But underneath sits an extremely dense layer of compacted glacial material called a densic contact. This layer is so hard that roots cannot penetrate it and water cannot drain through it at any useful rate. It is not bedrock, but for practical purposes it behaves like a concrete floor buried under your garden. The result is a soil that creates two opposite problems in the same year. In winter, rain percolates quickly through the upper layer, hits the dense layer, and has nowhere to go. Water pools above it, creating a perched water table that saturates the root zone from roughly December through April. In summer, because the effective root zone is only two to three feet deep, there is very little stored moisture to draw on. Plants that survived waterlogged roots in February are drought-stressed by August. This wet-winter, dry-summer cycle is the defining challenge of gardening on Alderwood soil.

TThe Bellingham Series represents the classic heavy clay soil found in parts of western Washington. High clay content gives the soil vertic properties, meaning it expands when wet and shrinks as it dries. This seasonal movement presses soil aggregates, called peds, against each other and forms smooth surfaces known as pressure faces. During dry periods the shrinking clay often produces visible surface cracks. For gardeners, this soil behaves exactly like what most people mean when they say “heavy clay.” It drains slowly in winter and spring because water moves poorly through the fine particles. In summer it dries hard and dense. Organic matter, mulch, and careful timing when working the soil can gradually improve structure and make it easier to manage.'

The Briscot Series is a floodplain soil with a twist: its layers are not uniform. Rivers deposit sediment in pulses, and each flood event leaves a different texture behind. One layer might be silt loam, the next fine sand, the next fine sandy loam. Soil scientists call this stratified alluvium, and it creates a soil profile that behaves differently at different depths. This layering is what separates Briscot from Woodinville, the other major poorly drained floodplain soil in the region. Woodinville is consistently fine-textured (silt loam throughout), so its drainage behavior is predictable: slow everywhere. Briscot is unpredictable. A coarse sandy layer sandwiched between two fine silt layers can create a perched water pocket that holds moisture long after the surrounding soil has drained. Dig a test hole in one spot and the soil drains in hours; dig another ten feet away and it stays wet for days. For gardeners, this variability is the central challenge. Standard drainage advice assumes the soil behaves the same way across a site, but Briscot does not. You need to test drainage at each specific planting location rather than assuming the entire yard will behave like one test hole. The good news is that the pH is nearly neutral (6.6-6.8), which means most plants will not need pH adjustment. Like all floodplain soils, Briscot is subject to occasional flooding. The same rivers that deposited the stratified sediment still flood these valleys periodically. The flat terrain and high water table during the wet season (November through April) mean the soil stays saturated for months.

The Everett Series is the opposite of Alderwood. Where Alderwood has a dense impermeable layer that traps water, Everett has no restrictive layer at all. Water drains straight through, and that is both its greatest strength and its biggest limitation. This soil formed from glacial outwash, the gravelly, sandy material deposited by meltwater rivers as the glaciers retreated. The texture is coarse: imagine sandy loam with 35 to 65 percent gravel mixed in. It looks and feels more like a gravel pit than a garden bed. Because of all that gravel and sand, water moves through very quickly. There is no perched water table, no winter waterlogging, and no risk of root rot from sitting water. The tradeoff is that this soil holds almost nothing. Water drains through before roots can take it up. Nutrients wash out just as fast. In the dry summer months, plants on Everett soil run out of moisture weeks before plants on heavier soils nearby. Without supplemental irrigation, most ornamental plantings will struggle from July through September. The soil is also strongly acid (pH around 5.3 to 5.6), which limits nutrient availability further. If you garden on Everett soil, your entire strategy revolves around holding onto water and nutrients that the soil naturally lets go of.

If you live on Alderwood soil, there is a good chance that Norma soil is somewhere nearby, sitting in the low spot where water collects. This is one of the most common soil pairings in Puget Sound residential neighborhoods: Alderwood on the slopes and ridges, Norma in the depressions and drainageways between them. The water that runs off the dense layer in Alderwood has to go somewhere, and it flows downhill into the low spots where Norma formed. Norma is poorly drained, but it has one advantage over the other poorly drained soils in this region: volcanic ash. The surface horizon contains ash from ancient Cascade eruptions, and that ash gives the soil better nutrient-holding capacity than you would expect from a wet, low-lying soil. Soil scientists call this an aquandic property. In practical terms, it means the topsoil is darker, more fertile, and more responsive to organic matter additions than comparable soils like Bellingham or Briscot. The texture is ashy loam in the upper layer over sandy loam below. The sandy loam subsoil is actually a hidden asset: once water is managed at the surface, the subsoil drains reasonably well. This makes Norma more fixable than soils like Woodinville or Bellingham, where the fine texture persists all the way down. For gardeners, the key insight is that Norma soil sits where water naturally collects. You cannot change the topography that sends water to these spots, but you can work with it. Rain gardens, French drains to intercept slope runoff, and raised beds for plants that need drainage all work well here. The volcanic ash in the topsoil gives you a better starting point for fertility than most poorly drained soils offer.

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.

The Seattle Series is not soil in the way most people think of soil. It is organic material all the way down, at least five feet deep, with no mineral soil underneath. Soil scientists classify it as a Histosol, which means the entire profile is made of decomposed plant matter rather than the sand, silt, and clay particles that make up mineral soils. If you dig into it, it looks and feels like dark, spongy peat. These soils formed in bogs and marshes where plant material accumulated faster than it decomposed. The waterlogged, oxygen-poor conditions preserved organic matter over thousands of years, building up thick deposits of muck and mucky peat. You can still see the history in the profile: the upper layers are highly decomposed muck (less than 10 percent recognizable plant fibers), while deeper layers retain more structure as mucky peat (25 to 70 percent fibers). For gardeners, the most important thing to understand is that organic soil behaves nothing like mineral soil. It holds enormous amounts of water but drains almost none of it. It is very strongly acid (pH as low as 4.8). It has high organic matter by definition, but that does not mean it is fertile in the way gardeners expect, because the nutrient chemistry of organic soils is fundamentally different from mineral soils. Micronutrients like copper, zinc, and boron are often deficient even though the soil looks rich and dark. There is also a practical reality that goes beyond gardening: many areas mapped as Seattle Series are now classified as jurisdictional wetlands. Federal and state regulations may restrict what you can do on these sites, including drainage, grading, and planting. Check your property's wetland status before planning any significant work.

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.

The Woodinville Series is what people in the Kent and Green River valley are actually talking about when they say 'Kent clay.' The name is misleading, though, because the texture is silt loam, not true clay. Silt particles are larger than clay particles, and the distinction matters: silt loam does not shrink and swell the way true clay does (for that, see the Bellingham Series). But it still drains poorly, compacts easily, and feels heavy and sticky when wet. This soil formed from layers of sediment deposited by river flooding over thousands of years. It sits on the flat valley floor and collects water from surrounding slopes. During the wet season, the water table rises to the surface or very near it, and the soil stays saturated for months. In summer it dries slowly and can become quite firm. For gardeners, the practical reality is that this soil is difficult to work. If you grab a handful when it is wet and squeeze it into a wire or worm shape, it is too wet to dig or till. Working silt loam in that condition destroys the soil structure and creates hard, compacted layers that persist for years. Timing is everything: there is a narrow window in late spring and early fall when the soil is moist but not saturated, and that is when you can work it without causing damage.