The Complicated Story of Vashon Soils
If you want to know where you stand on stewarding your property, you need to know something about what you stand on, which is some version of the Island’s infamous soils.
The soil layer landowners think about most is the top one, because it’s obvious but also because it matters a lot to growing things. The sad news is that this layer of topsoil, which contains the precious component known as humus (decomposed plant and animal life that retains water, feeds small organisms, and fights erosion) isn’t very thick on Vashon. The historically dominant conifer forests just never provided much material to make humus, which elsewhere comes from fallen deciduous tree leaves and litter from plants that can’t grow under a dense conifer stand. It’s hard to have tilth (soil amenable to growing things like animal fodder, native plants, your strawberries) without humus-rich topsoil.
Underneath this lamentably thin layer is, in many parts of the island, outwash from the melting glacier that carved the Island millennia ago: sand, gravel, cobbles, and some boulders. Water moves quickly through this soil horizon because these materials don’t absorb much moisture. Making matter worse, the moving water leaches nutrients from the soil. That’s the double whammy you face when you try to grow things in recessional outwash. But, if there’s clay (finer particles than sand) above the outwash or intermixed with it, water will stay
around, caught in the denser soil structure clay’s presence provides. The presence of some clay can mean abundant groundwater for plants to use, in turn supporting wildlife. Much clay can mean standing water—maybe in the form of wetlands to be conserved and stewarded or maybe in the form of a drowned new stand of trees and woody natives you planted in the spring, when conditions were drier and you weren’t thinking about soils.
Still farther below ground in many places is the notorious Vashon Till, a soil layer hard-packed by the weight of the glacier mentioned a moment ago. Little can penetrate this layer except water, which penetrates only slowly. This relative impermeability is good news because it prevents many contaminants (e.g., a failing septic system’s discharge) from reaching the aquifers below, which are the Island’s sole source of drinking water.
The potentially bad news: In some places on the Island, the till sits near the ground’s surface. In those places, its hardness challenges plant growth. If its locale is a depression, though, the till’s closeness to the surface supports wetlands by preventing water from percolating downward too rapidly. So that’s good. But, in still other areas, surface-lying till allows water to race across it in winter, headed for the Sound and moving too quickly for the ground to absorb or streams to accommodate, which may lead to stream blowout. None of these till-related dynamics is necessarily a critical issue in an undisturbed landscape, but the Island is far from such.
What this soil situation means for a landowner can be as complicated as the soil itself. Later posts will address working with the soil you’ve inherited when you took on your place.
