Seattle Series

Key Challenges

• Very poorly drained. Without artificial drainage, water sits at or near the surface year-round. These were bogs and marshes before development, and the soil still behaves like one.
• Subsidence is the hidden problem. When organic soils are drained for agriculture or development, the organic material begins to decompose in the presence of oxygen. As it decomposes, the ground surface literally drops. Some drained peat soils in western Washington have subsided several feet over decades. This process cannot be reversed.
• The soil looks rich but may not be fertile. Organic soils have different nutrient chemistry than mineral soils. Micronutrients like copper, zinc, and boron are often deficient even though the soil appears dark, organic, and healthy. Standard fertilizer programs designed for mineral soils may not address the actual deficiencies.
• Very strongly acid (pH as low as 4.8). Most vegetables, fruit trees, and common landscape plants need a higher pH. Lime can raise it, but the organic material naturally re-acidifies over time, so liming is an ongoing commitment rather than a one-time fix.
• Wetland regulations may apply. Many areas mapped as Seattle Series are jurisdictional wetlands under federal and state law, which restricts drainage, grading, filling, and sometimes even planting. Check before investing in any site work.
• The ground cannot support heavy loads. Organic soil compresses under weight, which means patios, retaining walls, and other hardscape may shift or sink over time. Structural foundations require special engineering on these soils.

Amendment & Management Strategy

• The most successful approach is to work with what this soil does well rather than trying to make it behave like mineral soil. Acid-loving plants like blueberries thrive here without amendment because the naturally low pH and organic substrate are exactly what they need.
• If you want to grow vegetables or other plants that need a higher pH, lime is necessary. But organic soils re-acidify faster than mineral soils, so plan on liming every year or two rather than once. Soil testing annually is worth the investment here.
• Micronutrient supplementation is often more important than standard NPK fertilizer. Have the soil tested specifically for copper, zinc, and boron. These are commonly deficient in organic soils even when nitrogen, phosphorus, and potassium are adequate.
• Bog garden and rain garden plantings are the highest-value use of this soil in a landscape setting. The conditions that make it difficult for conventional gardening are exactly what bog plants, native sedges, and wetland species need.

Drainage Solutions

• Tile drainage has historically been used to convert these sites for agriculture, particularly blueberry production. But drainage comes with a cost: once drained, the organic material oxidizes and the ground surface subsides. This is a one-way process.
• Raised beds filled with imported mineral soil are the most practical approach for vegetable and ornamental gardening. Even 12 inches of mineral soil mix above grade creates a completely different growing environment for roots.
• Before installing any drainage, check whether your property falls within a mapped wetland. State and federal regulations may prohibit or restrict drainage, and the penalties for unpermitted wetland alteration are significant.

Plant Suitability