County soil summary
Soil types in King County, Washington
The most widespread soils are Alderwood, Playco and Everett; the most common drainage class is well drained (41% of the land), and typical topsoil pH is 5.6.
- Well or moderately well drained
- 84% of land · state 82%
- Typical topsoil pH
- 5.6 moderately acidic · state 6.5
- Septic fields very limited
- 93% of land · state 81%
- Subject to flooding
- 5% of land, any frequency · state 4%
Most common soils in King County
Share of mapped land
A county is a patchwork of many soils. These 8 cover 48% of the mapped land; 104 named soil series are mapped here in all.
| Soil | Share of land | Drainage | Topsoil pH |
|---|---|---|---|
| Alderwood | 15.6% | Moderately well drained | 5.8 |
| Playco | 6.2% | Well drained | 5.0 |
| Urban landLand type, not a soil series | 5.1% | — | — |
| Everett | 4.8% | Somewhat excessively drained | 5.3 |
| Pitcher | 4.3% | Well drained | 5.6 |
| Nimue | 4.2% | Well drained | 4.8 |
| Tokul | 4.1% | Moderately well drained | 5.8 |
| Kaleetan | 3.5% | Well drained | 4.8 |
What this means
What the soils in King County mean for gardens, lawns and septic systems
For your garden
Typical topsoil in King County is moderately acidic: pH 5.6 (most land between 4.8 and 5.8). Many vegetables grow well here, and some gardens benefit from a little lime. Test before adding any.
Loamy topsoil (loam, silt loam and similar) covers 70% of the land: generally the easiest soil to garden, holding moisture without staying waterlogged. The most common surface texture is gravelly sandy loam (21% of the land).
84% of the land is moderately well to excessively drained, which suits most garden plants.
For your lawn and yard
Most land is in hydrologic groups B and C, where rain soaks in at a moderate pace.
For a septic system
For conventional septic absorption fields, NRCS rates 93% of the land in King County very limited, <1% somewhat limited and 0% not limited (7% not rated, such as urban land, pits or water).
Statewide, 81% of the land is very limited, so King County is more restrictive than typical.
Where the soil is very limited (slow permeability, a high water table, flooding, shallow bedrock or steep slopes), alternative systems such as mound or sand-filter designs are common and cost more.
Only a licensed site evaluation or perc test at the actual lot decides whether a system is allowed and which design.
Drainage
How long the soil stays wet
The most common drainage class is well drained (41%). 84% of the land is moderately well drained or better, compared with 82% across Washington.
Show as a table
| Category | King County | Washington |
|---|---|---|
| Excessively drained | 4.4% | 2.1% |
| Somewhat excessively drained | 9.8% | 5.7% |
| Well drained | 41.1% | 65.7% |
| Moderately well drained | 28.6% | 8.7% |
| Somewhat poorly drained | 0.7% | 2.4% |
| Poorly drained | 5.0% | 2.3% |
| Very poorly drained | 2.1% | 0.7% |
| Not rated | 8.3% | 12.4% |
Septic absorption fields
NRCS rating for conventional drain fields
The rating looks at the soil from 24 to 60 inches deep: permeability, depth to a seasonal water table or bedrock, flooding and slope. “Very limited” means at least one feature is unfavorable enough that a conventional field is hard to make work.
Show as a table
| Category | King County | Washington |
|---|---|---|
| Not limited | 0.0% | 0.6% |
| Somewhat limited | 0.1% | 9.4% |
| Very limited | 93.0% | 81.4% |
| Not rated | 6.9% | 8.6% |
Rain, runoff and water table
Hydrologic soil group, flooding, seasonal water table
Hydrologic soil group
How fast rain soaks in when the soil is already wet. Group A soaks in fastest, D slowest.
Show as a table
| Category | King County | Washington |
|---|---|---|
| A: fast infiltration (mostly sandy or gravelly) | 17.2% | 15.6% |
| B: moderate infiltration | 44.2% | 37.1% |
| C: slow infiltration | 16.1% | 19.9% |
| D: very slow infiltration (clayey, shallow or wet) | 0.3% | 8.7% |
| A/D: D as is, A if drained | 0.5% | 0.8% |
| B/D: D as is, B if drained | 3.3% | 1.6% |
| C/D: D as is, C if drained | 2.6% | 2.2% |
| Not rated | 15.7% | 14.2% |
Flooding
Show as a table
| Category | King County | Washington |
|---|---|---|
| No flooding | 80.6% | 84.2% |
| Rare or very rare | 0.3% | 0.8% |
| Occasional | 3.4% | 2.3% |
| Frequent or very frequent | 1.1% | 1.1% |
| Not reported | 14.6% | 11.6% |
Seasonal high water table
Show as a table
| Category | King County | Washington |
|---|---|---|
| Within 18 inches at some time of year | 7.6% | 5.3% |
| 18 inches to 3.3 feet | 30.0% | 8.1% |
| Deeper than 3.3 feet or none reported | 47.7% | 75.1% |
| Not reported | 14.6% | 11.5% |
Topsoil pH and texture
First mineral layer of each soil
Typical topsoil pH is 5.6 (moderately acidic); 80% of rated land falls between 4.8 and 5.8. Median topsoil organic matter is 9%.
Show as a table
| Category | King County | Washington |
|---|---|---|
| Strongly acidic (below 5.5) | 37.3% | 17.7% |
| Moderately to slightly acidic (5.5–6.4) | 46.5% | 22.4% |
| Near neutral (6.5–7.3) | 1.3% | 37.6% |
| Alkaline (7.4 and up) | 0.0% | 8.2% |
| Not reported | 14.9% | 14.1% |
Texture
Show as a table
| Category | King County | Washington |
|---|---|---|
| Sandy | 17.4% | 7.9% |
| Loamy (loam, silt loam and similar) | 70.0% | 77.9% |
| Clayey (40%+ clay) | 0.0% | 0.0% |
| Organic (muck or peat) | 1.6% | 0.6% |
| Not reported (rock, urban land, etc.) | 11.0% | 13.6% |
Sources & data dates
- Source
- USDA NRCS Soil Survey Geographic Database (SSURGO), queried through Soil Data Access. Public domain.
- Data date
- SSURGO as published on November 3, 2026 (most recent survey area update October 26, 2026). NRCS refreshes SSURGO each October.
- Scope
- King County, Washington (FIPS 53033): 1,224,967 acres of mapped land, 16,686 acres of open water excluded. State comparisons pool all mapped land in Washington.
- Method
- Shares are by mapped land area: each map unit’s acres in the county times each soil component’s percentage. Open water is excluded. Topsoil values use the first mineral horizon of each component. Full methodology.