Soil series profile

Alderwood soil series

Alderwood is a moderately well drained soil with a gravelly sandy loam surface, mapped on 533,386 acres in 9 counties in WA. Hydrologic group B.

#504 by mapped area loamy-skeletal Aquic Dystroxerepts (Inceptisols)

Drainage class: Moderately well drained
Hydrologic group: B
Typical topsoil pH: 5.8
Mapped area: 533k

Typical Alderwood profile

Texture and pH by depth

Depth to scale. More strongly shaded layers hold more clay. Horizon names are the soil survey's (A = topsoil, B = subsoil, C = parent material).

Show as a table

Typical Alderwood horizons (representative values)
Depth	Texture	pH	Clay	Sand	Organic matter
0–7 in (A)	Gravelly sandy loam	5.8	8%	65%	9%
7–21 in (Bw1)	Very gravelly sandy loam	5.8	8%	70%	2.5%
21–30 in (Bw2)	Very gravelly sandy loam	5.8	8%	65%	2.5%
30–35 in (Bg)	Very gravelly sandy loam	6.0	8%	65%	0.5%
35–43 in (2Cd1)	Very gravelly sandy loam	6.0	5%	70%	0.05%
43–59 in (2Cd2)	Very gravelly sandy loam	6.0	5%	70%	0.05%

Representative values from the most extensive Alderwood map unit (Alderwood gravelly sandy loam, 8 to 15 percent slopes, survey area WA633). Other map units of this series vary within the series range.

Stock photo for illustration, not a sample of this soil series. Credit on the Sources & method page.

What this means

What Alderwood soil means for gardens, lawns and septic systems

For gardens

Typical topsoil in Alderwood soils is moderately acidic: pH 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 >99% of mapped Alderwood acres: generally the easiest soil to garden, holding moisture without staying waterlogged. The most common surface texture is gravelly sandy loam (90% of mapped Alderwood acres).

100% of mapped Alderwood acres is moderately well to excessively drained, which suits most garden plants.

For lawns and yards

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 >99% of the acres in mapped Alderwood soils very limited, 0% somewhat limited and 0% not limited.

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.

Generated from the mapped numbers above using fixed rules, not a site visit. See how these notes are written.

Where Alderwood soil is found

Mapped acres

Alderwood is mapped in Washington.

Counties with the most mapped Alderwood soil
County	Acres	Share of county land
King County, WA	190,769	15.6%
Mason County, WA	85,563	14.9%
Snohomish County, WA	82,340	13.9%
Pierce County, WA	54,800	5.0%
Kitsap County, WA	47,754	18.9%
Thurston County, WA	39,051	7.9%
Jefferson County, WA	32,929	5.0%
Skagit County, WA	154	0.0%
Lewis County, WA	25	0.0%

Septic absorption fields

NRCS rating, share of Alderwood acres

Not limited 0%

Somewhat limited 0%

Very limited >99%

Not rated <1%

Septic tank absorption field rating across mapped Alderwood acres. Scale 0 to 100% of mapped land.

Show as a table

Septic tank absorption field rating across mapped Alderwood acres.
Category	Share of acres
Not limited	0.0%
Somewhat limited	0.0%
Very limited	99.7%
Not rated	0.3%

Drainage, water table and flooding

Share of Alderwood acres

Depth to seasonal high water table: About 2.2 feet (26 in) (median where reported).
Flooding: Not mapped as flooding on most acres.
Slope: Typically 2–20% (median 10%).
Topsoil organic matter: Median 9%.

Drainage class

Excessively drained 0%

Somewhat excessively drained 0%

Well drained 0%

Moderately well drained 100%

Somewhat poorly drained 0%

Poorly drained 0%

Very poorly drained 0%

Drainage class across mapped Alderwood acres. Scale 0 to 100% of mapped land.

Show as a table

Drainage class across mapped Alderwood acres.
Category	Share of acres
Excessively drained	0.0%
Somewhat excessively drained	0.0%
Well drained	0.0%
Moderately well drained	100.0%
Somewhat poorly drained	0.0%
Poorly drained	0.0%
Very poorly drained	0.0%

Hydrologic soil group

A: fast infiltration (mostly sandy or gravelly) 6%

B: moderate infiltration 89%

C: slow infiltration 0%

D: very slow infiltration (clayey, shallow or wet) 0%

A/D: D as is, A if drained 0%

B/D: D as is, B if drained 0%

C/D: D as is, C if drained 0%

Not rated 5%

Hydrologic soil group across mapped Alderwood acres. Scale 0 to 100% of mapped land.

Show as a table

Hydrologic soil group across mapped Alderwood acres.
Category	Share of acres
A: fast infiltration (mostly sandy or gravelly)	5.5%
B: moderate infiltration	89.3%
C: slow infiltration	0.0%
D: very slow infiltration (clayey, shallow or wet)	0.0%
A/D: D as is, A if drained	0.0%
B/D: D as is, B if drained	0.0%
C/D: D as is, C if drained	0.0%
Not rated	5.2%

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: All map unit components named Alderwood (component kind "Series") in SSURGO: 533,386 acres.
Method: Shares are by mapped acres of Alderwood components. The profile comes from the single largest Alderwood component. Full methodology.

Independent reference site. No government affiliation or endorsement.

Full narrative description: USDA Official Soil Series Description for Alderwood.