Soil series profile

Doubtful soil series

Doubtful is a well drained soil with a gravelly ashy sandy loam surface, mapped on 67,789 acres in 4 counties in WA. Hydrologic group B.

#4736 by mapped area ashy over sandy or sandy-skeletal Typic Vitricryands (Andisols)

Drainage class: Well drained
Hydrologic group: B
Typical topsoil pH: 6.0
Mapped area: 68k

Typical Doubtful 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).

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Typical Doubtful horizons (representative values)
Depth	Texture	pH	Clay	Sand	Organic matter
0–1 in (Oi)	Slightly decomposed plant material	4.5	4%	70%	75%
1–4 in (A)	Gravelly ashy sandy loam	6.0	4%	70%	7%
4–11 in (Bw1)	Gravelly ashy sandy loam	6.4	4%	70%	3%
11–23 in (Bw2)	Gravelly ashy sandy loam	6.2	4%	70%	3%
23–28 in (2C)	Extremely gravelly loamy coarse sand	6.1	0%	80%	0.5%
28–60 in (R)	Unweathered bedrock	Not reported	Not reported	Not reported	Not reported

Representative values from the most extensive Doubtful map unit (Doubtful-Treen-Rock outcrop complex, 35 to 100 percent slopes, survey area WA774). 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 Doubtful soil means for gardens, lawns and septic systems

For gardens

Typical topsoil in Doubtful soils is moderately acidic: pH 6.0. Many vegetables grow well here, and some gardens benefit from a little lime. Test before adding any.

Sandy topsoil covers 100% of mapped Doubtful acres. It drains fast and warms early in spring, but dries out and loses nutrients quickly: mulch, compost and steady watering help. The most common surface texture is gravelly ashy sandy loam (100% of mapped Doubtful acres).

100% of mapped Doubtful 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 100% of the acres in mapped Doubtful 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 Doubtful soil is found

Mapped acres

Doubtful is mapped in Washington.

Counties with the most mapped Doubtful soil
County	Acres	Share of county land
Whatcom County, WA	41,603	4.0%
Chelan County, WA	13,237	0.7%
Skagit County, WA	12,594	1.6%
Okanogan County, WA	355	0.0%

Septic absorption fields

NRCS rating, share of Doubtful acres

Not limited 0%

Somewhat limited 0%

Very limited 100%

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

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Septic tank absorption field rating across mapped Doubtful acres.
Category	Share of acres
Not limited	0.0%
Somewhat limited	0.0%
Very limited	100.0%

Drainage, water table and flooding

Share of Doubtful acres

Depth to seasonal high water table: None reported within about 6.5 feet (median where reported).
Flooding: Not mapped as flooding on most acres.
Slope: Typically 65–75% (median 75%).
Topsoil organic matter: Median 7%.

Drainage class

Excessively drained 0%

Somewhat excessively drained 0%

Well drained 100%

Moderately well drained 0%

Somewhat poorly drained 0%

Poorly drained 0%

Very poorly drained 0%

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

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

Hydrologic soil group

A: fast infiltration (mostly sandy or gravelly) 0%

B: moderate infiltration 100%

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%

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

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Hydrologic soil group across mapped Doubtful acres.
Category	Share of acres
A: fast infiltration (mostly sandy or gravelly)	0.0%
B: moderate infiltration	100.0%
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%

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 Doubtful (component kind "Series") in SSURGO: 67,789 acres.
Method: Shares are by mapped acres of Doubtful components. The profile comes from the single largest Doubtful component. Full methodology.

Independent reference site. No government affiliation or endorsement.

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