Soil series profile

Purdam soil series

Purdam is a well drained soil with a silt loam surface, mapped on 256,271 acres in 12 counties in ID. Hydrologic group C.

#1364 by mapped area fine-silty Haploxerollic Durargids (Aridisols)

Drainage class: Well drained
Hydrologic group: C
Typical topsoil pH: 7.0
Mapped area: 256k

Typical Purdam 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 Purdam horizons (representative values)
Depth	Texture	pH	Clay	Sand	Organic matter
0–6 in (A)	Silt loam	7.0	23%	10%	1.5%
6–15 in (Bt1)	Silty clay loam	7.5	29%	7%	0.75%
15–21 in (Bt2)	Silt loam	7.9	19%	12%	0.25%
21–40 in (Bkq)	Cemented silt loam	7.9	19%	12%	0.25%
40–60 in (C)	Stratified loamy sand to loam	7.9	10%	53%	0.25%

Representative values from the most extensive Purdam map unit (Purdam silt loam, 0 to 4 percent slopes, survey area ID685). 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 Purdam soil means for gardens, lawns and septic systems

For gardens

Typical topsoil in Purdam soils is neutral: pH 7.0 (most land between 6.7 and 7.0), the range most vegetables, flowers and lawn grasses prefer.

Loamy topsoil (loam, silt loam and similar) covers 100% of mapped Purdam acres: generally the easiest soil to garden, holding moisture without staying waterlogged. The most common surface texture is silt loam (>99% of mapped Purdam acres).

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

For lawns and yards

On 98% of mapped acres, rain soaks in slowly (hydrologic groups C and D, including dual groups that are D until drained). Expect runoff and puddles after heavy rain: core-aerate compacted lawns and keep ground sloping away from foundations.

For a septic system

For conventional septic absorption fields, NRCS rates 98% of the acres in mapped Purdam soils very limited, 2% 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 Purdam soil is found

Mapped acres

Purdam is mapped in Idaho.

Counties with the most mapped Purdam soil
County	Acres	Share of county land
Elmore County, ID	69,447	5.7%
Owyhee County, ID	68,673	1.7%
Ada County, ID	39,963	7.0%
Twin Falls County, ID	39,222	3.9%
Canyon County, ID	17,331	5.4%
Payette County, ID	9,959	4.4%
Gem County, ID	5,846	2.1%
Gooding County, ID	2,296	0.6%
Washington County, ID	2,052	0.3%
Jerome County, ID	1,403	0.4%
Adams County, ID	42	0.0%
Lincoln County, ID	36	0.0%

Septic absorption fields

NRCS rating, share of Purdam acres

Not limited 0%

Somewhat limited 2%

Very limited 98%

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

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Septic tank absorption field rating across mapped Purdam acres.
Category	Share of acres
Not limited	0.0%
Somewhat limited	2.3%
Very limited	97.7%

Drainage, water table and flooding

Share of Purdam 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 1–7% (median 2%).
Topsoil organic matter: Median 1.5%.

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 Purdam acres. Scale 0 to 100% of mapped land.

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Drainage class across mapped Purdam 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 2%

C: slow infiltration 98%

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 Purdam acres. Scale 0 to 100% of mapped land.

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Hydrologic soil group across mapped Purdam acres.
Category	Share of acres
A: fast infiltration (mostly sandy or gravelly)	0.0%
B: moderate infiltration	2.3%
C: slow infiltration	97.7%
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 Purdam (component kind "Series") in SSURGO: 256,271 acres.
Method: Shares are by mapped acres of Purdam components. The profile comes from the single largest Purdam component. Full methodology.

Independent reference site. No government affiliation or endorsement.

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