County soil summary
Soil types in Polk County, Iowa
The most widespread soils are Clarion, Webster and Nicollet; the most common drainage class is well drained (38% of the land), and typical topsoil pH is 6.5.
- Well or moderately well drained
- 48% of land · state 55%
- Typical topsoil pH
- 6.5 slightly acidic · state 6.3
- Septic fields very limited
- 66% of land · state 91%
- Subject to flooding
- 16% of land, any frequency · state 15%
Most common soils in Polk County
Share of mapped land
A county is a patchwork of many soils. These 8 cover 61% of the mapped land; 76 named soil series are mapped here in all.
| Soil | Share of land | Drainage | Topsoil pH |
|---|---|---|---|
| Clarion | 18.4% | Well drained | 6.2 |
| Webster | 8.8% | Poorly drained | 6.5 |
| Nicollet | 8.7% | Somewhat poorly drained | 6.2 |
| Urban landLand type, not a soil series | 8.5% | — | — |
| Canisteo | 6.4% | Poorly drained | 7.5 |
| Coland | 4.4% | Poorly drained | 6.7 |
| Storden | 3.3% | Well drained | 7.7 |
| Lester | 2.8% | Well drained | 6.6 |
What this means
What the soils in Polk County mean for gardens, lawns and septic systems
For your garden
Typical topsoil in Polk County is slightly acidic: pH 6.5 (most land between 6.2 and 7.5), the range most vegetables, flowers and lawn grasses prefer.
Loamy topsoil (loam, silt loam and similar) covers 89% of the land: generally the easiest soil to garden, holding moisture without staying waterlogged. The most common surface texture is loam (44% of the land).
42% of the land is somewhat poorly to very poorly drained, so soil stays soggy for long spells after rain and in wet seasons. Raised beds, plants that tolerate wet feet, and not working the soil while wet all help.
For your lawn and yard
On 59% of the land, 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.
A seasonal high water table comes within 18 inches of the surface on 33% of the land, often in spring. Basements, sump pumps and lawns in low spots feel it first.
For a septic system
For conventional septic absorption fields, NRCS rates 66% of the land in Polk County very limited, 23% somewhat limited and 0% not limited (11% not rated, such as urban land, pits or water).
Statewide, 91% of the land is very limited, so Polk County is less 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.
Flooding is mapped on 16% of the land (12% occasional, 3% frequent).
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 (38%). 48% of the land is moderately well drained or better, compared with 55% across Iowa.
Show as a table
| Category | Polk County | Iowa |
|---|---|---|
| Excessively drained | 0.4% | 1.2% |
| Somewhat excessively drained | 1.4% | 0.7% |
| Well drained | 37.7% | 35.5% |
| Moderately well drained | 8.0% | 17.2% |
| Somewhat poorly drained | 15.0% | 19.9% |
| Poorly drained | 25.0% | 21.9% |
| Very poorly drained | 1.8% | 2.7% |
| Not rated | 10.7% | 0.9% |
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 | Polk County | Iowa |
|---|---|---|
| Not limited | 0.0% | 0.0% |
| Somewhat limited | 23.1% | 8.0% |
| Very limited | 66.1% | 91.2% |
| Not rated | 10.8% | 0.8% |
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 | Polk County | Iowa |
|---|---|---|
| A: fast infiltration (mostly sandy or gravelly) | 2.1% | 2.4% |
| B: moderate infiltration | 28.2% | 13.2% |
| C: slow infiltration | 16.8% | 36.9% |
| D: very slow infiltration (clayey, shallow or wet) | 0.6% | 11.4% |
| A/D: D as is, A if drained | 0.2% | 0.2% |
| B/D: D as is, B if drained | 13.5% | 4.9% |
| C/D: D as is, C if drained | 27.8% | 30.0% |
| Not rated | 10.8% | 1.0% |
Flooding
Show as a table
| Category | Polk County | Iowa |
|---|---|---|
| No flooding | 82.5% | 84.5% |
| Rare or very rare | 0.6% | 3.2% |
| Occasional | 12.4% | 9.8% |
| Frequent or very frequent | 2.7% | 2.2% |
| Not reported | 1.8% | 0.3% |
Seasonal high water table
Show as a table
| Category | Polk County | Iowa |
|---|---|---|
| Within 18 inches at some time of year | 33.4% | 43.3% |
| 18 inches to 3.3 feet | 8.5% | 6.5% |
| Deeper than 3.3 feet or none reported | 56.3% | 49.9% |
| Not reported | 1.8% | 0.3% |
Topsoil pH and texture
First mineral layer of each soil
Typical topsoil pH is 6.5 (slightly acidic); 80% of rated land falls between 6.2 and 7.5. Median topsoil organic matter is 3.5%.
Show as a table
| Category | Polk County | Iowa |
|---|---|---|
| Strongly acidic (below 5.5) | 0.1% | 0.3% |
| Moderately to slightly acidic (5.5–6.4) | 38.2% | 52.6% |
| Near neutral (6.5–7.3) | 40.0% | 37.3% |
| Alkaline (7.4 and up) | 10.8% | 8.6% |
| Not reported | 10.9% | 1.2% |
Texture
Show as a table
| Category | Polk County | Iowa |
|---|---|---|
| Sandy | 0.5% | 1.1% |
| Loamy (loam, silt loam and similar) | 88.6% | 96.5% |
| Clayey (40%+ clay) | 0.0% | 1.1% |
| Organic (muck or peat) | 0.1% | 0.3% |
| Not reported (rock, urban land, etc.) | 10.9% | 1.0% |
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
- Polk County, Iowa (FIPS 19153): 367,118 acres of mapped land, 11,750 acres of open water excluded. State comparisons pool all mapped land in Iowa.
- 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.