County soil summary
Soil types in Winnebago County, Illinois
The most widespread soils are Comfrey, Osco and Ogle; the most common drainage class is well drained (62% of the land), and typical topsoil pH is 6.6.
- Well or moderately well drained
- 71% of land · state 41%
- Typical topsoil pH
- 6.6 neutral · state 6.5
- Septic fields very limited
- 70% of land · state 85%
- Subject to flooding
- 10% of land, any frequency · state 13%
Most common soils in Winnebago County
Share of mapped land
A county is a patchwork of many soils. These 8 cover 33% of the mapped land; 109 named soil series are mapped here in all.
| Soil | Share of land | Drainage | Topsoil pH |
|---|---|---|---|
| GriswoldGeneral soil group, not a named series | 6.2% | — | — |
| Comfrey | 5.2% | Poorly drained | 7.0 |
| Osco | 4.8% | Well drained | 6.0 |
| Ogle | 3.9% | Well drained | 6.3 |
| Jasper | 3.5% | Well drained | 6.2 |
| Plano | 3.4% | Well drained | 6.7 |
| Greenbush | 3.1% | Well drained | 6.4 |
| Flagler | 2.5% | Somewhat excessively drained | 6.5 |
What this means
What the soils in Winnebago County mean for gardens, lawns and septic systems
For your garden
Typical topsoil in Winnebago County is neutral: pH 6.6 (most land between 6.0 and 7.0), the range most vegetables, flowers and lawn grasses prefer.
Loamy topsoil (loam, silt loam and similar) covers 92% of the land: generally the easiest soil to garden, holding moisture without staying waterlogged. The most common surface texture is silt loam (55% of the land).
71% 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.
A seasonal high water table comes within 18 inches of the surface on 27% 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 70% of the land in Winnebago County very limited, 27% somewhat limited and 0% not limited (3% not rated, such as urban land, pits or water).
Statewide, 85% of the land is very limited, so Winnebago 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 10% of the land (2% occasional, 8% 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 (62%). 71% of the land is moderately well drained or better, compared with 41% across Illinois.
Show as a table
| Category | Winnebago County | Illinois |
|---|---|---|
| Excessively drained | 3.3% | 1.0% |
| Somewhat excessively drained | 4.5% | 0.5% |
| Well drained | 61.7% | 25.9% |
| Moderately well drained | 1.0% | 13.7% |
| Somewhat poorly drained | 12.3% | 30.5% |
| Poorly drained | 14.6% | 26.0% |
| Very poorly drained | 0.9% | 1.3% |
| Not rated | 1.7% | 1.0% |
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 | Winnebago County | Illinois |
|---|---|---|
| Not limited | 0.0% | 0.0% |
| Somewhat limited | 26.7% | 12.5% |
| Very limited | 69.9% | 85.2% |
| Not rated | 3.3% | 2.3% |
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 | Winnebago County | Illinois |
|---|---|---|
| A: fast infiltration (mostly sandy or gravelly) | 7.3% | 2.3% |
| B: moderate infiltration | 52.0% | 21.2% |
| C: slow infiltration | 8.2% | 16.1% |
| D: very slow infiltration (clayey, shallow or wet) | 3.5% | 5.9% |
| A/D: D as is, A if drained | 1.0% | 1.0% |
| B/D: D as is, B if drained | 23.5% | 20.1% |
| C/D: D as is, C if drained | 1.3% | 31.2% |
| Not rated | 3.3% | 2.1% |
Flooding
Show as a table
| Category | Winnebago County | Illinois |
|---|---|---|
| No flooding | 88.2% | 84.6% |
| Rare or very rare | 0.0% | 1.4% |
| Occasional | 1.5% | 4.0% |
| Frequent or very frequent | 8.4% | 8.0% |
| Not reported | 2.0% | 2.0% |
Seasonal high water table
Show as a table
| Category | Winnebago County | Illinois |
|---|---|---|
| Within 18 inches at some time of year | 26.9% | 54.1% |
| 18 inches to 3.3 feet | 1.7% | 17.1% |
| Deeper than 3.3 feet or none reported | 69.4% | 26.9% |
| Not reported | 2.0% | 1.9% |
Topsoil pH and texture
First mineral layer of each soil
Typical topsoil pH is 6.6 (neutral); 80% of rated land falls between 6.0 and 7.0. Median topsoil organic matter is 3.5%.
Show as a table
| Category | Winnebago County | Illinois |
|---|---|---|
| Strongly acidic (below 5.5) | 0.6% | 1.3% |
| Moderately to slightly acidic (5.5–6.4) | 29.5% | 45.4% |
| Near neutral (6.5–7.3) | 63.4% | 48.7% |
| Alkaline (7.4 and up) | 2.8% | 2.1% |
| Not reported | 3.7% | 2.5% |
Texture
Show as a table
| Category | Winnebago County | Illinois |
|---|---|---|
| Sandy | 3.9% | 1.5% |
| Loamy (loam, silt loam and similar) | 92.0% | 93.6% |
| Clayey (40%+ clay) | 0.0% | 2.4% |
| Organic (muck or peat) | 0.8% | 0.3% |
| Not reported (rock, urban land, etc.) | 3.3% | 2.3% |
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
- Winnebago County, Illinois (FIPS 17201): 312,598 acres of mapped land, 5,450 acres of open water excluded. State comparisons pool all mapped land in Illinois.
- 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.