County soil summary
Soil types in Cook County, Illinois
The most widespread soils are Ozaukee, Markham and Ashkum; the most common drainage class is moderately well drained (30% of the land), and typical topsoil pH is 6.7.
- Well or moderately well drained
- 40% of land · state 41%
- Typical topsoil pH
- 6.7 neutral · state 6.5
- Septic fields very limited
- 69% of land · state 85%
- Subject to flooding
- 2% of land, any frequency · state 13%
Most common soils in Cook County
Share of mapped land
A county is a patchwork of many soils. These 8 cover 75% of the mapped land; 77 named soil series are mapped here in all.
| Soil | Share of land | Drainage | Topsoil pH |
|---|---|---|---|
| Urban landLand type, not a soil series | 29.9% | — | — |
| OrthentsGeneral soil group, not a named series | 18.4% | — | — |
| Ozaukee | 6.3% | Moderately well drained | 6.6 |
| Markham | 5.6% | Moderately well drained | 5.8 |
| Ashkum | 5.5% | Poorly drained | 7.0 |
| Anthroportic UdorthentsGeneral soil group, not a named series | 4.2% | — | — |
| Milford | 3.4% | Poorly drained | 6.1 |
| Elliott | 2.1% | Somewhat poorly drained | 6.8 |
What this means
What the soils in Cook County mean for gardens, lawns and septic systems
For your garden
Typical topsoil in Cook County is neutral: pH 6.7 (most land between 5.9 and 7.3), the range most vegetables, flowers and lawn grasses prefer.
Loamy topsoil (loam, silt loam and similar) covers 53% of the land: generally the easiest soil to garden, holding moisture without staying waterlogged. The most common surface texture is silt loam (25% of the land).
For your lawn and yard
On 67% 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 30% 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 69% of the land in Cook County very limited, <1% somewhat limited and <1% not limited (31% not rated, such as urban land, pits or water).
Statewide, 85% of the land is very limited, so Cook 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.
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 moderately well drained (30%). 40% of the land is moderately well drained or better, compared with 41% across Illinois.
Show as a table
| Category | Cook County | Illinois |
|---|---|---|
| Excessively drained | 1.2% | 1.0% |
| Somewhat excessively drained | 0.1% | 0.5% |
| Well drained | 7.9% | 25.9% |
| Moderately well drained | 30.4% | 13.7% |
| Somewhat poorly drained | 12.7% | 30.5% |
| Poorly drained | 14.4% | 26.0% |
| Very poorly drained | 2.7% | 1.3% |
| Not rated | 30.6% | 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 | Cook County | Illinois |
|---|---|---|
| Not limited | 0.1% | 0.0% |
| Somewhat limited | 0.5% | 12.5% |
| Very limited | 68.5% | 85.2% |
| Not rated | 31.0% | 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 | Cook County | Illinois |
|---|---|---|
| A: fast infiltration (mostly sandy or gravelly) | 0.7% | 2.3% |
| B: moderate infiltration | 1.8% | 21.2% |
| C: slow infiltration | 20.0% | 16.1% |
| D: very slow infiltration (clayey, shallow or wet) | 20.7% | 5.9% |
| A/D: D as is, A if drained | 2.3% | 1.0% |
| B/D: D as is, B if drained | 5.0% | 20.1% |
| C/D: D as is, C if drained | 18.5% | 31.2% |
| Not rated | 31.0% | 2.1% |
Flooding
Show as a table
| Category | Cook County | Illinois |
|---|---|---|
| No flooding | 92.7% | 84.6% |
| Rare or very rare | 0.0% | 1.4% |
| Occasional | 0.0% | 4.0% |
| Frequent or very frequent | 1.7% | 8.0% |
| Not reported | 5.6% | 2.0% |
Seasonal high water table
Show as a table
| Category | Cook County | Illinois |
|---|---|---|
| Within 18 inches at some time of year | 29.6% | 54.1% |
| 18 inches to 3.3 feet | 30.4% | 17.1% |
| Deeper than 3.3 feet or none reported | 34.4% | 26.9% |
| Not reported | 5.6% | 1.9% |
Topsoil pH and texture
First mineral layer of each soil
Typical topsoil pH is 6.7 (neutral); 80% of rated land falls between 5.9 and 7.3. Median topsoil organic matter is 2.5%.
Show as a table
| Category | Cook County | Illinois |
|---|---|---|
| Strongly acidic (below 5.5) | 0.4% | 1.3% |
| Moderately to slightly acidic (5.5–6.4) | 15.4% | 45.4% |
| Near neutral (6.5–7.3) | 45.9% | 48.7% |
| Alkaline (7.4 and up) | 6.3% | 2.1% |
| Not reported | 31.9% | 2.5% |
Texture
Show as a table
| Category | Cook County | Illinois |
|---|---|---|
| Sandy | 1.7% | 1.5% |
| Loamy (loam, silt loam and similar) | 52.6% | 93.6% |
| Clayey (40%+ clay) | 13.6% | 2.4% |
| Organic (muck or peat) | 1.2% | 0.3% |
| Not reported (rock, urban land, etc.) | 31.0% | 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
- Cook County, Illinois (FIPS 17031): 603,063 acres of mapped land, 12,982 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.