Williamson County, Illinois
The most common soil here is Ava, on about 12% of the land: a silt loam, smooth, like flour. Belknap and Bluford come next.
About 20% of the land drains well, and 31% stays wet for long stretches; groundwater comes within two feet of the surface under 33% of it.
USDA rates 85% of the land very limited for a standard septic drain field, and 63% very limited for homes with basements. That means extra cost or special design, not that it can't be done.
Worth knowing: 17% of the land floods occasionally or more, and bedrock lies within 40 inches under 7% of it.
Soils change over short distances, so look up an address below for the soil at that exact spot and the reasons behind each rating.
The soils you're most likely standing on
The biggest named soils in Williamson County, Illinois, each drawn five feet deep from its official USDA description, in its recorded colors.
Dots mark gravel; white flecks mark lime. Shares are by acres across the survey area, and each mapped patch usually mixes several of these soils.
The biggest mapped areas
The dominant drainage class is Moderately well drained and the dominant hydrologic group is C (slow infiltration, moderate runoff). The most common soil order is Alfisols — moderately leached forest soils with a clay-enriched subsoil. This report summarizes the major soil map units across the survey area to help you understand what to expect when buying, building, or gardening in Williamson County, Illinois. Search your address to find the exact soil composition, drainage, and series details at your specific location.
| Soil Map Unit | Acres | Drainage | Hydro Group | Dwellings | Septic |
|---|---|---|---|---|---|
| Belknap silt loam, 0 to 2 percent slopes, frequently flooded | 29K | Somewhat poorly drained | B/D | Very limited | Very limited |
| Ava silt loam, 2 to 5 percent slopes | 19K | Moderately well drained | C | Very limited | Very limited |
| Water | 15K | Not rated | Not rated | ||
| Plumfield silty clay loam, 5 to 10 percent slopes | 11K | Moderately well drained | D | Somewhat limited | Very limited |
| Ava silt loam, 5 to 10 percent slopes, eroded | 9K | Moderately well drained | C | Very limited | Very limited |
| Ava silty clay loam, 5 to 10 percent slopes, severely eroded | 8K | Moderately well drained | C | Somewhat limited | Very limited |
| Rend silty clay loam, 5 to 10 percent slopes, severely eroded | 8K | Moderately well drained | C | Very limited | Very limited |
| Bonnie silt loam, 0 to 2 percent slopes, frequently flooded | 7K | Poorly drained | C/D | Very limited | Very limited |
| Bluford silt loam, 2 to 5 percent slopes | 7K | Somewhat poorly drained | C/D | Very limited | Very limited |
| Hickory-Kell silt loams, 18 to 35 percent slopes | 7K | Well drained | B | Very limited | Very limited |
| Grantsburg silt loam, 2 to 5 percent slopes | 7K | Moderately well drained | C | Somewhat limited | Very limited |
| Bluford silt loam, bench, 2 to 5 percent slopes | 6K | Somewhat poorly drained | C/D | Very limited | Very limited |
| Hickory clay loam, 10 to 18 percent slopes, severely eroded | 6K | Well drained | B | Somewhat limited | Somewhat limited |
| Plumfield silty clay loam, 10 to 18 percent slopes | 6K | Moderately well drained | D | Somewhat limited | Very limited |
| Rend silt loam, 5 to 10 percent slopes, eroded | 6K | Moderately well drained | C | Very limited | Very limited |
| Sharon silt loam, 0 to 2 percent slopes, frequently flooded | 5K | Moderately well drained | C | Very limited | Very limited |
| Fairpoint gravelly silt loam, 7 to 20 percent slopes | 5K | Well drained | C | Somewhat limited | Very limited |
| Hickory-Kell silt loams, 10 to 18 percent slopes, eroded | 5K | Well drained | B | Somewhat limited | Somewhat limited |
| Hickory-Kell clay loams, 10 to 18 percent slopes, severely eroded | 5K | Well drained | B | Somewhat limited | Somewhat limited |
| Rend silt loam, 2 to 5 percent slopes | 5K | Moderately well drained | C | Very limited | Very limited |
Soil Orders in This Area
The USDA classifies every soil into one of 12 soil orders. Here are the dominant orders found in this survey area.
What This Means
Building & Foundations
About 63% of soils are rated "very limited" for dwellings with basements. Specific challenges include steep slopes, poor drainage in some areas, flood-prone areas. A geotechnical assessment is recommended before building.
Septic Systems
About 85% of soils are rated "very limited" for septic absorption fields. Despite good drainage overall, limitations may stem from shallow bedrock, steep slopes, or seasonal conditions. Engineered or alternative systems are frequently required. Always get a professional perc test before purchasing land that needs septic.
Gardening & Agriculture
Well-drained soils on sloped terrain — good for most plants but watch for erosion. Terracing, contour planting, and mulching help retain moisture and topsoil. Drip irrigation is more effective than sprinklers on slopes.