Soil series profile
Opal soil series
Opal is a well drained soil with a clay surface, mapped on 1,052,750 acres in 22 counties across 2 states. Hydrologic group D.
- Drainage class
- Well drained 100% of mapped acres
- Hydrologic group
- D 100% of mapped acres
- Typical topsoil pH
- 7.6 slightly alkaline · range 7.6–7.6
- Mapped area
- 1.1M acres in 22 counties, 2 states
Typical Opal profile
Texture and pH by depth
Show as a table
| Depth | Texture | pH | Clay | Sand | Organic matter |
|---|---|---|---|---|---|
| 0–5 in (A) | Clay | 7.6 | 55% | 6% | 3% |
| 5–15 in (Bss) | Clay | 7.6 | 65% | 4% | 2.4% |
| 15–24 in (Bkss) | Clay | 7.9 | 65% | 6% | 1.6% |
| 24–33 in (Cyz) | Clay | 7.6 | 55% | 6% | 0.9% |
| 33–53 in (Cr) | Bedrock | Not reported | Not reported | Not reported | Not reported |
What this means
What Opal soil means for gardens, lawns and septic systems
For gardens
Typical topsoil in Opal soils is slightly alkaline: pH 7.6. Acid-loving plants such as blueberries and azaleas struggle without amended beds, and some trees and shrubs show iron chlorosis (yellow leaves with green veins).
Clayey topsoil covers 100% of mapped Opal acres. Clay holds water and nutrients but drains slowly and compacts if worked wet: add compost, avoid tilling soggy soil and consider raised beds. The most common surface texture is clay (96% of mapped Opal acres).
100% of mapped Opal acres is moderately well to excessively drained, which suits most garden plants.
For lawns and yards
On 100% 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 100% of the acres in mapped Opal soils very limited, 0% 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.
Where Opal soil is found
Mapped acres
Opal is mapped in South Dakota and Nebraska.
| County | Acres | Share of county land |
|---|---|---|
| Dewey County, SD | 282,314 | 19.2% |
| Stanley County, SD | 161,009 | 17.3% |
| Lyman County, SD | 153,856 | 14.8% |
| Jones County, SD | 74,344 | 12.1% |
| Brule County, SD | 50,062 | 6.1% |
| Buffalo County, SD | 50,062 | 6.1% |
| Haakon County, SD | 49,913 | 4.3% |
| Corson County, SD | 47,622 | 3.0% |
| Mellette County, SD | 40,468 | 4.9% |
| Ziebach County, SD | 34,759 | 2.8% |
| Walworth County, SD | 22,064 | 4.9% |
| Potter County, SD | 19,503 | 3.5% |
| Tripp County, SD | 18,265 | 1.8% |
| Hyde County, SD | 16,575 | 3.0% |
| Campbell County, SD | 12,312 | 2.6% |
| Hughes County, SD | 5,602 | 1.2% |
| Sully County, SD | 5,410 | 0.8% |
| Jackson County, SD | 4,467 | 0.4% |
| Gregory County, SD | 2,474 | 0.4% |
| Hand County, SD | 889 | 0.1% |
| Charles Mix County, SD | 669 | 0.1% |
| Boyd County, NE | 108 | 0.0% |
Septic absorption fields
NRCS rating, share of Opal acres
Show as a table
| Category | Share of acres |
|---|---|
| Not limited | 0.0% |
| Somewhat limited | 0.0% |
| Very limited | 100.0% |
Drainage, water table and flooding
Share of Opal 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 4–15% (median 9%).
- Topsoil organic matter
- Median 3%.
Drainage class
Show as a table
| 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
Show as a table
| Category | Share of acres |
|---|---|
| A: fast infiltration (mostly sandy or gravelly) | 0.0% |
| B: moderate infiltration | 0.0% |
| C: slow infiltration | 0.0% |
| D: very slow infiltration (clayey, shallow or wet) | 100.0% |
| 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 Opal (component kind "Series") in SSURGO: 1,052,750 acres.
- Method
- Shares are by mapped acres of Opal components. The profile comes from the single largest Opal component. Full methodology.