County soil summary
Soil types in Ada County, Idaho
The most widespread soils are Chilcott, Purdam and Power; the most common drainage class is well drained (84% of the land), and typical topsoil pH is 7.2.
- Well or moderately well drained
- 92% of land · state 88%
- Typical topsoil pH
- 7.2 neutral · state 6.7
- Septic fields very limited
- 89% of land · state 86%
- Subject to flooding
- 6% of land, any frequency · state 3%
Most common soils in Ada County
Share of mapped land
A county is a patchwork of many soils. These 8 cover 41% of the mapped land; 124 named soil series are mapped here in all.
| Soil | Share of land | Drainage | Topsoil pH |
|---|---|---|---|
| Chilcott | 9.6% | Well drained | 7.2 |
| Purdam | 7.0% | Well drained | 7.0 |
| Power | 5.5% | Well drained | 7.2 |
| Scism | 4.3% | Well drained | 7.9 |
| Tadpole | 4.1% | Well drained | 7.8 |
| Elijah | 3.8% | Well drained | 7.4 |
| Colthorp | 3.5% | Well drained | 7.2 |
| Corder | 3.1% | Well drained | 7.4 |
What this means
What the soils in Ada County mean for gardens, lawns and septic systems
For your garden
Typical topsoil in Ada County is neutral: pH 7.2 (most land between 6.7 and 7.8), the range most vegetables, flowers and lawn grasses prefer.
Loamy topsoil (loam, silt loam and similar) covers 90% of the land: generally the easiest soil to garden, holding moisture without staying waterlogged. The most common surface texture is silt loam (58% of the land).
92% of the land is moderately well to excessively drained, which suits most garden plants.
For your lawn and yard
On 77% 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.
For a septic system
For conventional septic absorption fields, NRCS rates 89% of the land in Ada County very limited, 6% somewhat limited and 2% not limited (3% not rated, such as urban land, pits or water).
That is close to the state as a whole (86% very 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.
Flooding is mapped on 6% of the land (1% occasional, <1% 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 (84%). 92% of the land is moderately well drained or better, compared with 88% across Idaho.
Show as a table
| Category | Ada County | Idaho |
|---|---|---|
| Excessively drained | 1.0% | 1.0% |
| Somewhat excessively drained | 6.0% | 6.2% |
| Well drained | 83.9% | 77.5% |
| Moderately well drained | 1.0% | 3.3% |
| Somewhat poorly drained | 3.2% | 1.5% |
| Poorly drained | 0.1% | 1.5% |
| Very poorly drained | 0.1% | 0.6% |
| Not rated | 4.5% | 8.3% |
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 | Ada County | Idaho |
|---|---|---|
| Not limited | 2.3% | 1.4% |
| Somewhat limited | 5.9% | 7.0% |
| Very limited | 88.5% | 85.5% |
| Not rated | 3.3% | 6.2% |
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 | Ada County | Idaho |
|---|---|---|
| A: fast infiltration (mostly sandy or gravelly) | 9.7% | 11.4% |
| B: moderate infiltration | 10.7% | 37.7% |
| C: slow infiltration | 38.8% | 26.4% |
| D: very slow infiltration (clayey, shallow or wet) | 37.4% | 18.8% |
| A/D: D as is, A if drained | 0.3% | 0.3% |
| B/D: D as is, B if drained | 0.1% | 0.8% |
| C/D: D as is, C if drained | 0.0% | 1.9% |
| Not rated | 2.9% | 2.8% |
Flooding
Show as a table
| Category | Ada County | Idaho |
|---|---|---|
| No flooding | 93.9% | 91.2% |
| Rare or very rare | 4.7% | 0.7% |
| Occasional | 1.1% | 1.1% |
| Frequent or very frequent | 0.3% | 1.1% |
| Not reported | 0.0% | 5.8% |
Seasonal high water table
Show as a table
| Category | Ada County | Idaho |
|---|---|---|
| Within 18 inches at some time of year | 0.3% | 3.2% |
| 18 inches to 3.3 feet | 2.0% | 2.5% |
| Deeper than 3.3 feet or none reported | 97.7% | 88.9% |
| Not reported | 0.0% | 5.4% |
Topsoil pH and texture
First mineral layer of each soil
Typical topsoil pH is 7.2 (neutral); 80% of rated land falls between 6.7 and 7.8. Median topsoil organic matter is 1.5%.
Show as a table
| Category | Ada County | Idaho |
|---|---|---|
| Strongly acidic (below 5.5) | 0.0% | 3.4% |
| Moderately to slightly acidic (5.5–6.4) | 4.8% | 26.8% |
| Near neutral (6.5–7.3) | 58.8% | 41.3% |
| Alkaline (7.4 and up) | 31.9% | 20.1% |
| Not reported | 4.5% | 8.4% |
Texture
Show as a table
| Category | Ada County | Idaho |
|---|---|---|
| Sandy | 5.5% | 3.4% |
| Loamy (loam, silt loam and similar) | 90.0% | 87.6% |
| Clayey (40%+ clay) | 0.1% | 0.3% |
| Organic (muck or peat) | 0.0% | 0.4% |
| Not reported (rock, urban land, etc.) | 4.4% | 8.3% |
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
- Ada County, Idaho (FIPS 16001): 569,650 acres of mapped land, 5,669 acres of open water excluded. State comparisons pool all mapped land in Idaho.
- 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.