County soil summary
Soil types in Napa County, California
The most widespread soils are Bressa, Henneke and Maymen; the most common drainage class is well drained (59% of the land), and typical topsoil pH is 6.5.
- Well or moderately well drained
- 79% of land · state 81%
- Typical topsoil pH
- 6.5 slightly acidic · state 6.7
- Septic fields very limited
- 92% of land · state 79%
- Subject to flooding
- 11% of land, any frequency · state 11%
Most common soils in Napa County
Share of mapped land
A county is a patchwork of many soils. These 8 cover 55% of the mapped land; 82 named soil series are mapped here in all.
| Soil | Share of land | Drainage | Topsoil pH |
|---|---|---|---|
| Bressa | 15.4% | Well drained | 6.7 |
| Henneke | 11.2% | Well drained | 6.9 |
| Rock outcropLand type, not a soil series | 8.6% | — | — |
| ForwardGeneral soil group, not a named series | 5.8% | — | — |
| Maymen | 3.8% | Somewhat excessively drained | 5.8 |
| Dibble | 3.6% | Well drained | 6.1 |
| Sobrante | 3.5% | Well drained | 6.1 |
| Hambright | 3.3% | Well drained | 6.3 |
What this means
What the soils in Napa County mean for gardens, lawns and septic systems
For your garden
Typical topsoil in Napa County is slightly acidic: pH 6.5 (most land between 5.8 and 6.7), the range most vegetables, flowers and lawn grasses prefer.
Loamy topsoil (loam, silt loam and similar) covers 86% 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).
79% of the land is moderately well to excessively drained, which suits most garden plants.
For your lawn and yard
On 90% 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 92% of the land in Napa County very limited, 4% somewhat limited and 0% not limited (3% not rated, such as urban land, pits or water).
Statewide, 79% of the land is very limited, so Napa County is more 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 11% of the land (<1% occasional, 3% 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 (59%). 79% of the land is moderately well drained or better, compared with 81% across California.
Show as a table
| Category | Napa County | California |
|---|---|---|
| Excessively drained | 11.3% | 7.7% |
| Somewhat excessively drained | 5.9% | 17.6% |
| Well drained | 58.5% | 50.7% |
| Moderately well drained | 2.8% | 4.7% |
| Somewhat poorly drained | 5.5% | 2.5% |
| Poorly drained | 3.1% | 2.0% |
| Very poorly drained | 1.3% | 0.7% |
| Not rated | 11.5% | 14.2% |
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 | Napa County | California |
|---|---|---|
| Not limited | 0.0% | 4.3% |
| Somewhat limited | 4.3% | 3.6% |
| Very limited | 92.4% | 79.2% |
| Not rated | 3.3% | 12.9% |
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 | Napa County | California |
|---|---|---|
| A: fast infiltration (mostly sandy or gravelly) | 0.6% | 20.5% |
| B: moderate infiltration | 5.9% | 12.5% |
| C: slow infiltration | 42.8% | 22.7% |
| D: very slow infiltration (clayey, shallow or wet) | 44.7% | 30.3% |
| A/D: D as is, A if drained | 0.0% | 0.3% |
| B/D: D as is, B if drained | 0.0% | 0.3% |
| C/D: D as is, C if drained | 2.8% | 0.9% |
| Not rated | 3.3% | 12.6% |
Flooding
Show as a table
| Category | Napa County | California |
|---|---|---|
| No flooding | 85.5% | 77.1% |
| Rare or very rare | 7.5% | 8.2% |
| Occasional | 0.3% | 1.6% |
| Frequent or very frequent | 3.0% | 1.0% |
| Not reported | 3.7% | 12.1% |
Seasonal high water table
Show as a table
| Category | Napa County | California |
|---|---|---|
| Within 18 inches at some time of year | 2.9% | 2.6% |
| 18 inches to 3.3 feet | 3.4% | 1.9% |
| Deeper than 3.3 feet or none reported | 90.9% | 86.9% |
| Not reported | 2.8% | 8.6% |
Topsoil pH and texture
First mineral layer of each soil
Typical topsoil pH is 6.5 (slightly acidic); 80% of rated land falls between 5.8 and 6.7. Median topsoil organic matter is 2%.
Show as a table
| Category | Napa County | California |
|---|---|---|
| Strongly acidic (below 5.5) | 2.0% | 3.1% |
| Moderately to slightly acidic (5.5–6.4) | 41.8% | 22.8% |
| Near neutral (6.5–7.3) | 40.5% | 25.2% |
| Alkaline (7.4 and up) | 3.9% | 30.6% |
| Not reported | 11.8% | 18.2% |
Texture
Show as a table
| Category | Napa County | California |
|---|---|---|
| Sandy | 0.0% | 21.3% |
| Loamy (loam, silt loam and similar) | 85.6% | 56.7% |
| Clayey (40%+ clay) | 2.6% | 3.9% |
| Organic (muck or peat) | 0.0% | 0.2% |
| Not reported (rock, urban land, etc.) | 11.8% | 18.0% |
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
- Napa County, California (FIPS 06055): 441,255 acres of mapped land, 20,831 acres of open water excluded. State comparisons pool all mapped land in California.
- 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.