Napa County, California
The dominant drainage class is Well drained and the dominant hydrologic group is D (very slow infiltration, high runoff). 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 Napa County, California.
| Soil Map Unit | Acres | Drainage | Hydro Group | Dwellings | Septic |
|---|---|---|---|---|---|
| Bressa-Dibble complex, 30 to 50 percent slopes | 60K | Well drained | C | Very limited | Very limited |
| Henneke gravelly loam, 30 to 75 percent slopes | 53K | Excessively drained | D | Very limited | Very limited |
| Maymen-Millsholm-Lodo association, 30-75 percent slopes | 30K | Somewhat excessively drained | D | Very limited | Very limited |
| Bressa-Dibble complex, 50 to 75 percent slopes | 22K | Well drained | C | Very limited | Very limited |
| Water | 21K | D | Not rated | Not rated | |
| Hambright rock-Outcrop complex, 30 to 75 percent slopes | 18K | Well drained | D | Very limited | Very limited |
| Forward silt loam, 12 to 57 percent slopes, MLRA 15 | 18K | Well drained | C | Very limited | Very limited |
| Sobrante loam, 30 to 50 percent slopes | 15K | Well drained | C | Very limited | Very limited |
| Rock outcrop-Kidd complex, 50 to 75 percent slopes | 14K | D | Very limited | Very limited | |
| Bale clay loam, 0 to 2 percent slopes | 12K | Somewhat poorly drained | B | Very limited | Somewhat limited |
| Rock outcrop | 11K | D | Very limited | Very limited | |
| Maymen-Los Gatos complex, 50 to 75 percent slopes | 9K | Somewhat excessively drained | D | Very limited | Very limited |
| Boomer-Forward-Felta complex, 30 to 50 percent slopes | 9K | Well drained | B | Very limited | Very limited |
| Boomer gravelly loam, volcanic bedrock, 14 to 60 percent slopes, MLRA 15 | 9K | Well drained | C | Very limited | Very limited |
| Haire loam, 2 to 9 percent slopes | 9K | Moderately well drained | D | Very limited | Very limited |
| Cole silt loam, 0 to 2 percent slopes, MLRA 14 | 8K | Somewhat poorly drained | C | Very limited | Very limited |
| Rock outcrop-Hambright complex, 50 to 75 percent slopes | 8K | D | Very limited | Very limited | |
| Bressa-Dibble complex, 15 to 30 percent slopes | 8K | Well drained | C | Very limited | Very limited |
| Reyes silty clay loam | 7K | Poorly drained | C/D | Very limited | Very limited |
| Forward-Kidd complex, 11 to 60 percent slopes, MLRA 15 | 7K | Well drained | C | Very limited | Very limited |
What This Means
Building & Foundations
About 94% of soils are rated "very limited" for dwellings with basements. Specific challenges include shallow bedrock, steep slopes, poor drainage in some areas, flood-prone areas. A geotechnical assessment is recommended before building.
Septic Systems
About 90% 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.