Napa County, California
The most common soil here is Bressa, on about 13% of the land: a silt loam, smooth, like flour. Henneke and Forward come next.
About 77% of the land drains well, and 10% stays wet for long stretches; groundwater comes within two feet of the surface under 3% of it.
USDA rates 91% of the land very limited for a standard septic drain field, and 90% very limited for homes with basements. That means extra cost or special design, not that it can't be done.
Worth knowing: 3% of the land floods occasionally or more, and bedrock lies within 40 inches under 71% 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 Napa County, California, 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 Well drained and the dominant hydrologic group is D (very slow infiltration, high runoff). The most common soil order is Mollisols — grassland soils with a thick, dark, fertile topsoil rich in organic matter. 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. 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 |
|---|---|---|---|---|---|
| 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 |
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 90% 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 91% 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.