Napa County, California

Survey Area CA055• California

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.

Mollisols

Fertile grassland soils with thick, dark topsoil. Among the best for agriculture, building, and gardening.

Inceptisols

Young but developing soils found in mountains and river terraces. Variable properties — check drainage and bedrock depth.

Alfisols

Moderately weathered forest soils with clay-enriched subsoil. Productive for agriculture, generally good for building and septic.

Ultisols

Strongly weathered clay soils of the Southeast. Acidic and naturally low in nutrients. Clay subsoil can slow drainage.

Vertisols

High-clay soils that shrink and swell dramatically. Fertile but challenging for foundations and septic systems.

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.