Colusa County, California
The most common soil here is Willows, on about 15% of the land: a clay, sticky when wet and rock-hard when dry. Millsholm and Capay come next.
About 60% of the land drains well, and 30% stays wet for long stretches; groundwater comes within two feet of the surface under 6% of it.
USDA rates 95% of the land very limited for a standard septic drain field, and 94% very limited for homes with basements. That means extra cost or special design, not that it can't be done.
Worth knowing: 19% of the land floods occasionally or more, and bedrock lies within 40 inches under 39% 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 Colusa 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 Colusa 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 |
|---|---|---|---|---|---|
| Millsholm-Contra Costa association, 30 to 75 percent slopes | 74K | Well drained | D | Very limited | Very limited |
| Willows silty clay, 0 to 1 percent slopes, occasionally flooded | 58K | Poorly drained | D | Very limited | Very limited |
| Willows silty clay, 0 to 1 percent slopes | 37K | Poorly drained | D | Very limited | Very limited |
| Goldeagle-Positas-Balcom complex, 30 to 75 percent slopes | 33K | Well drained | C | Very limited | Very limited |
| Willows silty clay, 0 to 1 percent slopes, frequently flooded | 30K | Poorly drained | D | Very limited | Very limited |
| Capay clay loam, 0 percent slopes, low precip, MLRA 17 | 29K | Moderately well drained | D | Very limited | Very limited |
| Westfan loam, 0 to 2 percent slopes | 24K | Well drained | B | Very limited | Very limited |
| Scribner silt loam, 0 to 1 percent slopes | 19K | Poorly drained | C/D | Very limited | Very limited |
| Etsel-Maymen-Marpa association, 30 to 50 percent slopes | 18K | Somewhat excessively drained | D | Very limited | Very limited |
| Clear Lake clay, 0 to 1 percent slopes, occasionally flooded, MLRA 17 | 18K | Poorly drained | C/D | Very limited | Very limited |
| Vina loam, silt loam substratum, 0 to 2 percent slopes, MLRA 17 | 17K | Well drained | B | Very limited | Somewhat limited |
| Mallard clay loam, 0 to 1 percent slopes | 16K | Somewhat poorly drained | D | Very limited | Very limited |
| Neuns-Bamtush-Goulding association, 30 to 50 percent slopes | 16K | Well drained | C | Very limited | Very limited |
| Hillgate clay loam, 0 to 2 percent slopes | 15K | Well drained | C | Very limited | Very limited |
| Henneke-Okiota complex, 50 to 75 percent slopes | 15K | Well drained | D | Very limited | Very limited |
| Henneke-Montara-Rock outcrop complex, 10 to 50 percent slopes, MLRA 15 | 15K | Well drained | D | Very limited | Very limited |
| Moonbend silt loam, 0 to 2 percent slopes | 14K | Moderately well drained | C | Very limited | Very limited |
| Contra Costa-Millsholm complex, 50 to 75 percent slopes | 14K | Well drained | D | Very limited | Very limited |
| Neuns-Marpa-Goulding complex, 30 to 50 percent slopes | 13K | Well drained | C | Very limited | Very limited |
| Sehorn-Altamont complex, 30 to 50 percent slopes | 12K | Well drained | D | 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 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 95% 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.