County soil summary
Soil types in Alameda County, California
The most widespread soils are Vallecitos, Los Osos and Millsholm; the most common drainage class is well drained (53% of the land), and typical topsoil pH is 6.7.
- Well or moderately well drained
- 65% of land · state 81%
- Typical topsoil pH
- 6.7 neutral · state 6.7
- Septic fields very limited
- 76% of land · state 79%
- Subject to flooding
- 12% of land, any frequency · state 11%
Most common soils in Alameda County
Share of mapped land
A county is a patchwork of many soils. These 8 cover 47% of the mapped land; 86 named soil series are mapped here in all.
| Soil | Share of land | Drainage | Topsoil pH |
|---|---|---|---|
| Vallecitos | 9.3% | Well drained | 6.3 |
| Los Osos | 8.1% | Well drained | 6.5 |
| Millsholm | 6.5% | Well drained | 6.5 |
| Altamont | 5.3% | Well drained | 7.0 |
| Diablo | 5.1% | Well drained | 7.3 |
| Linne | 4.5% | Well drained | 8.2 |
| Los Gatos | 4.4% | Well drained | 6.5 |
| Positas | 4.3% | Well drained | 6.1 |
What this means
What the soils in Alameda County mean for gardens, lawns and septic systems
For your garden
Typical topsoil in Alameda County is neutral: pH 6.7 (most land between 5.8 and 8.2), the range most vegetables, flowers and lawn grasses prefer.
Loamy topsoil (loam, silt loam and similar) covers 59% of the land: generally the easiest soil to garden, holding moisture without staying waterlogged. The most common surface texture is clay (18% of the land).
65% of the land is moderately well to excessively drained, which suits most garden plants.
For your lawn and yard
On 75% 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 76% of the land in Alameda County very limited, 3% somewhat limited and <1% not limited (21% not rated, such as urban land, pits or water).
That is close to the state as a whole (79% very limited).
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 12% of the land (<1% occasional, 6% 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 (53%). 65% of the land is moderately well drained or better, compared with 81% across California.
Show as a table
| Category | Alameda County | California |
|---|---|---|
| Excessively drained | 1.7% | 7.7% |
| Somewhat excessively drained | 7.3% | 17.6% |
| Well drained | 53.1% | 50.7% |
| Moderately well drained | 2.5% | 4.7% |
| Somewhat poorly drained | 1.6% | 2.5% |
| Poorly drained | 8.5% | 2.0% |
| Very poorly drained | 3.5% | 0.7% |
| Not rated | 21.9% | 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 | Alameda County | California |
|---|---|---|
| Not limited | 0.1% | 4.3% |
| Somewhat limited | 3.0% | 3.6% |
| Very limited | 75.7% | 79.2% |
| Not rated | 21.1% | 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 | Alameda County | California |
|---|---|---|
| A: fast infiltration (mostly sandy or gravelly) | 2.4% | 20.5% |
| B: moderate infiltration | 5.9% | 12.5% |
| C: slow infiltration | 33.9% | 22.7% |
| D: very slow infiltration (clayey, shallow or wet) | 39.7% | 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 | 1.4% | 0.9% |
| Not rated | 16.8% | 12.6% |
Flooding
Show as a table
| Category | Alameda County | California |
|---|---|---|
| No flooding | 79.1% | 77.1% |
| Rare or very rare | 6.0% | 8.2% |
| Occasional | 0.4% | 1.6% |
| Frequent or very frequent | 5.7% | 1.0% |
| Not reported | 8.8% | 12.1% |
Seasonal high water table
Show as a table
| Category | Alameda County | California |
|---|---|---|
| Within 18 inches at some time of year | 5.2% | 2.6% |
| 18 inches to 3.3 feet | 3.4% | 1.9% |
| Deeper than 3.3 feet or none reported | 85.4% | 86.9% |
| Not reported | 5.9% | 8.6% |
Topsoil pH and texture
First mineral layer of each soil
Typical topsoil pH is 6.7 (neutral); 80% of rated land falls between 5.8 and 8.2. Median topsoil organic matter is 2%.
Show as a table
| Category | Alameda County | California |
|---|---|---|
| Strongly acidic (below 5.5) | 1.0% | 3.1% |
| Moderately to slightly acidic (5.5–6.4) | 33.4% | 22.8% |
| Near neutral (6.5–7.3) | 25.2% | 25.2% |
| Alkaline (7.4 and up) | 18.7% | 30.6% |
| Not reported | 21.7% | 18.2% |
Texture
Show as a table
| Category | Alameda County | California |
|---|---|---|
| Sandy | 2.2% | 21.3% |
| Loamy (loam, silt loam and similar) | 58.7% | 56.7% |
| Clayey (40%+ clay) | 18.2% | 3.9% |
| Organic (muck or peat) | 0.0% | 0.2% |
| Not reported (rock, urban land, etc.) | 20.9% | 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
- Alameda County, California (FIPS 06001): 470,940 acres of mapped land, 59,304 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.