County soil summary
Soil types in Boulder County, Colorado
The most widespread soils are Nunn and Ascalon; the most common drainage class is well drained (36% of the land), and typical topsoil pH is 6.8.
About 448,000 acres of Boulder County lie in multi-county soil survey areas (CO643, CO645) that the soil survey does not split by county. Those acres, often public forest or desert land, are counted in the Colorado totals but not in the figures below.
- Well or moderately well drained
- 75% of land · state 84%
- Typical topsoil pH
- 6.8 neutral · state 7.2
- Septic fields very limited
- 76% of land · state 68%
- Subject to flooding
- 6% of land, any frequency · state 5%
Most common soils in Boulder County
Share of mapped land
A county is a patchwork of many soils. These 8 cover 42% of the mapped land; 81 named soil series are mapped here in all.
| Soil | Share of land | Drainage | Topsoil pH |
|---|---|---|---|
| Rock outcropLand type, not a soil series | 8.5% | — | — |
| Leighcan familyGeneral soil group, not a named series | 6.5% | — | — |
| Nunn | 6.0% | Well drained | 7.0 |
| Cypher familyGeneral soil group, not a named series | 4.8% | — | — |
| Bullwark familyGeneral soil group, not a named series | 4.7% | — | — |
| Ascalon | 4.2% | Well drained | 7.0 |
| Ratake familyGeneral soil group, not a named series | 4.2% | — | — |
| Catamount familyGeneral soil group, not a named series | 3.3% | — | — |
What this means
What the soils in Boulder County mean for gardens, lawns and septic systems
For your garden
Typical topsoil in Boulder County is neutral: pH 6.8 (most land between 6.0 and 7.9), the range most vegetables, flowers and lawn grasses prefer.
Loamy topsoil (loam, silt loam and similar) covers 58% of the land: generally the easiest soil to garden, holding moisture without staying waterlogged. The most common surface texture is very gravelly sandy loam (10% of the land).
39% of the land is excessively or somewhat excessively drained: water moves through fast, so gardens need more frequent watering in dry spells.
For your lawn and yard
On 62% 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 Boulder County very limited, 6% somewhat limited and 4% not limited (14% not rated, such as urban land, pits or water).
Statewide, 68% of the land is very limited, so Boulder County is more restrictive than typical.
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 6% of the land (6% occasional, <1% 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 (36%). 75% of the land is moderately well drained or better, compared with 84% across Colorado.
Show as a table
| Category | Boulder County | Colorado |
|---|---|---|
| Excessively drained | 5.9% | 5.1% |
| Somewhat excessively drained | 32.9% | 8.0% |
| Well drained | 35.6% | 70.2% |
| Moderately well drained | 0.1% | 0.9% |
| Somewhat poorly drained | 0.2% | 1.0% |
| Poorly drained | 5.5% | 1.5% |
| Very poorly drained | 0.4% | 0.5% |
| Not rated | 19.3% | 12.8% |
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 | Boulder County | Colorado |
|---|---|---|
| Not limited | 3.8% | 6.3% |
| Somewhat limited | 6.0% | 15.4% |
| Very limited | 76.3% | 68.0% |
| Not rated | 14.0% | 10.4% |
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 | Boulder County | Colorado |
|---|---|---|
| A: fast infiltration (mostly sandy or gravelly) | 9.6% | 13.8% |
| B: moderate infiltration | 15.9% | 22.0% |
| C: slow infiltration | 26.5% | 32.3% |
| D: very slow infiltration (clayey, shallow or wet) | 32.1% | 23.2% |
| A/D: D as is, A if drained | 0.0% | 0.4% |
| B/D: D as is, B if drained | 1.5% | 0.6% |
| C/D: D as is, C if drained | 2.0% | 0.7% |
| Not rated | 12.4% | 7.0% |
Flooding
Show as a table
| Category | Boulder County | Colorado |
|---|---|---|
| No flooding | 80.7% | 86.2% |
| Rare or very rare | 0.1% | 1.6% |
| Occasional | 5.8% | 1.8% |
| Frequent or very frequent | 0.1% | 1.1% |
| Not reported | 13.2% | 9.3% |
Seasonal high water table
Show as a table
| Category | Boulder County | Colorado |
|---|---|---|
| Within 18 inches at some time of year | 3.6% | 1.8% |
| 18 inches to 3.3 feet | 2.7% | 1.4% |
| Deeper than 3.3 feet or none reported | 84.5% | 91.7% |
| Not reported | 9.3% | 5.0% |
Topsoil pH and texture
First mineral layer of each soil
Typical topsoil pH is 6.8 (neutral); 80% of rated land falls between 6.0 and 7.9. Median topsoil organic matter is 1.5%.
Show as a table
| Category | Boulder County | Colorado |
|---|---|---|
| Strongly acidic (below 5.5) | 2.8% | 3.7% |
| Moderately to slightly acidic (5.5–6.4) | 20.1% | 8.7% |
| Near neutral (6.5–7.3) | 42.2% | 42.6% |
| Alkaline (7.4 and up) | 11.7% | 30.7% |
| Not reported | 23.3% | 14.4% |
Texture
Show as a table
| Category | Boulder County | Colorado |
|---|---|---|
| Sandy | 16.1% | 8.8% |
| Loamy (loam, silt loam and similar) | 58.0% | 75.0% |
| Clayey (40%+ clay) | 2.6% | 1.3% |
| Organic (muck or peat) | 0.1% | 0.2% |
| Not reported (rock, urban land, etc.) | 23.2% | 14.6% |
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
- Boulder County, Colorado (FIPS 08013): 463,460 acres of mapped land, 8,140 acres of open water excluded. State comparisons pool all mapped land in Colorado.
- 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.