County soil summary
Soil types in Johnson County, Iowa
The most widespread soils are Fayette, Colo and Ladoga; the most common drainage class is well drained (37% of the land), and typical topsoil pH is 6.3.
- Well or moderately well drained
- 66% of land · state 55%
- Typical topsoil pH
- 6.3 slightly acidic · state 6.3
- Septic fields very limited
- 99% of land · state 91%
- Subject to flooding
- 18% of land, any frequency · state 15%
Most common soils in Johnson County
Share of mapped land
A county is a patchwork of many soils. These 8 cover 54% of the mapped land; 98 named soil series are mapped here in all.
| Soil | Share of land | Drainage | Topsoil pH |
|---|---|---|---|
| Fayette | 17.9% | Well drained | 6.2 |
| Colo | 8.1% | Poorly drained | 6.3 |
| Ladoga | 7.7% | Moderately well drained | 6.6 |
| Tama | 5.6% | Well drained | 6.5 |
| Clinton | 4.6% | Moderately well drained | 6.3 |
| Downs | 3.7% | Well drained | 6.2 |
| Ely | 3.3% | Somewhat poorly drained | 6.5 |
| TamaGeneral soil group, not a named series | 2.7% | — | — |
What this means
What the soils in Johnson County mean for gardens, lawns and septic systems
For your garden
Typical topsoil in Johnson County is slightly acidic: pH 6.3 (most land between 6.0 and 6.6), the range most vegetables, flowers and lawn grasses prefer.
Loamy topsoil (loam, silt loam and similar) covers 95% of the land: generally the easiest soil to garden, holding moisture without staying waterlogged. The most common surface texture is silt loam (57% of the land).
66% of the land is moderately well to excessively drained, which suits most garden plants.
For your lawn and yard
On 87% 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.
A seasonal high water table comes within 18 inches of the surface on 34% of the land, often in spring. Basements, sump pumps and lawns in low spots feel it first.
For a septic system
For conventional septic absorption fields, NRCS rates 99% of the land in Johnson County very limited, <1% somewhat limited and 0% not limited.
Statewide, 91% of the land is very limited, so Johnson 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 18% of the land (8% 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 (37%). 66% of the land is moderately well drained or better, compared with 55% across Iowa.
Show as a table
| Category | Johnson County | Iowa |
|---|---|---|
| Excessively drained | 4.7% | 1.2% |
| Somewhat excessively drained | 0.2% | 0.7% |
| Well drained | 37.3% | 35.5% |
| Moderately well drained | 24.2% | 17.2% |
| Somewhat poorly drained | 15.9% | 19.9% |
| Poorly drained | 15.8% | 21.9% |
| Very poorly drained | 1.8% | 2.7% |
| Not rated | 0.2% | 0.9% |
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 | Johnson County | Iowa |
|---|---|---|
| Not limited | 0.0% | 0.0% |
| Somewhat limited | 0.8% | 8.0% |
| Very limited | 99.0% | 91.2% |
| Not rated | 0.2% | 0.8% |
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 | Johnson County | Iowa |
|---|---|---|
| A: fast infiltration (mostly sandy or gravelly) | 6.8% | 2.4% |
| B: moderate infiltration | 5.6% | 13.2% |
| C: slow infiltration | 52.7% | 36.9% |
| D: very slow infiltration (clayey, shallow or wet) | 4.2% | 11.4% |
| A/D: D as is, A if drained | 0.4% | 0.2% |
| B/D: D as is, B if drained | 4.7% | 4.9% |
| C/D: D as is, C if drained | 25.3% | 30.0% |
| Not rated | 0.2% | 1.0% |
Flooding
Show as a table
| Category | Johnson County | Iowa |
|---|---|---|
| No flooding | 82.1% | 84.5% |
| Rare or very rare | 3.4% | 3.2% |
| Occasional | 8.1% | 9.8% |
| Frequent or very frequent | 6.4% | 2.2% |
| Not reported | 0.0% | 0.3% |
Seasonal high water table
Show as a table
| Category | Johnson County | Iowa |
|---|---|---|
| Within 18 inches at some time of year | 34.3% | 43.3% |
| 18 inches to 3.3 feet | 2.5% | 6.5% |
| Deeper than 3.3 feet or none reported | 63.2% | 49.9% |
| Not reported | 0.0% | 0.3% |
Topsoil pH and texture
First mineral layer of each soil
Typical topsoil pH is 6.3 (slightly acidic); 80% of rated land falls between 6.0 and 6.6. Median topsoil organic matter is 3%.
Show as a table
| Category | Johnson County | Iowa |
|---|---|---|
| Strongly acidic (below 5.5) | 0.7% | 0.3% |
| Moderately to slightly acidic (5.5–6.4) | 54.7% | 52.6% |
| Near neutral (6.5–7.3) | 44.3% | 37.3% |
| Alkaline (7.4 and up) | 0.0% | 8.6% |
| Not reported | 0.3% | 1.2% |
Texture
Show as a table
| Category | Johnson County | Iowa |
|---|---|---|
| Sandy | 4.6% | 1.1% |
| Loamy (loam, silt loam and similar) | 95.1% | 96.5% |
| Clayey (40%+ clay) | 0.0% | 1.1% |
| Organic (muck or peat) | 0.0% | 0.3% |
| Not reported (rock, urban land, etc.) | 0.2% | 1.0% |
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
- Johnson County, Iowa (FIPS 19103): 389,519 acres of mapped land, 9,381 acres of open water excluded. State comparisons pool all mapped land in Iowa.
- 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.