Soil series profile
Colby soil series
Colby is a well drained soil with a silt loam surface, mapped on 1,873,484 acres in 56 counties across 6 states. Hydrologic group B.
- Drainage class
- Well drained >99% of mapped acres
- Hydrologic group
- B 96% of mapped acres
- Typical topsoil pH
- 7.9 moderately alkaline · range 7.9–7.9
- Mapped area
- 1.9M acres in 56 counties, 6 states
Typical Colby profile
Texture and pH by depth
Show as a table
| Depth | Texture | pH | Clay | Sand | Organic matter |
|---|---|---|---|---|---|
| 0–6 in (A) | Silt loam | 7.9 | 21% | 11% | 1.25% |
| 6–79 in (C) | Silt loam | 7.9 | 21% | 10% | 0.5% |
What this means
What Colby soil means for gardens, lawns and septic systems
For gardens
Typical topsoil in Colby soils is moderately alkaline: pH 7.9. Acid-loving plants such as blueberries and azaleas struggle without amended beds, and some trees and shrubs show iron chlorosis (yellow leaves with green veins).
Loamy topsoil (loam, silt loam and similar) covers >99% of mapped Colby acres: generally the easiest soil to garden, holding moisture without staying waterlogged. The most common surface texture is silt loam (80% of mapped Colby acres).
>99% of mapped Colby acres is moderately well to excessively drained, which suits most garden plants.
For lawns and yards
Most land is in hydrologic groups B and C, where rain soaks in at a moderate pace.
For a septic system
For conventional septic absorption fields, NRCS rates 16% of the acres in mapped Colby soils very limited, 83% somewhat limited and <1% not limited.
Only a licensed site evaluation or perc test at the actual lot decides whether a system is allowed and which design.
Where Colby soil is found
Mapped acres
Colby is mapped in Kansas, Colorado, South Dakota, Montana, Wyoming and Nebraska.
| County | Acres | Share of county land |
|---|---|---|
| Hamilton County, KS | 203,072 | 31.8% |
| Cheyenne County, KS | 181,733 | 27.8% |
| Rawlins County, KS | 179,158 | 26.2% |
| Yuma County, CO | 134,929 | 9.2% |
| Washington County, CO | 132,361 | 8.2% |
| Kit Carson County, CO | 128,038 | 9.3% |
| Wallace County, KS | 91,384 | 15.6% |
| Logan County, KS | 74,328 | 10.8% |
| Sherman County, KS | 61,801 | 9.1% |
| Weld County, CO | 58,856 | 2.3% |
| Adams County, CO | 50,796 | 6.7% |
| Big Horn County, MT | 48,130 | 1.1% |
| Sheridan County, KS | 44,948 | 7.8% |
| Oglala Lakota County, SD | 44,441 | 3.1% |
| Prowers County, CO | 41,068 | 3.9% |
| Thomas County, KS | 37,509 | 5.5% |
| Kearny County, KS | 34,494 | 6.2% |
| Greeley County, KS | 33,671 | 6.8% |
| Cheyenne County, CO | 32,563 | 2.8% |
| Morgan County, CO | 28,693 | 3.6% |
| Baca County, CO | 21,390 | 1.3% |
| Kiowa County, CO | 20,107 | 1.7% |
| Arapahoe County, CO | 19,900 | 3.8% |
| Jackson County, SD | 15,795 | 1.3% |
| Stanton County, KS | 14,127 | 3.2% |
| Scott County, KS | 13,556 | 3.0% |
| Wichita County, KS | 12,272 | 2.7% |
| Bennett County, SD | 10,639 | 1.4% |
| Lincoln County, CO | 10,430 | 0.6% |
| Carbon County, MT | 10,396 | 0.8% |
| Goshen County, WY | 10,184 | 0.7% |
| Logan County, CO | 9,488 | 0.8% |
| Boulder County, CO | 8,411 | 1.8% |
| Seward County, KS | 8,141 | 2.0% |
| Gove County, KS | 7,852 | 1.1% |
| Decatur County, KS | 6,509 | 1.1% |
| Elbert County, CO | 6,085 | 0.5% |
| Fall River County, SD | 5,093 | 0.5% |
| Custer County, SD | 4,958 | 0.6% |
| Grant County, KS | 4,416 | 1.2% |
| Finney County, KS | 2,142 | 0.3% |
| Dawes County, NE | 2,033 | 0.2% |
| Haskell County, KS | 1,909 | 0.5% |
| Lane County, KS | 966 | 0.2% |
| Sedgwick County, CO | 849 | 0.2% |
| Larimer County, CO | 754 | 0.0% |
| Laramie County, WY | 680 | 0.0% |
| Sheridan County, NE | 563 | 0.0% |
| Gray County, KS | 413 | 0.1% |
| Todd County, SD | 394 | 0.0% |
| Denver County, CO | 345 | 0.3% |
| Pennington County, SD | 268 | 0.0% |
| Broomfield County, CO | 187 | 0.7% |
| Trego County, KS | 176 | 0.0% |
| Butte County, SD | 10 | 0.0% |
| Weston County, WY | 1 | 0.0% |
Septic absorption fields
NRCS rating, share of Colby acres
Show as a table
| Category | Share of acres |
|---|---|
| Not limited | 0.4% |
| Somewhat limited | 82.8% |
| Very limited | 16.0% |
| Not rated | 0.8% |
Drainage, water table and flooding
Share of Colby acres
- Depth to seasonal high water table
- None reported within about 6.5 feet (median where reported).
- Flooding
- Not mapped as flooding on most acres.
- Slope
- Typically 2–18% (median 9%).
- Topsoil organic matter
- Median 1.2%.
Drainage class
Show as a table
| Category | Share of acres |
|---|---|
| Excessively drained | 0.0% |
| Somewhat excessively drained | 0.0% |
| Well drained | 99.2% |
| Moderately well drained | 0.0% |
| Somewhat poorly drained | 0.0% |
| Poorly drained | 0.0% |
| Very poorly drained | 0.0% |
| Not rated | 0.8% |
Hydrologic soil group
Show as a table
| Category | Share of acres |
|---|---|
| A: fast infiltration (mostly sandy or gravelly) | 0.0% |
| B: moderate infiltration | 96.2% |
| C: slow infiltration | 3.0% |
| D: very slow infiltration (clayey, shallow or wet) | 0.0% |
| A/D: D as is, A if drained | 0.0% |
| B/D: D as is, B if drained | 0.0% |
| C/D: D as is, C if drained | 0.0% |
| Not rated | 0.8% |
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
- All map unit components named Colby (component kind "Series") in SSURGO: 1,873,484 acres.
- Method
- Shares are by mapped acres of Colby components. The profile comes from the single largest Colby component. Full methodology.