Soil classification
Soils are typically classified based on a specific property, such as infiltration characteristics or texture. This page provides a summary of the more commonly used classification systems.
Natural Resource Conservation Service Hydrologic Soil Groups
Reference: Urban Hydrology for Small Watersheds - TR-55 (USDA)
Soils are classified by the Natural Resource Conservation Service into four Hydrologic Soil Groups (HSG) based on the soil's runoff potential. The four Hydrologic Soils Groups are A, B, C and D. Where A's generally have the smallest runoff potential and Ds the greatest.
- Group A: sand, loamy sand or sandy loam types of soils. It has low runoff potential and high infiltration rates even when thoroughly wetted. They consist chiefly of deep, well to excessively drained sands or gravels and have a high rate of water transmission.
- Group B: silt loam or loam. It has a moderate infiltration rate when thoroughly wetted and consists chiefly or moderately deep to deep, moderately well to well drained soils with moderately fine to moderately coarse textures.
- Group C: sandy clay loam. They have low infiltration rates when thoroughly wetted and consist chiefly of soils with a layer that impedes downward movement of water and soils with moderately fine to fine structure.
- Group D: clay loam, silty clay loam, sandy clay, silty clay or clay. This HSG has the highest runoff potential. They have very low infiltration rates when thoroughly wetted and consist chiefly of clay soils with a high swelling potential, soils with a permanent high water table, soils with a claypan or clay layer at or near the surface and shallow soils over nearly impervious material.
This classification system is the most widely used system in the Minnesota Stormwater Manual and in most stormwater applications in the U.S. BMP selection and design are typically based on soils at a site. For example, infiltration bmps are preferred on HSG A and B soils when there are no other site restrictions (e.g. contamination, presence of active karst).