Hydrologic Groups (A, B, C, D)

The USDA assigns every mapped soil to one of four hydrologic groups based on how quickly it absorbs rainfall:

Group A — High infiltration rate. Sandy, gravelly, or well-structured soils. Less than 10% of rainfall runs off. Best for septic and rain gardens.

Group B — Moderate infiltration. Silt loams and loams. 10-20% runoff. Good for most uses.

Group C — Slow infiltration. Clay loams and shallow soils. 20-40% runoff. Septic may need engineering.

Group D — Very slow infiltration. Heavy clays, high water table, or shallow bedrock. Over 40% of rainfall runs off. Significant challenges for septic, drainage, and stormwater management.

Dual groups like A/D or B/D mean the soil behaves like the first letter when artificially drained, and the second when undrained.

Look up your address to see the specific soil conditions at your property, or browse the full glossary for more terms.