Soil Drainage Class
Soil drainage class is a way of communicating internal soil drainage conditions together with the depth in the soil that reducing conditions were/are prevalent (as determined by soil morphological features). There are seven such classes.
The Seven Soil Drainage Classes
- Excessively drained:
- Water is removed very rapidly (gravelly or coarse sand textures). The occurrence of free water is very deep (> 150 cm [or 60 inches]). Free of redoximorphic features related to wetness.
- Somewhat excessively drained:
- Water is removed from the soil rapidly (sands and loamy sand textures). The occurrence of free water commonly is very deep (> 150 cm [or 60 inches]). Free of redoximorphic features related to wetness.
- Well drained:
- Water is removed readily but not rapidly. Free water occurrence is deep (> 150 cm [or 60 inches]). Well drained soils are generally free of redoximorphic features related to wetness in the upper 150 cm (or 60 inches). There may be redoximorphic features deeper in the soil profile.
- Moderately well drained:
- Water is removed from the soil somewhat slowly during some periods of the year. Free water occurrence is moderately deep (50 cm to 100 cm) and transitory (1 to 3 months) to permanent (continuous). The soils are wet for only a short time within the rooting depth during the growing season. Moderately well drained soils may have redoximorphic features below a depth of 50 cm (about 20 inches). It is common for moderately well drained soils to have a slowly permeable layer within or immediately beneath the solum or a relatively high water table.
- Somewhat poorly drained:
- Water is removed slowly so that the soil is wet at a shallow depth for significant periods during the growing season. The occurrence of free water is shallow (25 cm to 50 cm [or 10 to 20 inches]) and transitory (1 to 3 months) or common (present 3 to 6 months). Wetness markedly restricts the growth of many common crop plants unless artificial drainage is provided. They commonly have a high water table, and redoximorphic features occur below a depth of 25 cm (about 10 inches). These soils often have thick dark A horizons high in organic matter.
- Poorly drained:
- Water is removed so slowly that the soil is wet at shallow depths periodically during the growing season or remains wet for long periods. The occurrence of free water is shallow or very shallow (< 25 cm [about 10 inches]) and common (3 to 6 months) or persistent (6 through 12 months). Free water is commonly at or near the surface long enough during the growing season so that mesophytic crops cannot be grown without artificial drainage. These soils are commonly gleyed rather than redoximorphic features because they remain anoxic for long periods.
- Very poorly drained:
- Water is removed from the soil so slowly that free water remains at or very near the ground surface during much of the growing season. The occurrence of free water is very shallow and persistent or permanent.