Drainage class
Amber Anderson; Lee Burras; and Gerald Miller
Internal Drainage
Internal soil drainage takes effect after the water has entered the soil and is a measure of the amount of water held and the rate at which water moves through or sits in the soil profile. Excessive soil water may be caused by ponded or slow surface drainage, by high infiltration rates, or by seepage from adjoining soils. Too much water from any of these sources may cause the soil to be waterlogged and poorly aerated or anaerobic. This means soil pores are holding stagnant water and gleyed conditions (gray colors due to wetness) will result in the soil. These colors are visible even if the soil isn’t currently saturated. Depletions are small spots of these reduced conditions, while gleyed conditions refers to these conditions (and corresponding gray color) making up more than half of the soil area. Additionally, anaerobic conditions will slow decomposition, resulting in accumulation of organic material levels above the surrounding area’s levels. Dark colors from organic accumulation mean depletions, or gray spots, won’t be clearly visible.
The five internal drainage classes used in the contest are excessively drained, well drained, somewhat poorly drained, poorly drained, and very poorly drained. Features should be identified based upon visibility in the soil pit and samples, although this may not represent current or future wetness status.
| Drainage class | Redox or required features | Common landscape positions |
| Excessively drained | No redox features, requires coarse textures | Coarse textured pockets such as glacial outwash or coarse alluvium |
| Well drained | None within 40″, brown color in the B horizon | Areas with slope suitable for water movement, generally higher on the landscape |
| Moderately well drained | Few concentrations near 40″, no significant depletions present | Gently sloping areas, generally higher in the landscape |
| Somewhat poorly drained | Concentrations and depletions begin between 20″ and 40″ | Areas that may temporarily accumulate water such as footslopes, stream terraces, or gently sloping uplands |
| Poorly or very poorly drained
*If a dark A extends below 40″, soil should be examined for concentrations as depletions will not be visible. If present, use the depth at which they occur to determine drainage class |
Gleyed matrix (grey due to wetness) under a dark A horizon, concentrations expected
|
Depressions or other areas where water cannot easily escape. |
EXCESSIVELY DRAINED
These soils are well aerated and have low water holding capacity. The texture of the subsoil or C horizon is coarse or moderately coarse, and the color of the B or C horizon is usually
uniform brown to include yellowish-brown or strong brown. The base of the A horizon has Munsell chroma three or higher and a value four or higher.
WELL DRAINED
Aeration is adequate. The color of the A may be dark or black and likely one color group lighter than that of nearby wetter soils. The B horizon has a uniform brown color with no redox features. The base of the A horizon has Munsell chroma three or higher and a value of three or higher.
MODERATELY WELL DRAINED
Common across Iowa, this area is mostly well-aerated with the same bright colors as a well-drained soil. It will contain a few redox concentrations at the base of the profile, but no depletions within 40 inches.
SOMEWHAT POORLY DRAINED
The soil may be waterlogged for several days or a few weeks at a time during wet seasons but is aerated at other times. Soils occupying level or nearly level slopes may require tile drainage to achieve their agronomic potential. The A is likely to be relatively dark or black in color, and the B is usually grayish- brown or olive gray with or without redox features. The base of the A horizon has Munsell chroma two and a value 3-5.
POORLY DRAINED
The soil is waterlogged for several weeks during wet seasons and usually cannot be satisfactorily cropped without artificial drainage. Ponding of water on the surface occurs for short periods of time. The A horizon is black in color, and the B horizon is almost entirely uniform gray with no or few grayish brown redox features or uniform gray with rust redox features. The base of the A horizon has Munsell chroma 0-1 and value 2-6.
VERY POORLY DRAINED
These soils often occur in depressions on uplands, terraces, and bottomlands. The soil is covered with ponded water part of the time and is water-logged most of the time unless it is artificially drained. The surface soil may be muck or peat, the A horizon is black, and the subsoil has a uniform gray color or mostly olive gray. The base of the A horizon or the lower A horizon has Munsell chroma 0-1 and a value 2-6. A few rust-colored redox features may be present within the black A horizon and the B horizon. For contest purposes, poorly and very poorly drained will be grouped.
