Drainage Class
Amber Anderson; Lee Burras; and Gerald Miller
Surface Drainage
Adequate drainage to dispose of excess water within a reasonable time is essential for many uses of soil. Inadequate drainage produces waterlogged soil that is likely to prevent growth of plant roots because of poor aeration. Plants adapted to growth in wet soils, for instance cattails or swamp smartweed, will tend to predominate. Also, wet soils, when exposed to mechanical disturbance, become muddy and may fail to support loads placed on them. A building may settle, or a vehicle may get stuck when soil drainage is inadequate.
Excess water received beyond what the soil can store for plant growth must either pond, run off the surface, or pass through the soil. The surface drainage depends greatly on slope, texture, surface roughness, and vegetative cover. Soils with coarse and moderately coarse textures in the surface horizon usually have a greater potential for increased surface drainage when compared to soils with medium, moderately fine, or fine surface textures on similar slope gradients. The rate that water can escape by flowing across the soil surface is evaluated as rapid, medium, slow, or ponded surface drainage.
For purposes of the contest, judges will select soils that 1) are not saturated across the depth of the entire profile and 2) have potential for infiltration of water in the upper part of the profile. The classes may be defined as follows:
RAPID
The soil surface is steep enough and smooth enough that water can flow rapidly across it. The water flow is so fast that water remains on the soil surface only a few minutes after rainfall ceases. Much water runs off that could have been stored for plant growth if it had infiltrated. Soils occurring on slopes greater than 9% gradient will experience rapid surface drainage unless they have a sod-based healthy cover. All soils occupying slopes greater than 14% will have rapid surface drainage.
MEDIUM
Water is held on the soil surface long enough to give it an opportunity to infiltrate but not long enough to cause excessive wetness. Gentle and moderate slopes are likely to have medium surface drainage. Soils occurring on nearly level slopes classified with excessive internal drainage are included in this category of medium.
SLOW
Water has an avenue to escape across the soil surface, but its movement is slow because the slopes are nearly level, the surface is quite rough, or vegetation impedes its flow. Water may stand on the soil surface for several hours after a rain.
PONDED
Water has no avenue to escape across the soil surface, so it accumulates in a pond. The water remains in the pond until it either infiltrates or evaporates. Soils formed in closed landforms or depressions are included in this category.
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 ponding or slow surface drainage, 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.
Key Takeaways
- Surface drainage depends greatly on slope, texture, surface roughness, and vegetative cover.
- Rapid, water flow is so fast that water remains on the soil surface only a few minutes after rainfall ceases.
- Medium, water is held on the soil surface long enough to give it an opportunity to infiltrate but not long enough to cause excessive wetness.
- Slow, water has an avenue to escape across the soil surface, but its movement is slow because the slopes are nearly level, the surface is quite rough, or vegetation impedes its flow.
- Ponded, water has no avenue to escape across the soil surface, so it accumulates in a pond.
- 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.
- Excessively drained soils are well aerated and have a low water holding capacity. The color of the B or C horizon is usually uniform brown to include yellowish-brown or strong brown.
- Well drained soils have adequate aeration. The color of the A horizon may be dark or black and the B horizon commonly has a uniform brown color with no redox features.
- Moderately well drained soils are mostly well-aerated with the same bright colors as a well-drained soil, however, they will contain a few redox concentrations at the base of the profile, but no depletions within 40 inches.
- Somewhat poorly drained soil may be waterlogged for several days to a few weeks at a time during wet seasons but is aerated at other times. The A horizon will be relatively dark or black in color, and the B horizon is usually grayish-brown or olive gray with or without redox features.
- Poorly drained soils are waterlogged for several weeks during wet seasons, cannot be cropped without artificial drainage, and ponding of water can occur for short periods of time on the surface. The A horizon is black in color, and the B horizon is almost entirely uniform gray with no to a few grayish brown redox features or uniform gray with rust redox features.
- Very poorly drained soils occur in depressions on uplands, terraces, and bottomlands. The soil is typically covered with ponded water part of the time or is water-logged most of the time unless it is artificially drained.
