Hydric Soils

Wetland criteria

• Hydrology• Hydric soils• Hydrophytic plants

Hydric soil

• soil that is saturated, flooded, or ponded long enough during the growing season to develop anaerobic conditions in the upper part.– Oxygen is removed from groundwater by

respiration of microbes, roots, soil fauna – Biological zero = 5°C

Why is “during growing season” important part of definition?

• If wet period is during COLD time of year (too cold for microbial growth and plant root respiration), might not have anaerobic conditions.

• It is anaerobic conditions that cause a soil to be hydric, not just saturation!!!

• Hydric soils support growth and regeneration of hydrophytic plants.

Hydric Soils and Taxonomy• Histosols – (all Histosols except Folists)– (all Histels except Folistels)

• Aquic suborders and subgroups– Definition of aquic soil moisture regime:

“reducing regime in soil virtually free of dissolved oxygen because it is saturated. Some soils are saturated at times while dissolved oxygen is present, either because the water is moving or the environment is unfavorable for microorganisms; such a regime is NOT considered aquic”.

Organic soils made up mostly of forest litter’ not saturated

Aquic Conditions:

• Periodic or continuous saturation• Redoximorphic features• Verify by measuring saturation or reduction

Exception to Aquic conditions:

• Artificial drainage– Removal of free water from soils with aquic

conditions– Artificially drained soils are included with aquic

soils• Because soil Taxonomy is based on soil GENESIS and

minimizes human disturbance– Pertains to Hydric soils also

• Artificially wet soils are considered hydric

• Artificially “dry” (drained) soils are considered hydric

Types of saturation

• endosaturation: all soil layers sat’d to 2 m depth

• Episaturation: sat’d layers in upper 2 m (perched)

• Anthric saturation: controlled flooding (rice, cranberries)

Hydric soil indicators:

• Color– Chroma 1or 2 or gley (Fe++2 grey or green)

• May have redox concentrations or concretions

• Sulfidic materials (odor of rotten eggs)– Sulfate reduction

Plate 30  Dark (black) humic accumulation and gray humus depletion spots in the A horizon are indicators of a hydric soil. Water table is 30 cm below the soil surface.

Figure 7.11 The relationship between the occurrence of some soil features and the annual duration of water-saturated conditions. The absence of iron concentrations (mottles) with colors of chroma >4, and the presence of strong expressions of the other features are indications that a soil may be hydric. [Adapted from Veneman et al. (1999)]

List of hydric soils

http://soils.usda.gov

Click on hydric soils

Oxidized rhizosphere

• In some poorly aerated soils:

– Red, oxidized iron in root channels• Oxygen diffused out of plant roots• Some plants transport oxygen through aerenchyma

tissue in stems and leaves to roots (hydrophytic plants)

Plate 29 Oxidized (red) root zones in the A and E horizons indicate a hydric soil. They result from oxygen diffusion out from roots of wetland plants having aerenchyma tissues (air passages).

Black spruce

Pitcher plant