Vegetation

Soil

Photo by: Jessen Book

The composition of wetland soil is one of the characteristics that define a wetland. Wetland soil is often described as hydric soil, a soil that is saturated, flooded, or ponded long enough during the growing season to develop anaerobic conditions in the upper part (Conservation Service, 1987 in Mitsch and Gosselink, 1993).

There are primarily two types of wetland soil, mineral soil and organic soil. Almost all wetlands have organic soil, but if the organic matter content of the soil is less than 20-35 percent organic matter, it is considered a mineral soil (Mitsch and Gosselink, 1993).

A table comparing mineral and organic soils in wetlands reprinted from Mitsch and Gosselink (1993).

Description Mineral Soil Organic soil

Organic Content, percent Less than 20-35 Greater than 20- 35

Organic Carbon, percent Less than 12 to 20 Greater than 12- 20

pH Usually circumneutral Acidic

Bulk Density High Low

Porosity Low (45-55%) High (80%)

Hydraulic Conductivity High (except for clays) Low to high

Water holding capacity Low High

Nutrient availability Generally high Often low

Cation exchange capacity Low, dominated by major cations High,dominated by hydrogen ion

Typical Wetland Riparian forest, some marshes Northern Peatland

The composition of wetland soil can challenge restoration projects. Among other things, wetland soils need to be evaluated based on the:

available nutrient levels
proportions of sand, silt, clay
gravel content
organic material
permeability
drainage potential
erodibility
soil chemistry (from Heid, 1997).

These factors are important in wetlands for creating an environment capable of sustaining water for at least part of the year (i.e. gravel content, permeability, drainage potential, and erodibility), and for providing the nutrients and substrates necessary for wetland vegetative growth (e.g. nutrient levels, organic matter, and soil chemistry).

The composition of wetland seedbanks can be important in wetland restoration projects. The seedbank of a wetland can be very beneficial to restoration projects because it can contain seed remnants of native flora. Establishing native vegetation is often difficult in restoration projects due to threat of invasive or alien species. For more information on wetland seed banks, click here!

CASE STUDIES

CASE STUDY 1: A long term assessment of wetland restoration efforts comparing created and natural wetlands. (Confer and Niering, 1992; Moore, et al., 1999).

CASE STUDY 2: Restoring wetland vegetation using transplanted soil (Brown and Bedford, 1997).

CASE STUDY 3: Limited response of cordgrass (Spartina foliosa) to soil amendments in a constructed marsh. (Gibson et al., 1994).