Wetland Soils

Definition of Soil

• A collection of natural bodies, made up of mineral and organic materials, that supports or is capable of supporting the growth of land plants ‘out of doors.’

• Soil is different from substrate

• Upper limit of soil is air or shallow water

Composition

• Inorganic material– Sand– Silt– Clay– Gravel, stones, rock

• Organic matter• Water• Gasses (air)• The percentages of each are highly variable

across space. For example, 10% to 60% of the volume of soils supporting upland plants is gas.

Soil Texture

Pedogenesis

• Five soil forming factors– Parent material– Climate– Topography– Organic activity– Time

Soil Classifications

• Wetland soils can be either primarily mineral or organic

• Organic– High in plant remains

• Mineral– High in inorganic material

Soil properties

• Porosity• Infiltration• Erodibility• Available Water Holding Capacity• Shrink-Swell• Permeability• Soil Texture (used more with Mineral soils)

– Based on percentages of sand, silt, and clay– Loam is soil with relatively even mixtures of sand, silt,

and clay– Best for conductivity AND water holding capacity

Soil Orders

Hydric Soils

• Soils that experience flooded conditions which lead to anaerobic conditions during at least part of the year.

• Features– Flooding creates anaerobic conditions; lack of oxygen– Organic materials accumulate– Hydrarch succession; peat buildup

• Drainage classes– excessively drained– somewhat excessively drained– well drained– moderately well drained– somewhat poorly drained– poorly drained: saturated for extended time– very poorly drained: waterlogged

Mineral Hydric Soils

• “Mineral soils, when flooded for extended periods, develop certain characteristics that allow for their identification” (Mitsch & Gosselink, 1993, p.119).

• Organic content is typically less than 20% to 35%.• Porosity between 45% and 55%.• They typically have relatively low water holding

capacity due to high hydraulic conductivity (the speed with which water passes through soil).

• Nutrient availability for plants in these soils is relatively low

• Mineral soils in wet environments frequently are blue-grey in color due to chemical changes as a result of water.

Gleization

• Development of a blue-gray or greenish color

• Gleying: result of chemical reduction of iron

Mottle Formation

• Alternate wetting and drying

• Spots of orange/reddish-brown (iron) or dark reddish-brown/black (manganese) spots in a gray (gleyed) soil matrix

• Intermittently exposed soils in an otherwise reduced environment

Formation of Hydric Mineral Soils

• Depends on:– presence of anaerobic conditions– sufficient soil temperature (> 5° C)

– presence of organic matter

Organic Hydric Soils

• Most hydric soils, however, have a high percentage of organic matter.

• Heavily organic soils are called histosols because they contain at least 78% (by volume) water, which is filling the very large amount of pore space

• “Organic soil is composed primarily of the remains of plants in various stages of decomposition and accumulates in wetlands as a result of anaerobic conditions created by standing water or poorly drained conditions” (Mitsch & Gosselink, 1993, p.117).

• Anaerobic conditions greatly reduce decomposition because decomposers in this environment are fewer in number and far less efficient than their aerobic brethren.

Characteristics

• Determined by the botanical origin of organic material and the degree of decomposition

– Porosity is about 80%– Bulk density is low (dry weight of soil material per unit volume)– Water holding capacity is relatively high– Hydraulic conductivity can be either high or low (depending on

the degree of decomposition of the plant material).– High cation exchange capacity, dominated by Ca, Mg, K, and

Na– But since many nutrients are tied up in organic material,

nutrient availability to plants is often low– Organic soils are generally colored dark, from dark black to

dark brown

Basic Sources of Organic Material

• 1- Mosses– Sphagnum

Basic Sources of Organic Material

• 2- Herbaceous material– Grasses– Reed grass (Phragmites)– Wild rice (Zizania)– Salt marsh cord grass (Spartina)– Sedges– Carex– Cladium– Cattails (Typha)– Water lilies (Nymphaea)

Basic Sources of Organic Material

• 3- Wood and leaf litter– Birch (Betula)– Pine (Pinus)– Tamarack (Larix)– Cypress (Taxodium)– Water tupelo (Nyssa)

Decomposition

• Soil scientists distinguish suborders of histosols based on the amount of organic decay.

– Fibrists (also called peat)• less than one-third of the organic material is decomposed

and more than two-thirds of the plant material is identifiable.

– Saprists (muck)• at least two-thirds of the material is decomposed and less

than one-third of the plants fibers are identifiable.– Hemists (mucky peat)

• conditions of decomposition between saprists and fibrists.– Folist: organic soils caused by excessive moisture

that accumulate in tropical and boreal mountains

Identifying Hydric Soils

• Organic soils– Hydric organic soils are typified by only one layer:

O-horizon• decomposed remains of plants• peat, muck, or combination

– Identified by black muck or black to orange-brown peat, usually thicker than 16 inches

– Form in:• waterlogged depressions• cold, wet climates• low-lying areas along coastal waters

Identifying Mineral Hydric Soils

• Indicators– Histic epipedon: layer of organic material on the surface 8-16”

thick– Smell of rotten eggs (hydrogen sulfide)– Mottles- lenses of non-dominant soil type– Abundance, size, and color of mottles reflects duration of

saturation period and whether soil is hydric– Stratified layers within 6 inches of the soil surface– Presence of 2% or more organic bodies of muck or mucky

material within 6” of soil surface– Mucky modified mineral surface layer 2” or thicker within 6” of

soil surface– Muck layer 0.5” or thicker with 6” of soil surface– Loamy mucky material 4” or thicker within 6” of soil surface

Identifying Mineral Hydric Soils

• Organic layer– Oa- mucky material (sapric)– Organic matter that breaks down into a greasy mass

upon rubbing– Oi- peaty material (fibric)– More than 2/3s of the organic material is identifiable

(leaves, stems, roots)– Oe- peaty muck or mucky peat– Folists- nonhydric organic soil forming in high

mountains and boreal/arctic regions• Dark A-horizon from organic matter• Spodic horizon in evergreen forests