UHF475, Wetland Ecology, Wetland Soils, Physical Properties

Handout notes for UHF475 Wetland Ecology
Wetland Soils, Physical Properties

Rob Harrison
Bloedel 218
5-7463 phone
mailto:robh@u.washington.edu

note to students: these figures will be included in a "note taking guide" that will be handed out to you when we start the wetland soils portion of UHF475. They are included here in case you lose them or for future reference.

Soil: (1) A dynamic natural body composed of mineral and organic materials and living forms in which plants grow. (2) The collection of natural bodies occupying parts of the earth's surface that support plants and that have properties due to the integrated effect of climate and living matter acting upon parent material, as conditioned by relief, over periods of time.

Wetland Soil (Hydric soil): A soil that is saturated with water long enough during the plant growing season to become anaerobic. This soil will usually be characterized by anaerobic soil zones and wetland vegetation.

Soil Texture: The size distribution of individual particles of a soil.

Soil Structure: The spatial distribution and total organization of the soil system as expressed by the degree and type of aggregation and the nature and distribution of pores and pore spaces.

Soil consistence: The resistance of the soil to deformation or rupture as determined by the degree of cohesion or adhesion of the soil particles to each other.

Restricting horizon: The soil horizon that most restricts movement of water or air movement vertically through the soil, or restricts root growth down into soil. Restricting horizons are often termed "PANS".

Pans: Soil horizons that are strongly compacted, cemented or have a high content of clay.

Fragipan: Brittle subsurface restricting soil horizon, usually loamy textured and weakly cemented.

Traffic pan: Compacted soil horizon created by the action of machinery, such as trucks, tractors or logging skidders, over the soil.

Soil aeration: The process by which atmospheric air enters the soil. The rate and amount of aeration depends on the size and continuity of pore spaces, the degree of water logging, and the activity of soil microorganisms.

21% O2

0.033 % CO2

Field Evaluation of several wetland soil properties

Accurate and comparable descriptions of soil properties is critical in developing a successful program of correlating site properties with other site characteristics. The properties commonly evaluated in the field include soil texture, soil structure, soil and rooting depth, permeability, depth to water and color. These properties are discussed briefly below.

1) Soil texture is the distribution of individual soil particle sizes. These include

Sand 2 - 0.05 mm

Silt 0.05-0.002 mm

Clay < 0.002 mm

Gravels > 2 mm are not included in evaluating texture, but should be noted in any description of the soil profile. Soils dominated by sand-sized particles typically have large pore sizes and rapid soil aeration and water percolation. However sand does not hold water or nutrients and may be droughty and infertile. Sand feels gritty when rubbed between the fingers. Silt sized soil particles generally offer adequate aeration and water penetration and reasonable water and nutrient holding capacities. Silt feels slippery when rubbed between the fingers, but is not sticky. Clay has the smallest grain size and pore size distribution. Clay generally offers high mosture and nutrient holding capacity. Clay soils that have not developed good structure do not allow good aeration or water penetration.

2) Soil structure is the aggregation of sand, silt and clay into larger structures called peds. Pore spaces between this aggregation promote water and air movement into soil, which is a highly desirable property of productive soils. Thus the presence of a clay texture in soils does not mean it will have low aeraton or water movement if the soil has a good structure. High degrees of soil structure are most common in finely-textured soils.

3) Soil and rooting depth is determined by the presence of bedrock or layers in soil that restrict root penetration into the soil. Very shallow soils are generally poorer for forestry because of lower nutrient and water availability for plant growth. When comparing similar soil material, forest growth is often strongly correlated with soil depth. Compacted or cemented horizons can restrict water penetration and aeration and can cause a soil with coarse textures to show slow water penetration and poor aeration.

4) Soil permeability is strongly related to soil texture and structure. Medium to coarse textured soils or soils with a high degree of structure generally show rapid water percolation and good aeration. This can be excessive in extreme cases. Fine textured soils with weak structure can have very slow permeability and low aeration.

5) Soil color can be an indication of depth to water table. Soils may be dry when pits are dug (this is certainly the case with NW summers), but the color of the soil may indicate a higher seasonal water table. Water tables restrict rooting depth and may indicate problems with trafficability in winter. Reddish colors indicate the horizon is drained during a large part of the year, while grayish hues denote presence of the water table. Mottling is the presence of both hues near each other, indicating a fluctuating water table.