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Marshneck Subaqueous Soils

Soil Map Unit Description from the RI Soil Survey Report (2010 Update)

WMa1: Marshneck loam, 1 to 2 meter water depth (2010 Update for the RI Coastal Zone Soil Survey Phase I - please download the latest copy of the spatial data).

This subaqueous soil is found on flood tidal delta slopes in coastal lagoons along the south shore of Washington COunty, RI. Mapped areas are irregular in shape found along the lagoonal side of flood tidal deltas along active or inactive lobes. Areas range in size from 6 to 10 hectares (17 to 27 acres). Slope ranges from 0 to 5 percent.

Typically, these Marshneck soils have a slightly fluid, black, silt loam surface layer, 10 to 20 cm thick, derived from estuarine and marine silts. The subsoil is dark gray sand down to a depth of 1 meter or more. The subsoil may contain buried black horizons or lenses representing old soil surfaces. A lithologic discontinuity is typically observed in these soils at a depth of 1 to 2 meters below the soil surface representing a buried Pishagqua soil. There may be concentrations of shell fragments in areas of this soil.

Included with this soil are small areas of Pishagqua silt loam at the lower edge of the slope where Marshneck soils grade into Pishagqua soils in the lagoon bottom. Small areas of Massapog soils are also included on the upper edge of the slope where the lithologic discontinuity is not encountered within 2 meters of the soil surface. Napatree and Anguilla soils are included on submerged headlands and submerged mainland beaches. Some areas are covered with shell fragments and macro algae.

Marshneck soils are permanently submerged beneath 1 to 2 meters of salt or brackish water. All horizons have a slightly alkaline pH in place and may have an acidic pH after oxidation. Sulfidic materials occur within 50 cm of the soil surface, typically the surface horizons and the buried surfaces containing more organic matter contain sulfidic materials that if drained or dredged and exposed to air, will oxidize and create acid sulfate drainage.

Marshneck soils provide an important habitat for eelgrass (Zostera marina) and often contain vegetative cover of 50% or more.

Link to Official Series Description - Marshneck | Soil Profile Photo of Marshneck Series

UPDATE: The Coastal Zone of Rhode Island is currently being updated and re-mapped, for more information on this update visit: