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.