Geoscience Reference
In-Depth Information
Wetland soil
5
5.1 Introduction
the crust that break down when exposed at the
surface to air, water, and living organisms (Fig.
5-1). Soils also may form by the accumulation
of unconsolidated sediment deposited by wind,
water, or ice. For example, i ne-grained deposits
on l oodplains are the basis for much alluvial
soil (Fig. 5-2). Wind-blown dust, known as loess,
is an important component of many upland soils
(Fig. 5-3), and till deposited by former glaciers
covers vast regions in northern Eurasia and
North America (Fig. 5-4). These deposits often
contain organic matter as part of the sedimen-
tary accumulation (Fig. 5-5).
From permafrozen tundra to hyper-arid
sabkha environments, soil traits vary tremen-
dously around the world. Wetland soils are
known as hydric soils, which were “formed
under conditions of saturation, l ooding or
ponding long enough during the growing season
to develop anaerobic conditions in the upper
part” (Natural Resources Conservation Service
2010a). Thus, it is specii cally the lack of oxygen
that inl uences development of wetland soils.
Three factors are key attributes of wetland soils;
when a dominant portion of the soil exhibits
all three factors it is classii ed as a hydric soil
(Welsch et al. 1995).
“Soil” is a common English word, both a noun
and verb, which is often synonymous with dirt,
earth or ground. To a highway engineer, soil is
any unconsolidated material that can be moved
by a bulldozer without the need for explosives.
For a farmer, soil is the medium in which crops
are rooted and draw their water and nutrients
for growth. For our purposes, soil is the complex
interface between the solid continental crust
below and the atmosphere, hydrosphere and
biosphere above. This interface contains organic
and inorganic components along with air, water
and living organisms, which are often distinctly
layered in so-called soil horizons. In most situ-
ations, soil represents the upper few to several
meters of the surface that is densely penetrated
by terrestrial plant roots. This common view of
soil is subject to wide variations, however. In
some soils, suri cial plants are virtually absent,
whereas in other cases plant roots may grow
many tens of meters deep.
Soils may form in two basic geomorphic situ-
ations. The i rst is by weathering, decomposi-
tion, and alteration of the underlying parent
material; in other words, in-place soil develop-
ment. The primary means of weathering are
oxidation and hydration of minerals, often aided
by microorganisms, as well as freezing-thawing,
wetting-drying, animal burrowing, root growth,
and other mechanical actions. The classic
example is granite or other crystalline rocks of
• Saturation - water saturates the soil for pro-
longed or extended periods (Fig. 5-6), par-
ticularly during the growing season, which
limits diffusion of air into the soil and allows
organic matter to accumulate at the surface.
