morphologies develop under conditions with sufficient
accumulation and/or iron segregation. (1) The entire
soil biomass to drive the redox chemistry definitive of
surface 15 cm (6 in.) is very dark gray to black from or-
hydric soils. Unvegetated playas are unlikely to pos-
ganic matter accumulation; this would most commonly
sess sufficient biomass to support such microbial activ-
result from nearly permanent saturation. (2) The surface
ity. Third, pH is usually higher in playa soils than in
15 cm (6 in.) of soil contains one or more thin layers of
wetland soils; consequently iron segregation features
very dark gray to black organic matter accumulation,
in part definitive of hydric soils (see below) are less dis-
which is within or underlain by a dominantly gray
tinct in playas than in lower pH wetlands, because iron
(chroma 2) matrix to 15 cm (6 in.); this most common
reduces more readily at pH below neutrality than above.
expression of the indicator would be found in season-
Until 1996, a fourth contrast existed between playa and
ally ponded or saturated soils. (3) The upper 15 cm (6
wetland soils: the concept of "classified soil" required
in.) contains dominantly gray soils with at least one layer
the possibility of natural vegetation, thereby excluding
with iron concentrations (red mottles); no organic matter
application to playas. At that time the NRCS definition
accumulations are present. (4) The wording also allows
of soil was broadened to include unvegetated areas if
pedogenesis occurs (cf Soil Survey Staff 1996, 1998).
segregation in dominantly gray matrixes.
The "changes in the character of the soil" (33 CFR
Although the above indicator is intended for identi-
329.11[a][1]) that are potentially useful in identifying the
fication of vegetated areas that saturate at least every
OHW elevation in playas include (1) accumulations of
other year for at least two continuous weeks, similar
organic matter (2) iron segregations (3) salt crystals, and
morphologies are seen in unvegetated playas with less
(4) soil structure and texture.
defined hydrologic regimes. Lines (1979) and Lichvar
and Sprecher (1998) have found thin black layers in pla-
Accumulations of organic matter
yas at White Sands Missile Range and Bonneville Salt
Investigators of playas have reported two kinds of
Flats, respectively. Those at White Sands Missile Range
shallow organic matter accumulation from which surfi-
have a greasy feel and H2S odor when wet (Lichvar and
cial hydrologic regimes may be inferred. First, the
Sprecher 1998); the black layers appear to have a very
USDANRCS is testing the hydric soil indicator men-
high abundance of bacteria, based on light microscopy,
tioned above (TA1, Playa Rim Stratified Layers, USDA
compared to other playa substrata (Brostoff, personal
NRCS 1998). The morphology described by this indica-
observation). At least twice we have observed green,
tor has already been observed in eight different playas
presumably chlorophyllous, matter within the surficial
in California (two), Utah (three), and Oregon (three).*
black layer at White Sands Missile Range in New Mex-
If further work confirms the initial observations, the
ico (Lichvar and Sprecher 1998). Akili and Torrance
required morphology will be as follows.
(1981), for continental sabkhas on the Arabian Penin-
Stratified layers starting within the upper 15 cm (6
sula, report a similar layering of fine-grained wind-blown
in.) of the soil surface. At least one layer has value 3 or
sand and cemented gray mud. These independent
less and chroma 1 or it has value 2 or more and chroma
observations of organic matter layers in the surface soils
2 or less with 2% or more distinct or prominent redox
of playas and playa-like entities from different parts of
concentrations as soft masses or pore linings. The upper
the world show the utility of this process for identify-
15 cm (6 in.) has dominant chroma 2 or less.
ing seasonally wet areas in arid regions. Most environ-
Playa Rim Stratified Layers User Notes. This indica-
ments in the arid West promote rapid decomposition of
tor is for the sparsely vegetated playas of the western
organic matter, so accumulation of organic layers should
United States.... [T]his indicator does not require con-
in theory be a reliable indication of unusually wet con-
tinuous chroma 2 or less. Thin layers of chroma 3 or
ditions. We are not aware of quantitative hydrologic
higher may occur as long as the upper 15 cm (6 in.) is
monitoring along drainage catenas, however, to deter-
dominantly chroma 2 or less. A minimum amount of or-
mine how long the soil must remain wet in order for such
ganic carbon is not required. A layer with redox con-
layers to accumulate.
centrations is substitutional for the dark layer. As inferred,
Iron segregations
this indicator occurs on sparsely vegetated playas and
playa rims adjacent to the non-vegetated playas.
Most hydric soil determinations in the United States
This indicator includes several possible morpholo-
are based on the process of soil iron segregating under
gies within the upper 15 cm (6 in.) of the playa rim soil,
low redox conditions into areas of iron depletion (gray
mottles or gray matrixes) and iron concentration (orange
mottles). This process operates more readily under acid
than alkaline conditions and also requires an active
* Personal communication, Wade Hurt, USDA NRCS, Gaines-
microbial community supported by moderate to large
ville, Florida, 1996.
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