amounts of labile organic matter. These conditions exist
exposure to winds and load of eroding sands or silts
in most wetlands; they do not in playas in the arid West
within those winds, as well as rate of crystal regrowth.
because organic matter contents are low and pHs are
These cautions notwithstanding, common sense dic-
high. Therefore, the standard hydric soil indicator is of
tates that delicate crystals protruding into the air will
little use in identifying OHW levels in playas. The NRCS
not remain intact for years in desert environments; they
hydric soil indicator described above includes provi-
must have formed relatively recently.
sions for iron concentrations within a soil matrix that is
The use of salt crystals to infer relative durations of
dominantly depleted of iron. This indicator was written
hydrologic input results from the relative solubilities of
for vegetated rims of playas rather than unvegetated
various crystal types. In general, playa carbonates are
centers.
less soluble than playa sulfates, which in turn are less
A review of the soil series descriptions of Aquisali-
soluble than playa chlorides. As inundating surface
ds (salt-enriched desert soils with wetness problems)
waters evaporate, the mineral solutes become more con-
shows that iron concentrations are present in several
centrated and only the most soluble species will remain
series, but at the bottoms of the profiles rather than the
in solution. This results in concentric bands of calcium
tops (Boettinger 1997). Iron reduction occurs more
carbonates around the playa rim, gypsiferous minerals
readily at low pHs than at high. Indeed, at pH 7.9, ferric
nearer the middle, and halite minerals in the center (Hunt
hydroxide (Fe[OH)]3) does not reduce to soluble fer-
1966, Lines 1979). Accurate identification of individual
rous iron but rather to siderite (FeCO3), and then only
mineral types requires use of laboratory equipment, but
at very low redox potentials (McBride 1994). Given the
gross changes in mineralogy can be seen in the field.
low quantities of organic matter in most playa soils, iron
Since these different crystal morphologies often occur
reduction is unlikely to be very common near the soil
in concentric zones on the playa surface, it is reason-
surface, and indeed has not been found to be a useful
able to assume that the innermost zone was where wa-
indicator in unvegetated areas. Experience at White
ter stood the longest and the outermost zone where
Sands Missile Range has shown that presence of iron
water stood the least. However, the research has not
concentrations as an indicator need not be confined to
been conducted yet to infer absolute durations of inun-
matrix chromas of 2 in all playa systems. The gypsifer-
dation associated with different crystal forms.
ous playas of that installation occasionally had 2.5Y to
Crystals develop on playas from both surface and
10YR 5/3 matrixes with 7.5 to 10 YR 5/4 to 5/6 concen-
subsurface hydrologic inputs. Significant surface inputs
trations, so rules may have to be tailored to locally
to playas can often be determined by the shape of crys-
unique environments. In the gypsum-rich playas of
talline regions on the playa surface. Frequently, the
White Sands Missile Range, iron concentrations were
playa surface will have faint zones of irregular shape
more often lacking than present. Work is under way in
that look like water has stood or flowed there because
eastern Oregon to determine whether small Fe/Mn nod-
of microtopographic differences. This is strong evidence
ules in playa soils may result from reduction processes
of inundation. Differences in subsurface inputs of saline
(Clausnitzer and Huddleston 1998).
waters will probably result in more diffuse boundaries
between mineralogical zones.
Salt crystals
Two other kinds of evidence for surficial inputs into
Salt crystals are present in the upper soil horizons
salt playas are (1) thin, horizontal layers of mud in the
and on the soil surface in some playas. There are two
upper part of the soil profile, and (2) solution cavities
kinds of information that can be inferred from salt crys-
(Lowenstein and Hardie 1985). Inundating waters usu-
tals: relative strength of destructive forces acting on
ally run off the surrounding landscape and carry fine
the playa surface, and duration of hydrologic input rel-
silts and clays with them to the playa center. These fines
ative to other parts of the playa. Unfortunately, abso-
will deposit as fine layers with each inundation event,
lute lengths of time for crystal growth and destruction
with thicknesses on the order of mm rather than cm or
have not been determined. Therefore, use of salt crys-
dm. It may be necessary to inspect the top of the soil
tals for OHW determinations will infer only time scales
profile with a hand lens to find the mud layers.
relative to other places on the playa and not infer abso-
Lowenstein and Hardie (1985) also described the mi-
lute durations of growth or erosion.
cromorphology of halite crystals and layers in the soils
The presence of delicate salt crystals on the playa
of playas that inundate periodically. When these soils
surface indicates that it has been inundated relatively
are viewed in thin section, and, less conspicuously, by
recently. Erosive forces destroy fine-textured crystals
hand lens, one can see mm-sized cavities (vughes) that
if they are exposed very long without continued nour-
result from dissolution of the halite mass. Individual
ishment from subsurface waters. Quantitative rates of
crystals have rounded rather than sharp edges, and
destruction have not been determined and depend on
horizontal truncation surfaces can be seen where crys-
11
to contents
Previous Page
