soil formation in dry climates
DESCRIPTION
Soil formation in dry climates. Calcification forms calcic horizons : Bk (if cemented: Bkm (K) horizons, aka petrocalcic horizon ). Ladies and germs, This is one helluva petrocalcic horizon!. Two factors to always consider: Dust is everywhere. Dust devil trails. Namibia. - PowerPoint PPT PresentationTRANSCRIPT
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Soil formation in dry climates
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Calcificationforms calcic horizons: Bk
(if cemented: Bkm (K) horizons, aka petrocalcic horizon)
Ladies and germs,This is one helluva petrocalcic horizon!
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Two factors to always consider:
1. Dust is everywhere
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Dust devil trails
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Namibia
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Approaching alkalai dust storm, off a playa to the left
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Infiltration vs percolation
CONCEPTS
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Potential evapotranspiration – max (potential)water that can be evaporated and transpiredfrom an area
PET
CONCEPTS
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Two factors to always consider:
1. Dust is everywhere
Two factors to always consider:
1. Dust is everywhere
2. Where PET > P, precip infiltrates but rarely does it percolate THROUGH the profile
Instead, the soil gets wetted to a given depth,but the moisture then wicks back up to the surface
Any soluble compounds in the wetted soil precipitate in the profile
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Calcification … in a nutshell
PET > precip
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Calcic (Bk) is thediagnostic horizon
Typic Haplocalcid
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In dry climates,B horizons accumulate soluble materials, translocated in percolating water:
By – gypsum – gypsic horizonBk – carbonates – calcic horizonBz – soluble salts – salic horizonBn – Na salts – natric horizonBq – silicaBkm – caliche, calcrete, petrocalcic horizonBqm – duripan, silcreteBym – gypcrete, petrogypsic horizon
Cemented versionsCemented versions
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Most desert soils have an “excess” accumulation of Ca, Na, gypsum, etc, in their B horizons.
Ca
Where did it all come from?What is the source?
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WHAT is IN the dust (and the groundwater)?Na salts and other saltsGypsum (CaSO4
.H2O)CaCO3Silica
Solubility decreases
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Depth
Bk
Bky
Byz
Cz
Na saltsGypsum CaCO3SilicaDe
crea
sing
solu
bilit
y
Illuvi
al c
arbo
nate
s (k)
Illuvi
al g
ypsu
m (y
)
Illuvi
al sa
lts (
z)
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Depth
Bk
Bky
Byz
Cz
Saline groundwater?
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Typic Haplosalid
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Depth
Btzn
Bz1
Bz2
Cz
Saline groundwater
A
When groundwateris shallow andvery saline
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Sooooooo,…. it all depends on 1. What is available (from dust, groundwater, etc)2. Solubility
Na saltsGypsum CaCO3Silica
Solubility decreases
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From here on in, our focus will be on carbonates
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Carbonates are normally translocated from the surface to depth – the per descensum model
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In upper solum, where wetter:CaCO3(dust) + H2CO3 Ca++ + 2(HCO3)-
In lower solum, where drier:Ca++ + 2(HCO3)- (Secondary)CaCO3 + H2O +CO2
CO2H2OSoluble:
translocatesin percolating
water
Precipitates:as secondary carbonate
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Thus, to get CaCO3 precipitation:-dry conditions (stoppage of wetting fronts)***-rise in ionic concentration of soil solution (cessation of percolation)-lowering of CO2 in soil air--warmer temps at depth cold water is able to dissolve more CaCO3
than warm water (important only regionally)
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Stage 1
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Carbonate filaments – Stage 1
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Early Stage 2
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Mid-Stage 2
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Carbonates - Stage 2
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Secondary carbonatesunder rocks – stages 1 and 2
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Stage 3 – Bkm develops. Carbonates plug pores.
Bkm becomes aquitard, then an aquiclude
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Stage 3
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Late Stage 3
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Late Stage 3
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Stage 4 – laminar Bkm forms on top on the Bkm aquiclude
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Solid Stage 4
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Stage 4 – this is as deep as the backhoe could go!
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Typic Petrocalcid
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Laminar Bkm
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Stages 5-6 – Bkm begins to break up
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Pisoliths
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