Soil formation in dry climates
Calcificationforms 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:
1. Dust is everywhere
Dust devil trails
Namibia
Approaching alkalai dust storm, off a playa to the left
Infiltration vs percolation
CONCEPTS
Potential evapotranspiration – max (potential)water that can be evaporated and transpiredfrom an area
PET
CONCEPTS
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
Calcification … in a nutshell
PET > precip
Calcic (Bk) is thediagnostic horizon
Typic Haplocalcid
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
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?
WHAT is IN the dust (and the groundwater)?Na salts and other saltsGypsum (CaSO4
.H2O)CaCO3Silica
Solubility decreases
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)
Depth
Bk
Bky
Byz
Cz
Saline groundwater?
Typic Haplosalid
Depth
Btzn
Bz1
Bz2
Cz
Saline groundwater
A
When groundwateris shallow andvery saline
Sooooooo,…. it all depends on 1. What is available (from dust, groundwater, etc)2. Solubility
Na saltsGypsum CaCO3Silica
Solubility decreases
From here on in, our focus will be on carbonates
Carbonates are normally translocated from the surface to depth – the per descensum model
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
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)
Stage 1
Carbonate filaments – Stage 1
Early Stage 2
Mid-Stage 2
Carbonates - Stage 2
Secondary carbonatesunder rocks – stages 1 and 2
Stage 3 – Bkm develops. Carbonates plug pores.
Bkm becomes aquitard, then an aquiclude
Stage 3
Late Stage 3
Late Stage 3
Stage 4 – laminar Bkm forms on top on the Bkm aquiclude
Solid Stage 4
Stage 4 – this is as deep as the backhoe could go!
Typic Petrocalcid
Laminar Bkm
Stages 5-6 – Bkm begins to break up
Pisoliths