Soil formation in dry climates

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