the “master” horizons o a e b c r organic horizon on the soil surface surface horizon high in...

The “Master” horizons

• O

• A

• E

• B

• C

• R

organic horizon on the soil surface

surface horizon high in organic matter, but composed of mineral soil – eluviation zone

subsurf. horizon light in color due to leaching – eluviation zone

subsurf horizon, from soil forming processes – illuviation zone

least weathered (and deepest) of all the soil horizons – from R or transported inbedrock

Subscripts: Processes occurring in these Horizons

O horizon

• Oi

• Oe

• Oa

A horizon

• Ap

• Ab

identifiable (recognizable) materialIntermediate

highly decomposed (almost all gone)

plowed surface

buried surface

More Subscripts

B horizons

• t

• g

• k

• s

• h

• w

• o

clay

“gleying” (very wet)

carbonates (dry regions)

“histic” humus (high organic content)

sesquioxides (iron, aluminum, red/yellow)

“cambic” – weakly altered by removal & chem rx, no illuviation

“oxic” highly wx’d, tropical soils (Fe, Al oxides, red)

Last One (whew) and Rules• C horizon

– r (weathered bedrock)– k (carbonates)– u (transported-in PM: unweathered)– ox (transported-in PM: weathered (“oxidized”)

• Subscript Rules– rarely use more than three– t (almost) always first

Examples: Btg, Cr, Bw, Ap, . . .

Types of Parent Material - Mode of transport

Type of P.M. Mode of transport Degree of sorting by size

Glacial till

Alluvium

Colluvium

Loess

Lacustrine

Residuum

Ice Low (all sizes)

Stream/river High (coarse dropped 1st)

Gravity Low

Ice + wind High (silt only)

Water (lakes) High

None NA

Effect of Parent Material on Soil Formation – Transported PM

Eolian sand

40cm

A

C 1m

Mixed alluvium

A

C

Bw

Effect of Parent Material on Soil Formation - Residuum

1.5m

Residual shale

A

AB

C

Bt1

Bt2

Granite

40cm

A

Bw

C

Influence of climate on soil pH

High (basic)

Low (acidic)

Why?

• Base cations soluble• Water = biomass (acids)

Influence of climate on Soil Organic Matter

Cold = low inputs

Hot= rapid losses

Influence of climate on Clay Formation

So where find the soil with more clay, Alabama, or Nevada?

Influence of climate on Carbonate Layer Depth

i.e., near surface

Depth