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
