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LECTURE 4
The soil profile
Definitions…
 Bioturbation
 (J. Winch)
 Acidophyte
 (T. Kinnell)
 Soil
monolith
 (R.P.
Solomons)
The soil profile…
The soil profile…
 A particular
combination of the 5 soil forming
factors will lead to different processes which
result in particular sets of layers (horizons).
 A vertical
exposure of these layers is known
as a soil profile.
Major (master) soil horizons…
O
– horizon:
 Organic
horizon
 Forms above mineral soil
 Dead plant and animal residues
 Often referred to as “forest floor”
– fibric (litter layer)
 Oe – hemic (fermentation layer)
 Oa – sapric (humified)
 Oi
A–
horizon
 Topmost
mineral horizon
 Organic material = darker colour
 Coarser texture (why?)
E
– horizon
 Zones
of maximim leaching or eluviation
 High concentration of resistant material (sand & silt)
 Usually under “A” horizon
 Lighter than horizons above and below it
 Common under forest vegetation
B
– horizon
 Below
O, A or E
 Changed to a point where parent material is no longer
discernable
 Zone of illuviation
 Where climate is humid, B horizons accumulate
materials such as iron/aluminium oxides, silicate clays
 Where climate is dry, B horizons accumulate calcium
carbonate and calcium sulphate
C
– horizon
 Unconsolidated
material
 May or may not be same as parent material from which
solum formed
 Below zones of greatest biological activity
 Loose enough to be dug with a shovel, but retains some
structural features of the parent material
 Upper layers may become part of the solum as
weathering and erosion continue
R
– horizon
 Unconsolidated
rock, little evidence of weathering
 Transition
horizons…
 Subordinate distinctions…
 E.G.
Oi, Oa, Oe, Bt, Bk
Test your knowledge so far…
Test your knowledge so far…
 Name
the 5 main functions of soil in
ecosystems
 Name the 5 main soil forming factors
 Name the 4 main soil forming processes
 What is a toposequence?
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