Agriculture Reference
In-Depth Information
Box 1.1 The Soil Profile
A vertical exposure of soil in a pit or road cutting is called a soil profile. Often there
are obvious changes in color and composition from the surface (enriched with
organic matter from plant litter) to the subsoil and parent material below, as can
be seen, for example, in figure 1.8. Where these changes in color and composition
are the result of soil formation in different parent materials, the profile is called
“layered.” Often a lower layer represents a buried or “fossil” soil.
Where the profile changes are due to different biophysical processes within
the same parent material, the layers are called horizons, labeled A, B, and C from
top to bottom. Horizons may be subdivided. For example, the upper zone of
an A horizon that contains dark-brown organic matter is labeled A1, as distinct
from the paler zone immediately below, labeled A2 (figure B1.1.1). The A and B
horizons comprise the soil proper, with the C horizon being weathering parent
material. An O horizon is a superficial organic horizon composed of partially
decomposed plant litter.
A horizon
B horizon
Figure B1.1.1
A vineyard soil in the Gippsland region Victoria, Australia, showing
distinct A and B horizons. The A horizon is a bleached sandy loam and the B horizon is
a pale orange-mottled clay. There is a thin layer of organic enrichment at the top of the
A horizon (A1). The major divisions in the scale are at 10 cm intervals. See color insert.
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