Agriculture Reference
In-Depth Information
Soil profile showing A, B, and C
horizons. Note the color change
among the sandy loam A horizon,
the clay-enriched B horizon with
Fe
2
O
3
accumulation, and the
pale-colored C horizon of
weathering siltstone. Photograph
by the author. See color insert.
Figure 1.2
Factors Affecting Soil Formation
1.3
Jenny (1941) formalized the idea that soil formation at a particular site depended
on the interaction among a soil's inheritance (the parent material), the environ-
ment (climate), and organisms (plants and animals), as well as the duration of this
interaction (time). He described these variables as the
soil-forming factors
. The
range of possible climates, parent materials, and values of the other soil-forming
factors is huge, so the range of soil types observed across the landscape is also very
large.
Most soil classifications rely to some extent on a scientific understanding of
how a set of values for the soil-forming factors can produce a particular soil type.
Although this understanding is far from complete, some useful generalizations can
be made about the range of soils produced as any one of these factors varies, on
the assumption that the other factors are constant. We briefly examine each fac-
tor in turn.
1.3.1
Parent Material
1.3.1.1
Rock Types
Consolidated rocks are of igneous, sedimentary, or metamorphic origin.
Igneous
rocks
are formed when molten magma solidifies in or on the earth's crust and are
the ultimate source of all other rocks. Material is released explosively from volca-
