Agriculture Reference
In-Depth Information
II.2
THE SOIL FORMING FACTORS
During the latter part of the nineteenth century, the Russian pedologist Dokuchaev
developed the integrating concept that soils form under the influence of the five
soil forming factors: climate, parent material, time, the biota and topography.
In the intervening years, considerable efforts have been made to further develop this
theory of soil formation and to quantify the effects of the individual factors (Jenny,
1941, 1980). However, these have only been partially successful. Jenny's ideas of
independence between the soil forming factors and his limited appreciation of
the biological influence (notably omission of the role of biomechanical processes) on
soil development much constrained its further development. In addition, no account was
taken of the capacities of soils to develop without corresponding changes in the soil
forming factors. Despite these drawbacks, development of this theory has provided
a stable conceptual basis for soil formation; its substantial contributions to soil and
related sciences are undoubted (Johnson and Hole, 1994).
With the exception of time, the soil forming factors are now considered as dependent
and mutually interacting with multiple feedback effects occurring between them (see, for
example, Chesworth, 1992). Furthermore, an hierarchy exists with climate playing
a dominant role over parent materials and topography. The biotic factor is now
considered to encompass the effects of all groups of the biota that impinge on soils either
by living within them, on their surfaces or contributing organic matter to them. Time is
a different sort of factor but of great significance because of the differing and extended
periods over which soil development occurs. Hole (1961) considered that a sixth factor,
space, should also be included in soil formation theory because of the importance of
lateral differentiation.
The soil forming factors operate wherever soil and soil-forming materials occur,
although their relative influences differ between soils and with location on the earth's
surface. The soil forming factors do not directly influence the soil but are considered to
act through the medium of soil processes. These processes act in potentially different
combinations in each environment and their myriad combinations and degrees of
expression are reflected in the wide diversity of soils found on the surface of the earth.
2.1
Climate
The regional climate is a dominant factor controlling the formation of all soils (Birkeland,
1984), since it circumscribes the forms and rates of local weathering, the translocation
of weathering products and other pedogenetic processes. It interacts with, and conditions
the effects of the other factors in determining the biota that can survive in particular
environments and the seasonality of its activities. Through its control of temperature and
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