Geoscience Reference
In-Depth Information
Retallack,
2001
). Following the early broad-scale mapping of soils in Russia, North
America and Europe (summarised by Joffe,
1949
, and by Kubiena,
1950
), many soil
scientists concluded that climate was
the
dominant factor in soil formation, and so a
zonal system of soil classification developed in which certain soils were considered
to be diagnostic of certain climates. For instance, lateritic soils were widely believed
to require a hot, wet tropical climate for their formation, and podzols were believed
to require a cool, wet climate characteristic of high northern latitudes. Difficulties
arose once podzols were found in tropical regions in association with sandy parent
materials and laterites were found actively forming at the present time on ultra-basic
rocks in temperate latitudes (Paton and Williams,
1972
). In the case of lateritic soils,
parent material plays a major role, as does an efficient leaching regime, as recognised
by Milne three-quarters of a century ago (Milne,
1938
,
1947
).
A second problem relates to the relative importance of vertical as opposed to lateral
processes in the development of soil horizons. The classic subdivision of soil profiles
was (and is) into A, B and C or R horizons, with some lower case suffix to qualify
the horizon further, such as a Bca-horizon rich in carbonate. The A-horizon was the
generally more organic surface horizon or top-soil, often with less clay content than
the underlying B-horizon. The A- and B-horizons form the solum. The C-horizon was
the slightly modified parent material in which the soil had formed, and R referred to
unaltered bedrock. Traditionally, texture contrast soils were thought to have formed by
eluviation of finer particles from the top-soil (or A-horizon) and their redeposition as
illuvial clay within the subsoil (or textural B-horizon). Sandy top-soils overlying clay-
rich B-horizons were thought to reflect the action of vertically operating processes of
surface eluviation (clay removal from the A-horizon) and subsurface illuviation (clay
deposition within the B-horizon). Brewer (
1955
) and Oertel (
1968
), among others,
challenged this model and rejected overly facile inferences about illuvial processes in
soils. In addition, a growing number of soil scientists became aware of the important
role played by insects such as ants, termites and other burrowing organisms in bringing
soil material to the surface, whence it is washed down the slope to form a sandy mantle
overlying an often clay-rich substrate (Nye,
1954
;Nye,
1955
; Watson,
1961
; Watson,
1962
; Watson,
1964
; Williams,
1968c
; Paton,
1978
; Williams;
1978
; Johnson,
1993
;
Paton et al.,
1995
).
A third and more subtle problem concerns polygenic soil profiles where the soil
horizons relate to several vertically stacked soils in which soil-forming processes
have continued to operate in both soil profiles. The result is that some of the original
properties of the underlying soil are camouflaged or even obliterated by the processes
operating in the overlying younger soil, making it hard to interpret the conditions
under which the underlying older soil had developed. One potentially useful way of
elucidating soil history is to study the microscopic evidence of past soil processes as
revealed by subtle changes in the arrangement of soil materials, otherwise known as
soil micromorphology.
