SOIL FORMATION - Soil Ecology - page 194

influenced by those further upslope. Thus, under defined conditions of climate, parent
material and soil age, suites of related soils (toposequences or catenas) will develop
across landscape assemblages of similar topography in which the soils will vary in
predictable ways between different landscape locations. This also implies that at
specific landscape locations within comparable toposequences, similarities in soil profile
morphology, the suites of clay minerals present and soil nutrient status may be expected.
The differences between soils within toposequences are therefore attributed to the
effects of gravity, drainage and relief. It should be noted that within toposequences,
biotic effects may not necessarily act equally on all member soils. This occurs because
of the mutual interdependence of the activities of the biota in soil formation and particular
soil properties. Thus, between the members within a particular toposequence, different
biotic communities may be expected, particularly where drainage extremes occur.
Toposequences form under general erosional influences and any processes (biotic or
abiotic) that lead to the detachment and transport of soil materials will contribute to their
formation. With downhill movement, coarser colluvial materials tend to be deposited
near the bases of slopes while finer materials are deposited further away. Of particular
importance are the movements of soluble soil materials and fine particles entrained in
lateral and downslope water flows, both below and above the soil surface. In addition,
local modification of Eh conditions often plays a significant part in their differentiation
(Duchaufour, 1997).
A great variety of toposequences have been formed due to variation in the balance
of the soil forming factors. Figure II.15 presents a toposequence of five related soils
comprising four spodosols and a peat (histosol) formed on greywacke (fine sandstone)
parent materials and supporting a vegetation dominated by Nothofagus cliffortioides
in an alpine, ice-cut, environment in South Island, New Zealand (Molloy, 1988).
Processes of downslope movement, leaching, eluviation:illuviation, creep and differential
movement have lead to a suite of related soils with many properties of the downslope
members depending on those further upslope.
Drainage is free in the upper slope positions but poor at the base of the slope leading
to some gleying and a reduced rate of organic matter decomposition. The predominantly
mineral soil is covered by an O horizon of decomposing litter comprising (with increasing
depth) the following three sub-horizons: Oi (slightly decayed), Oe (moderately decayed)
and Oa (highly decayed). The O horizon increases in thickness downslope and
the sequence culminates at the toeslope position in a series of deep peat sub-horizons
overlying the C horizon.
The A horizon only appears at the lower footslope position while the characteristic
E horizon lies immediately below the O in the upper three profiles. The E is subject
to reducing conditions at the lower footslope position and is absent in the histosol.
The B horizon has an upper thin (Bhs) sub-horizon enriched in illuvial humus and
the oxides and hydroxides of iron and aluminium (sesquioxides) in the upper three
profiles and, in the lower footslope position, this horizon also contains some illuviated
phyllosillicate clays (Bhst). At the summit and backslope positions, a weathered zone
(Bw) overlies an R layer of the fine sandstone parent material. At the footslope position,
the lower Bg horizon has some gleying indicating reducing conditions and overlies
a modified C horizon of parent rock weathering in situ. At the lower footslope position,