Agriculture Reference
In-Depth Information
content, consistent with its rainforest vegetation and elevation (620 m) (Spain, 1990).
The
mollisol
described in Avery (1990) was formed from doleritic parent materials in
a humid temperate climate (England). The A horizon is thick (28 cm) and humic, as
shown by the carbon figures presented (estimated from loss-on-ignition data using
formulae in Ball, 1964). It has a well-developed fine granular structure and many large
channels, a consequence of the substantial earthworm populations present.
The
oxisol
described in Spain
et al.
(1989a) was formed from basaltic parent materials
and occurs in a humid (3609 mm annual average rainfall) tropical environment in
northern Australia and supports a vegetation cover of lowland rainforest. Because of its
considerable age, the nature of its parent material and the environment, it has weathered
to produce a deep, permeable soil with a high proportion of clay-sized particles.
The surface soil is rich in organic matter, consistent with its rainforest vegetation and
high free iron (iron oxides soluble in dithionite) content (Spain, 1990). Despite its sub-
stantial clay contents it also transmits water readily because of micro-aggregation of the
smaller particles into the larger compound units (peds) that comprise the smallest unit of
structure. This is true of both infiltration and sub-surface lateral flow (Bonell
et al
., 1983).
The parent material of the Australian
vertisol
described by Reeve
et al.
(1963) is
a shale. The soil is alkaline in reaction with calcium carbonate nodules visible throughout
much of the profile. It is fine-textured and shrinks and swells depending on its state
of hydration, because of appreciable smectite among its clay minerals. Contraction on
drying, leads to the formation of extensive patterns of deep surface cracking. Because of
the smectite present, this soil has a higher organic matter concentration than most other
soils occurring under similar rainfall and temperature regimes. This soil has formed
under the influence of a sub-tropical climate with an average annual rainfall of 660 mm,
most of which falls in the summer months. It supports a savanna vegetation dominated
by
Acacia harpophylla
(Leguminosae) trees.
The English
spodosol
described in Avery (1990) was formed on a sandstone and
supports a vegetative cover of
Pinus sylvestris
and
Larix decidua
with an herbaceous
layer of grass and the fern
Pteridium esculentum.
It has a substantial litter layer of partly
decomposed vegetation and a typically organic A horizon. The bleached E horizon is
friable with particulate or single-grain structure. The B horizon shows characteristic
accumulations of clay, soil organic matter and the oxides and oxyhydroxides of iron
and aluminium.
Peats, or
histosols,
comprise a group of soils that possess a thick surface horizon
(known as an H or O horizon) consisting largely of often little-modified plant remains
overlying the mainly mineral soil. Peats usually form in situations where decomposition
is inhibited by impeded drainage, low temperatures or a combination of the two.
The histosol presented in Table I.4 was formed on rhyolitic parent materials in the north
of England at an altitude of 677 m (Avery, 1990) where it developed under a cold and
humid climate. It supports a vegetation of grasses, low shrubs and mosses and has
a surface layer of fresh and partially-decomposed litter. The upper peaty horizon and
the underlying mineral A horizon contain rhyolitic fragments, ascribed to the effects
of cryoturbation (frost heave).
