Fitting ~1000 m
2 of surface area into a gram of
soil
Surprisingly, because of the extremely small size of
the particles (between 3.5 and 5.0 nm (or
three-and-a-half to five millionths of a millimetre) in
diameter, the surface area of one gram of allophane
can extend to about 1000 m
2 . Imagine if you were to
take a sheet of very thin paper 40 m long and 20 m
wide and scrunch it up to make a tight ball, or perhaps
rip the paper into very small pieces and compress them
together very tightly. Could you reduce it to the size of
a gram of soil? With paper it is clearly impossible but
it can be imagined at a molecular scale. The enormous
surface area extends over the many tiny spherules of
allophane that invariably cluster together to form
nanoaggregates (small clusters 100 nm across of
spherules) or microaggregates (small groups of
nanoaggregates).
2.5 Distinguishing Between Allophanic Soils
and Related Soil Orders
Allophanic Soils are identified by the New Zealand Soil
Classification as containing ‘allophanic soil material’. The
minerals and compounds responsible for such properties are
allophane, imogolite, ferrihydrite, and metal-humus compounds (of Fe and/or Al). The presence of allophanic soil
materials can be estimated analytically (phosphate retention
of 85% or more, and dry bulk density of 0.9 t m
−3 ).
However, in keeping with the New Zealand Soil Classification principle that where possible class criteria should use
field observable properties, the criteria include greasy sensitivity, weak or very weak soil strength, non-sticky or
slightly sticky consistence, and a strong or very strong
reactive-aluminium field test for allophane (sometimes also
referred to as the Fieldes-Perrott or NaF test).
The presence of allophane in soils can be confirmed by a
simple field test, developed by Maurice Fieldes and Ken
Perrott, where a small sample of soil is placed on a filter
paper that has been soaked in phenolphthalein (a pH indicator) solution. A few drops of NaF are added and if allophane is present a resulting increase in pH (due to
dissociated hydroxyl groups) will cause the filter paper to
turn a dark red colour. The amount of allophane may be
indicated by the rate and strength of the reaction.
Granular Soils occur marginally to Allophanic Soils in
north and west Waikato, Auckland, and Northland, where
allophanic character is recognised in Allophanic subgroups
of the Melanic, Oxidic, and Orthic groups of the Granular
Soils.
Pumice Soils occur marginally to Allophanic Soils where
allophanic character is recognised in Allophanic and
Buried-allophanic subgroups of the Orthic Pumice Soils.
Allophanic Soils may occur in pumice parent materials
where the pumice has weathered to the extent that the volcanic glass component of the pumice has weathered to allophane to meet thickness and other criteria.
Podzols are distinguished from Allophanic Soils by the
presence of a dark humus- or sesquioxide-rich horizon
(podzolic-B horizon, Bh and/or Bs) in the Podzol Soil, or the
presence of a pale coloured E-horizon, and other features
including the site environment and pedological context.
2.6 Correlation with Other Classification
Systems
New Zealand, along with Japan and other Pacific Rim
countries, that have frequent volcanic eruptions of andesitic
and/or rhyolitic tephra, have large areas of allophane-rich
soils. However, such soils are rare in continental Europe
(though they do occur, for example, in Iceland and Italy) and
the contiguous USA where international soil classifications
were developed. Thus Allophanic Soils were not classed as
an order in early editions of Soil Taxonomy (an international
soil classification developed by the United States Department of Agriculture). Soils that contained allophane were
clearly recognised in the USA but many were under forest in
the Pacific Northwest, in very remote parts of Alaska, or on
the Hawaiian islands, and so Guy Smith, the architect for
Soil Taxonomy, took the view that the low abundance or
remoteness, and limited agricultural application, meant they
were best classed at a suborder level (Andepts). However
Mike Leamy, initially with Smith in 1978, proposed and
chaired a group who successfully developed the Andisol
order which was added to Soil Taxonomy in 1990 (supplanting the Andept suborder) (Table 2.3).
2.7 Use and Management of Allophanic Soils
2.7.1 Productive Use
Allophanic Soils underpin some of New Zealand’s most
productive agriculture and horticulture (Fig. 2.9). The
combination of high water-holding capacity, free drainage,
characteristically flat to gently sloping topography, low soil
dry bulk density, friability, good tilth, and thus ease of
cultivation and root penetration, ensure that Allophanic Soils
are excellent for agriculture and horticulture.
34
2 Allophanic Soils
2 of surface area into a gram of
soil
Surprisingly, because of the extremely small size of
the particles (between 3.5 and 5.0 nm (or
three-and-a-half to five millionths of a millimetre) in
diameter, the surface area of one gram of allophane
can extend to about 1000 m
2 . Imagine if you were to
take a sheet of very thin paper 40 m long and 20 m
wide and scrunch it up to make a tight ball, or perhaps
rip the paper into very small pieces and compress them
together very tightly. Could you reduce it to the size of
a gram of soil? With paper it is clearly impossible but
it can be imagined at a molecular scale. The enormous
surface area extends over the many tiny spherules of
allophane that invariably cluster together to form
nanoaggregates (small clusters 100 nm across of
spherules) or microaggregates (small groups of
nanoaggregates).
2.5 Distinguishing Between Allophanic Soils
and Related Soil Orders
Allophanic Soils are identified by the New Zealand Soil
Classification as containing ‘allophanic soil material’. The
minerals and compounds responsible for such properties are
allophane, imogolite, ferrihydrite, and metal-humus compounds (of Fe and/or Al). The presence of allophanic soil
materials can be estimated analytically (phosphate retention
of 85% or more, and dry bulk density of 0.9 t m
−3 ).
However, in keeping with the New Zealand Soil Classification principle that where possible class criteria should use
field observable properties, the criteria include greasy sensitivity, weak or very weak soil strength, non-sticky or
slightly sticky consistence, and a strong or very strong
reactive-aluminium field test for allophane (sometimes also
referred to as the Fieldes-Perrott or NaF test).
The presence of allophane in soils can be confirmed by a
simple field test, developed by Maurice Fieldes and Ken
Perrott, where a small sample of soil is placed on a filter
paper that has been soaked in phenolphthalein (a pH indicator) solution. A few drops of NaF are added and if allophane is present a resulting increase in pH (due to
dissociated hydroxyl groups) will cause the filter paper to
turn a dark red colour. The amount of allophane may be
indicated by the rate and strength of the reaction.
Granular Soils occur marginally to Allophanic Soils in
north and west Waikato, Auckland, and Northland, where
allophanic character is recognised in Allophanic subgroups
of the Melanic, Oxidic, and Orthic groups of the Granular
Soils.
Pumice Soils occur marginally to Allophanic Soils where
allophanic character is recognised in Allophanic and
Buried-allophanic subgroups of the Orthic Pumice Soils.
Allophanic Soils may occur in pumice parent materials
where the pumice has weathered to the extent that the volcanic glass component of the pumice has weathered to allophane to meet thickness and other criteria.
Podzols are distinguished from Allophanic Soils by the
presence of a dark humus- or sesquioxide-rich horizon
(podzolic-B horizon, Bh and/or Bs) in the Podzol Soil, or the
presence of a pale coloured E-horizon, and other features
including the site environment and pedological context.
2.6 Correlation with Other Classification
Systems
New Zealand, along with Japan and other Pacific Rim
countries, that have frequent volcanic eruptions of andesitic
and/or rhyolitic tephra, have large areas of allophane-rich
soils. However, such soils are rare in continental Europe
(though they do occur, for example, in Iceland and Italy) and
the contiguous USA where international soil classifications
were developed. Thus Allophanic Soils were not classed as
an order in early editions of Soil Taxonomy (an international
soil classification developed by the United States Department of Agriculture). Soils that contained allophane were
clearly recognised in the USA but many were under forest in
the Pacific Northwest, in very remote parts of Alaska, or on
the Hawaiian islands, and so Guy Smith, the architect for
Soil Taxonomy, took the view that the low abundance or
remoteness, and limited agricultural application, meant they
were best classed at a suborder level (Andepts). However
Mike Leamy, initially with Smith in 1978, proposed and
chaired a group who successfully developed the Andisol
order which was added to Soil Taxonomy in 1990 (supplanting the Andept suborder) (Table 2.3).
2.7 Use and Management of Allophanic Soils
2.7.1 Productive Use
Allophanic Soils underpin some of New Zealand’s most
productive agriculture and horticulture (Fig. 2.9). The
combination of high water-holding capacity, free drainage,
characteristically flat to gently sloping topography, low soil
dry bulk density, friability, good tilth, and thus ease of
cultivation and root penetration, ensure that Allophanic Soils
are excellent for agriculture and horticulture.
34
2 Allophanic Soils