2
Allophanic Soils
In the North Island where tephras fall
Allophanic Soils have it all,
Plants will thrive without much toil
Roots reach down to moist subsoil
Abstract
Allophanic Soils cover 5% of New Zealand, predominantly in the Taranaki, Waikato, and western Bay of
Plenty regions. Allophanic Soils are usually formed in
deep accumulations of tephra (volcanic ash) and other
volcanic debris, or occasionally on rapidly weathering
rocks, usually with good drainage and relatively high
rainfall. Thus they occur in the North Island around Mt
Taranaki and in areas peripheral to the Pumice Soils
where tephras from the central North Island volcanoes
and Mt Taranaki have accumulated, typically millimetre
by millimetre, over millennia. Most Allophanic Soils
originated by developmental and/or retardant upbuilding
pedogenesis. Many are 20,000 years old. Allophanic
Soils provide an excellent medium for plant growth
because of their friability, low bulk density, fine structure,
high water-storage capacity, and ability to support
deep-rooting plants. Allophanic Soils contain significant
amounts of a globally uncommon clay mineral, allophane,
which is formed mainly from the dissolution products of
volcanic glass. Allophane, an Al-rich aluminosilicate,
comprises tiny spherules with a huge surface area which
strongly adsorbs anions, notably of phosphorus and
sulphur. Allophane will adsorb a proportion of the
phosphorus applied as fertiliser, making it unavailable
for plant uptake, but can be used to advantage to adsorb
phosphorus from irrigated effluent.
2.1 Important Features of Allophanic Soils
2.1.1 Concept and Key Features of the Soil
Order
Allophanic Soils are deep, very friable, bright brown to
yellowish brown or, less commonly, reddish in colour
(Fig. 2.1). Allophanic soil material has a distinct ‘greasy’
feel when moist and rubbed between the thumb and fingers.
Most Allophanic Soils have formed as a result of gradual
accumulation of glass-rich tephra erupted from the major
active volcanoes in central and western North Island. Both
rhyolitic and andesitic volcanoes contribute, including
Okataina, Taupo, Ruapehu, Tongariro, Ngauruhoe, and
Taranaki Maunga (Mt Taranaki).
As the name suggests, Allophanic Soils have properties
that are strongly influenced by the presence of an aluminosilicate clay mineral called allophane. Allophane is common in soils formed on explosively erupted pyroclastic
(fragmental) deposits associated with recently active vol© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
A. E. Hewitt et al., The Soils of Aotearoa New Zealand, World Soils Book Series,
https://doi.org/10.1007/978-3-030-64763-6_2
21
Allophanic Soils
In the North Island where tephras fall
Allophanic Soils have it all,
Plants will thrive without much toil
Roots reach down to moist subsoil
Abstract
Allophanic Soils cover 5% of New Zealand, predominantly in the Taranaki, Waikato, and western Bay of
Plenty regions. Allophanic Soils are usually formed in
deep accumulations of tephra (volcanic ash) and other
volcanic debris, or occasionally on rapidly weathering
rocks, usually with good drainage and relatively high
rainfall. Thus they occur in the North Island around Mt
Taranaki and in areas peripheral to the Pumice Soils
where tephras from the central North Island volcanoes
and Mt Taranaki have accumulated, typically millimetre
by millimetre, over millennia. Most Allophanic Soils
originated by developmental and/or retardant upbuilding
pedogenesis. Many are 20,000 years old. Allophanic
Soils provide an excellent medium for plant growth
because of their friability, low bulk density, fine structure,
high water-storage capacity, and ability to support
deep-rooting plants. Allophanic Soils contain significant
amounts of a globally uncommon clay mineral, allophane,
which is formed mainly from the dissolution products of
volcanic glass. Allophane, an Al-rich aluminosilicate,
comprises tiny spherules with a huge surface area which
strongly adsorbs anions, notably of phosphorus and
sulphur. Allophane will adsorb a proportion of the
phosphorus applied as fertiliser, making it unavailable
for plant uptake, but can be used to advantage to adsorb
phosphorus from irrigated effluent.
2.1 Important Features of Allophanic Soils
2.1.1 Concept and Key Features of the Soil
Order
Allophanic Soils are deep, very friable, bright brown to
yellowish brown or, less commonly, reddish in colour
(Fig. 2.1). Allophanic soil material has a distinct ‘greasy’
feel when moist and rubbed between the thumb and fingers.
Most Allophanic Soils have formed as a result of gradual
accumulation of glass-rich tephra erupted from the major
active volcanoes in central and western North Island. Both
rhyolitic and andesitic volcanoes contribute, including
Okataina, Taupo, Ruapehu, Tongariro, Ngauruhoe, and
Taranaki Maunga (Mt Taranaki).
As the name suggests, Allophanic Soils have properties
that are strongly influenced by the presence of an aluminosilicate clay mineral called allophane. Allophane is common in soils formed on explosively erupted pyroclastic
(fragmental) deposits associated with recently active vol© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
A. E. Hewitt et al., The Soils of Aotearoa New Zealand, World Soils Book Series,
https://doi.org/10.1007/978-3-030-64763-6_2
21