buffer against storm surges. About 60% of the sediment
trapped by the mangroves is considered to have come from
sediment that was within the Firth of Thames by the 1960s
with the balance being carried down the rivers. Sediment can
be re-suspended by storms. Construction of stop-banks
along the margins of the Waihou and Piako Rivers prevents
flooding of adjacent land and means that sediment that was
formerly deposited on the flood plains is now all carried to
the sea. Between 1963 and 2007, the mangrove area
expanded by about 19 m yr
−1 . However, the rate is now
slowing and was about 8 m yr
−1 from 2002 to 2007, with no
marked expansion from 2007 to 2017.
When is a soil, a soil?
It might seem strange that something as common as
soil (from Latin, solum, ground) does not have a
universally agreed definition. Partly this is because of
the complexity and diversity of soil (in terms of origin,
character, functioning) and partly because it has different meanings to different people or disciplines. Prior
to 1999, the soil science definitions of soil, such as that
in Soil Taxonomy, generally required a ‘soil’ to include
an accumulation of organic matter in the topsoil and to
support plants. Thus many of the Raw Soils defined in
New Zealand, along with surface materials in ice-free
areas of Antarctica, the Moon, and planets such as
Mars, were not considered to qualify as ‘soil’. However, engineers use the word ‘soil’ to mean any disaggregated rock material that is not a solid or coherent
mass, and recognise three groups: sands and gravels,
silts and clays, and peat.
Scientists studying surface materials in Antarctica,
and on the Moon and Mars, felt strongly that such
materials should be considered soils and clearly come
within the interests of soil science. As a result, in
1999, the definition of a soil, in Soil Taxonomy,
was changed. Thus soil is now defined as ‘a
natural three-dimensional (Some define soil as
four-dimensional, the 4th dimension being time.) body
(pedon) typically *1–2 m thick comprised of solids
(minerals and organic matter), liquid, and gases, that
occurs on the land surface, occupies space, and is
characterised by one or both of the following: horizons, or layers, that are distinguishable from the initial material as a result of additions, losses, transfers,
and transformations of energy and matter or the
ability to support rooted plants in a natural
Fig. 13.10 Fluid Raw Soils and Saline Raw Soils under mangroves that have become established on the southern margin of the Firth of Thames
since building of the coastal stop bank was completed in 1963. Photo courtesy of Waikato Regional Council
210
13 Raw Soils
trapped by the mangroves is considered to have come from
sediment that was within the Firth of Thames by the 1960s
with the balance being carried down the rivers. Sediment can
be re-suspended by storms. Construction of stop-banks
along the margins of the Waihou and Piako Rivers prevents
flooding of adjacent land and means that sediment that was
formerly deposited on the flood plains is now all carried to
the sea. Between 1963 and 2007, the mangrove area
expanded by about 19 m yr
−1 . However, the rate is now
slowing and was about 8 m yr
−1 from 2002 to 2007, with no
marked expansion from 2007 to 2017.
When is a soil, a soil?
It might seem strange that something as common as
soil (from Latin, solum, ground) does not have a
universally agreed definition. Partly this is because of
the complexity and diversity of soil (in terms of origin,
character, functioning) and partly because it has different meanings to different people or disciplines. Prior
to 1999, the soil science definitions of soil, such as that
in Soil Taxonomy, generally required a ‘soil’ to include
an accumulation of organic matter in the topsoil and to
support plants. Thus many of the Raw Soils defined in
New Zealand, along with surface materials in ice-free
areas of Antarctica, the Moon, and planets such as
Mars, were not considered to qualify as ‘soil’. However, engineers use the word ‘soil’ to mean any disaggregated rock material that is not a solid or coherent
mass, and recognise three groups: sands and gravels,
silts and clays, and peat.
Scientists studying surface materials in Antarctica,
and on the Moon and Mars, felt strongly that such
materials should be considered soils and clearly come
within the interests of soil science. As a result, in
1999, the definition of a soil, in Soil Taxonomy,
was changed. Thus soil is now defined as ‘a
natural three-dimensional (Some define soil as
four-dimensional, the 4th dimension being time.) body
(pedon) typically *1–2 m thick comprised of solids
(minerals and organic matter), liquid, and gases, that
occurs on the land surface, occupies space, and is
characterised by one or both of the following: horizons, or layers, that are distinguishable from the initial material as a result of additions, losses, transfers,
and transformations of energy and matter or the
ability to support rooted plants in a natural
Fig. 13.10 Fluid Raw Soils and Saline Raw Soils under mangroves that have become established on the southern margin of the Firth of Thames
since building of the coastal stop bank was completed in 1963. Photo courtesy of Waikato Regional Council
210
13 Raw Soils