upslope migration of the gully head, and formation of a
major disrupting erosion feature (Fig. 10.12).
The Wither Hills near Blenheim is an area that has been
subject to repeated tunnel gully erosion events. Following
severe erosion in the early 1980s, the land was recontoured
to fill in the gullies and pasture was established with a policy
of only grazing cattle to ensure a reasonable grass cover was
maintained to prevent erosion. Sheep graze much closer to
the land surface and leave soil more exposed. However,
30 years later it was evident that new gullies were again
forming.
In the lower North Island, Immature or Argillic Pallic
Soils often form over a mudstone substrate. The mudstones
are particularly prone to landslide erosion—controlled more
by the nature of the rock than the soils. Severe erosion
events occasionally occur as a result of major storms. Forest
cover greatly reduces the erosion damage on the mudstone
hill country.
When Pallic Soils are ploughed they are particularly
prone to wind erosion, especially as most Pallic Soils occur
in regions that are often subject to strong dry north-westerly
winds. On sloping sites, water erosion is another hazard of
ploughed soils. Maintenance of vegetation cover, establishment of wind breaks, and use of no-till crop establishment
are important methods to prevent surface soil erosion.
10.7.4 Contaminant Behaviour
The Pallic Soils have low phosphate adsorbency indicated by
low P retention, and can be a source of P losses to groundwater.
Pallic Soils generally have a risk of dispersion due either to the
lack of stabilising soil organic matter or iron/aluminium oxides
(or both), or the presence of sufficient sodium in the cation
exchange complex to promote slaking when the soil is saturated. This potential dispersion may be exacerbated by inputs
Fig. 10.12 A collapsed tunnel forming tunnel gully erosion in Pallic Soils, in loess on exposed sunny slopes on the Wither Hills near Blenheim.
Photo courtesy of New Zealand Society of Soil Science
160
10 Pallic Soils
major disrupting erosion feature (Fig. 10.12).
The Wither Hills near Blenheim is an area that has been
subject to repeated tunnel gully erosion events. Following
severe erosion in the early 1980s, the land was recontoured
to fill in the gullies and pasture was established with a policy
of only grazing cattle to ensure a reasonable grass cover was
maintained to prevent erosion. Sheep graze much closer to
the land surface and leave soil more exposed. However,
30 years later it was evident that new gullies were again
forming.
In the lower North Island, Immature or Argillic Pallic
Soils often form over a mudstone substrate. The mudstones
are particularly prone to landslide erosion—controlled more
by the nature of the rock than the soils. Severe erosion
events occasionally occur as a result of major storms. Forest
cover greatly reduces the erosion damage on the mudstone
hill country.
When Pallic Soils are ploughed they are particularly
prone to wind erosion, especially as most Pallic Soils occur
in regions that are often subject to strong dry north-westerly
winds. On sloping sites, water erosion is another hazard of
ploughed soils. Maintenance of vegetation cover, establishment of wind breaks, and use of no-till crop establishment
are important methods to prevent surface soil erosion.
10.7.4 Contaminant Behaviour
The Pallic Soils have low phosphate adsorbency indicated by
low P retention, and can be a source of P losses to groundwater.
Pallic Soils generally have a risk of dispersion due either to the
lack of stabilising soil organic matter or iron/aluminium oxides
(or both), or the presence of sufficient sodium in the cation
exchange complex to promote slaking when the soil is saturated. This potential dispersion may be exacerbated by inputs
Fig. 10.12 A collapsed tunnel forming tunnel gully erosion in Pallic Soils, in loess on exposed sunny slopes on the Wither Hills near Blenheim.
Photo courtesy of New Zealand Society of Soil Science
160
10 Pallic Soils