sickness’ (vitamin B12 complex deficiency, see Chap. 12
Pumice Soils). Pasture may respond to lime where the pH is
less than *5.5.
The high capacity to adsorb anions is a challenge for soil
fertility management because of allophane’s potential to
adsorb significant proportions of fertiliser phosphate making
it unavailable to plants. Frequent additions of smaller
amounts of P fertiliser will help ensure that plants have some
phosphorus available. A newly recognised issue is the
accumulation of fluorine, derived from fertilisers, which has
been comparatively rapid and widespread in productive
Allophanic Soils which have a history of strong fertiliser
use. Nick Kim and colleagues reported that over 50 years,
average total fluorine concentrations in some surface soils
have doubled from *220 to 440 mg kg
−1 . They suggest
that chronic fluorosis in grazing animals could develop. The
addition of phosphate-containing fertiliser has also been
identified as enhancing the dissolution of volcanic glass,
consistent with phosphoric acid and fluorine-induced
removal of aluminium and silica. Thus, according to Matthew Taylor and others, soil weathering has likely been
accelerated because of the increases in fluorine.
The very high anion retention capacity in Allophanic
Soils can be useful to prevent loss of phosphate and nitrate to
surface or groundwater and a wide range of other contaminants may be adsorbed including heavy metals and pesticides. When the soil storage capacity holds water-soluble
nitrates, the nitrate may be detained for capture by root
systems for plant consumption. If the soil is poorly or
imperfectly drained, biochemical reduction can promote
denitrification which transforms nitrate to benign nitrogen
gas which returns to the atmosphere.
Topsoils are stable and can resist moderate impact of
machinery or grazing animals in wet weather. However, soil
quality monitoring shows that, like most soils, compaction
can result from heavy grazing when wet. Orthic Allophanic
Soils are free draining and water moves rapidly through the
soil after rainfall and the soil rarely becomes boggy or
pugged. However, Impeded Allophanic Soils may become
saturated for some time following heavy rain and hence, in
wet weather the use of short grazing periods and stand-off
pads for cattle, to prevent pugging and compaction, can be
an advantage.
Perch-gley and Gley Allophanic Soils have a slowly
permeable horizon which may perch water and provide a
barrier to root penetration. Where the slowly permeable
horizon is deeper in the soil (it may occur as deep as 90 cm)
it may not be a hindrance for most uses. However, where the
slowly permeable layer is less than about 50 cm deep, the
soil may benefit from installation of drainage.
Allophanic Soil materials with their high water-holding
capacity, especially Orthic Allophanic Soils, where plant
roots can penetrate to great depths, provide high profile
water-holding capacity. Most of the Allophanic Soils occur
in the Taranaki, Waikato, and western Bay of Plenty regions
where rainfall is moderate to high and so, generally, good
productivity can be achieved without need for irrigation.
Nevertheless, summer droughts do occur and for high value,
or shallow rooting crops, irrigation can underpin confidence
in successful production. Some of the sparse Allophanic
Soils on basaltic scoria cones in Northland are highly prized
in a region where many far less versatile soils, such as Ultic
Soils, prevail.
2.7.2 Erosion Risk
Allophanic Soils are generally characterised as having
‘slight to insignificant risk of erosion under pasture’ and the
soil will likely remain stable if rainfall is able to be absorbed
into, or rapidly drained through, the soil. However, if soil on
slopes does become saturated, then there is some risk of
hillslope movement. Where vegetation has been removed, or
soils have been cultivated, topsoils may be at risk of wind
erosion (Fig. 2.10). On slopes, cultivated soil is also vulnerable to surface sheet and rill erosion. Because of the low
bulk density, a rapid runoff event, or strong wind on a dry
soil, can quickly cause major loss of soil.
Work led by Les Basher and Craig Ross on the intensely
cultivated land used primarily for carrot production near
Ohakune showed that water erosion of cultivated soil had
caused extensive redistribution of soil within paddocks with
some areas eroded by up to 100 t ha
−1 year
−1 while other
areas had received deposition of 300–500 t ha
−1 year
−1 .
They observed that compacted wheel tracks were the source
of much of the runoff and subsequent water erosion as the
wheel tracks had much lower infiltration rates (4 mm h
−1 )
than the cultivated beds (800 mm h
−1 , which can easily
allow rainfall to infiltrate).
On central and eastern North Island hills and steeplands,
tephra, and thus potentially Allophanic Soil, is frequently
found on flatter ridge summits but not on associated slopes
>*25° where it has apparently been eroded preferentially. It
is likely that in some locations the erosion has occurred by
natural processes, including redistribution in the landscape
soon after tephra deposition. Overseas studies have shown
that differences in vegetation cover can affect the thickness
of tephra preserved, although it is evident that native forest
covered most, if not all, of the region during the Holocene
and until human arrival around 750 years ago. Work on the
36
2 Allophanic Soils
Pumice Soils). Pasture may respond to lime where the pH is
less than *5.5.
The high capacity to adsorb anions is a challenge for soil
fertility management because of allophane’s potential to
adsorb significant proportions of fertiliser phosphate making
it unavailable to plants. Frequent additions of smaller
amounts of P fertiliser will help ensure that plants have some
phosphorus available. A newly recognised issue is the
accumulation of fluorine, derived from fertilisers, which has
been comparatively rapid and widespread in productive
Allophanic Soils which have a history of strong fertiliser
use. Nick Kim and colleagues reported that over 50 years,
average total fluorine concentrations in some surface soils
have doubled from *220 to 440 mg kg
−1 . They suggest
that chronic fluorosis in grazing animals could develop. The
addition of phosphate-containing fertiliser has also been
identified as enhancing the dissolution of volcanic glass,
consistent with phosphoric acid and fluorine-induced
removal of aluminium and silica. Thus, according to Matthew Taylor and others, soil weathering has likely been
accelerated because of the increases in fluorine.
The very high anion retention capacity in Allophanic
Soils can be useful to prevent loss of phosphate and nitrate to
surface or groundwater and a wide range of other contaminants may be adsorbed including heavy metals and pesticides. When the soil storage capacity holds water-soluble
nitrates, the nitrate may be detained for capture by root
systems for plant consumption. If the soil is poorly or
imperfectly drained, biochemical reduction can promote
denitrification which transforms nitrate to benign nitrogen
gas which returns to the atmosphere.
Topsoils are stable and can resist moderate impact of
machinery or grazing animals in wet weather. However, soil
quality monitoring shows that, like most soils, compaction
can result from heavy grazing when wet. Orthic Allophanic
Soils are free draining and water moves rapidly through the
soil after rainfall and the soil rarely becomes boggy or
pugged. However, Impeded Allophanic Soils may become
saturated for some time following heavy rain and hence, in
wet weather the use of short grazing periods and stand-off
pads for cattle, to prevent pugging and compaction, can be
an advantage.
Perch-gley and Gley Allophanic Soils have a slowly
permeable horizon which may perch water and provide a
barrier to root penetration. Where the slowly permeable
horizon is deeper in the soil (it may occur as deep as 90 cm)
it may not be a hindrance for most uses. However, where the
slowly permeable layer is less than about 50 cm deep, the
soil may benefit from installation of drainage.
Allophanic Soil materials with their high water-holding
capacity, especially Orthic Allophanic Soils, where plant
roots can penetrate to great depths, provide high profile
water-holding capacity. Most of the Allophanic Soils occur
in the Taranaki, Waikato, and western Bay of Plenty regions
where rainfall is moderate to high and so, generally, good
productivity can be achieved without need for irrigation.
Nevertheless, summer droughts do occur and for high value,
or shallow rooting crops, irrigation can underpin confidence
in successful production. Some of the sparse Allophanic
Soils on basaltic scoria cones in Northland are highly prized
in a region where many far less versatile soils, such as Ultic
Soils, prevail.
2.7.2 Erosion Risk
Allophanic Soils are generally characterised as having
‘slight to insignificant risk of erosion under pasture’ and the
soil will likely remain stable if rainfall is able to be absorbed
into, or rapidly drained through, the soil. However, if soil on
slopes does become saturated, then there is some risk of
hillslope movement. Where vegetation has been removed, or
soils have been cultivated, topsoils may be at risk of wind
erosion (Fig. 2.10). On slopes, cultivated soil is also vulnerable to surface sheet and rill erosion. Because of the low
bulk density, a rapid runoff event, or strong wind on a dry
soil, can quickly cause major loss of soil.
Work led by Les Basher and Craig Ross on the intensely
cultivated land used primarily for carrot production near
Ohakune showed that water erosion of cultivated soil had
caused extensive redistribution of soil within paddocks with
some areas eroded by up to 100 t ha
−1 year
−1 while other
areas had received deposition of 300–500 t ha
−1 year
−1 .
They observed that compacted wheel tracks were the source
of much of the runoff and subsequent water erosion as the
wheel tracks had much lower infiltration rates (4 mm h
−1 )
than the cultivated beds (800 mm h
−1 , which can easily
allow rainfall to infiltrate).
On central and eastern North Island hills and steeplands,
tephra, and thus potentially Allophanic Soil, is frequently
found on flatter ridge summits but not on associated slopes
>*25° where it has apparently been eroded preferentially. It
is likely that in some locations the erosion has occurred by
natural processes, including redistribution in the landscape
soon after tephra deposition. Overseas studies have shown
that differences in vegetation cover can affect the thickness
of tephra preserved, although it is evident that native forest
covered most, if not all, of the region during the Holocene
and until human arrival around 750 years ago. Work on the
36
2 Allophanic Soils