CHAPTER FIVE
Clay Hills and Kauri
HE soils of Northland have always fascinated New Zealand's soil scientists,
just as they were the despair of pioneer farmers. Compared to the rest of
New Zealand with its youthful landforms, Northland has the subdued, rolling
topography typical of older landscapes. The contrasts are many: the region has
a warm, humid climate which seems almost subtropical to visitors from the
temperate south; the soils generally have clay-rich profiles over deeply weathered
rocks whereas most of New Zealand has coarse-textured or shallow soils, often
over weakly weathered bedrock or drift parent materials. Furthermore, in Northland
and Coromandel the forest was dominated by species such as kauri, taraire, puriri,
mangeao and pohutukawa which were confined to this warmer northern region.
Only scattered remnants of this kaurilpodocarplbroadleaf forest (Plate 5.1) now
remain, generally on pockets of uplands and hill country (including much of the
Coromandel Range). Yet the imprint of the forest on the soil was considerable.
In particular, kauri produced deep layers of highly acidic litter, which is implicated
in the podzolisation and gleying processes that have contributed to the poor physical
properties of many of the region's soils.
A number of other factors besides the warm, humid climate and kauri vegetation
contribute to the high degree of chemical and physical weathering of the rocks
of the region:
o most landforms are of greater age and/or stability than those outside the region;
o the rejuvenating effect of the Pleistocene glaciations was much less pronounced;
o soil rejuvenation by tephras from volcanic eruptions, was confined to only a
few small areas.
T
Influence of Parent Rocks on the Landscape
To understand Northland's very subtle pattern of landscapes and soils it is helpful
to consider first the distribution, mineral composition and stability of the sedimentary
and volcanic rocks of the region (Fig. 5. I). The o ldest rocks are greywackes which
are responSible for the irregular, wave-fissured nature of the eastern coastline.
Inland, the greywacke is weathered to considerable depths (up to IO m), sometimes
to a distinctive red colour. The softer sedimentary rocks extend from the Manukau
Harbour to Kaitaia, a highly heterogeneous matrix within which float discrete islands
of volcanic rocks.
Much of this softer material has been eroded and depOSited in the shallow
estuaries on the down-tilted western margin of the peninsula, especially around
Hokianga and Kaipara Harbours. The saline soils of these coastal mudflats (plate 5.2)
are ideal habitats for the New Zealand mangrove which is confined to this
northernmost soil landscape region, and is probably the most distant from the
equator of any of the world's mangrove communities.
Volcanic rocks stand out as isolated rugged uplands surrounded by the lower
hill country on the sedimentary rocks. The oldest volcanic rocks (Cretaceous-Lower
Tertiary) are the sheet lava flows of the north-west (Maungataniwha, Tauwhare
and Tangihua Ranges). Later, towards the end of the Tertiary Period, the
Coromandel and Northland region was the most active volcanic area in New
Zealand, with outpourings of andesitic rocks (Coromandel Range, Great Barrier
Island and Waitakere Range), flood basalts (Tutarnoe Range to the east ofWaipoua
Forest) and the rhyolitic rocks of the eastern part of the Coromandel Range (Fig. 5.1).
83
Plate 5.1 (opposite)
With its huge size and characteristic
many -branched crown, the kauri
(Agathis australis) dominated many of
the forests I)f Northland and
Coromandel. Mainly confined to drier,
less fertile ridges and spurs , kauri
formed a very complex mixture with
podocarps (totara, rimu and miro).
tanekaha, and hardwoods (towai ,
taraire, northern rata, puriri. tawa and
hard beech). The thick layers of litter
under kauri produce acidic humic
substances which promote soil leach ing
and can lead to gleying and
podzolisation of the soil. Only 2000 ha
of forest with a high proportion of kauri
is strictly protected, in forest
sanctuaries in Waipoua, Warawara, and
Omahuta State Forests and
Coromandel State Forest Park.
84
The Livina Mantle
Coastal sands: alluvium (Ouaternary)
Sandstone (Upper Tertiary)
Mudstones , siltstones, sandstones and limestones
(Lower Tertiary-Cretaceous)
Greywacke(Jurassic-Permian)
VOLCANIC ROCKS
~YoungbasaltS{Ouaternary)
~FloOdbasaltS(UpperTertiary)
~
Acid volcanics (Rhyolite, pumice, dacite: Upper Tertiary)
_Andesites(UpperTertiary)
D
Old sheet lava flows {Basalt and andesite; (lower Tertiary-Cretac eous)
Fig. 5 .1
Geolog ical sketch map of North land Auckland-Coromandel.
The younger basaltic volcanoes. however. are the most recognisable. These
basaltic eruptions began over a million years ago in the Whangarei and Bay of
Islands localities where they can be seen as strongly weathered lava tablelands
and cones. The youngest basalts. in the Auckland, Whangarei and Kerikeri districts
(Fig. 5.1). erupted from about 100000 years ago. many within the last 20000
years. The characteristic landforms of the latter group are well-preserved scoria
cones, such as the familiar AuckJand landmarks of Mt Eden, Mt Wellington, One
Tree Hill and Rangitoto [sland. The impressive symmetrical island of Rangitoto
(Plate 5.3) at the entrance to the Waitemata Harbour is the youngest and largest
of these volcanoes. With its ro ugh. black 00 lava flows and ratalpohutukawa forest,
Rangitoto is unique in the New Zealand landscape. It is a miniature version of
Hawaii island (the 'big island' of Hawaii).
The large number, smallness, and well-preserved form of the volcanoes of
Auckland (those that have been preserved from quarrying) is in sharp contrast
to the cataclysmic volcanism occurring at the same time in the Taupo Volcanic
Zone (Chapter 2). Although the dense residential development of Auckland city
has obscured much of this landscape. it is worth reflecting that over 50 volcanoes
have erupted within an area of 500 lan 2 - an average spacing of only 3 Ian between
each volcanic centre. Truly. Auckland is just as deserving of the title The City
of Volcanoes as other notable contenders such as Naples.
Soils of the Younger Basalt Landscapes
The youngest soils of the basalt landscapes are the raw volcanic soils of Rangitoto
Island but these are of very limited extent. Where the basalt rocks are older
(thousands rather than hundreds of years) they have weathered to friable. dayrich soils with a high content of allophane and crystalline oxides of aluminium
(gibbsite) and iron (goethite. haematite). In most of their chemical and physical
properties the less leached of these soils are similar to the volcanic loams of Taranaki
Clay Hills and Kauri
85
Mangroves fringe the shores of the
Kaipara Harbour near the delta of the
Araparera River. These Kaipara soils are
saline gley soi ls from estuarine clays
and sands eroded from the hinterland
of deeply weathered sedimentary
rocks. In the distance the pasturelands
have been establ ished on Puhoi and
Whangaripo hi ll soils which have
developed in the deeply weathered
banded sandstone
Plate 5.3
(Chapter 4). The most obvious difference is their stronger colour - usually red
or brown. A striking example is the Papakauri soil (plate 5.4) which Occurs around
the small scoria cones of the Auckland isthmus and in the Whangarei and Kaikohe
districts (plate 5.5). Previously these volcanic cones were covered in broadleaf forest
(puriri and taraire) but most of this forest has now been cleared for pastoral farming.
Aangitoto Island is New Zealand's
outstanding example of a basa lt shield
volcano. It is quite young; the gently
sloping aa lava flow basement probably
began forming around 800 years ago,
and the volcano was still active 200
years ago when the steeper summit
scoria cone was formed . The Rangitoto
soils are young and barely weathered.
yet they support over 200 species of
indigenous plants. The reason is
probably the combination of the warm
humid climate and the inherent fertility
of this thin. mineral soil.
86
The Living Mantle
Plate 5.4
The Papa kauri so its are weakly to
moderately leached volcanic loams
which have developed in basaltic sco ria
and ash. The bright red colour is due to
the iron oxide, haematite, which makes
up about 5 percent of the subsoil.
These soils are very friable, free
draining and have good structure
because of their high content of the
amorphous clay al1ophane, and oxides
of iron (haematite, ferrihyd rite ) and
aluminium (gibbsite). For the same
reasons they have very high phosphate
retention and require heavy applications
of phosphate ferti lisers for high
production levels
Closely associated with the Papakauri soils are the weakly to moderately leached
brown-coloured volcanic loams of the young basalt flows - the Kiripaka (plate 5.6)
and Ohaeawai soils. Use of the latter is limited by their variable nature: shallowness;
bouldery subsoils; and temporarily high groundwater tables after heavy rains. Many
areas have been manually cleared of these bo ulders which now remain as the
characteristic basalt dry stone walls around paddocks. Because of their excelle nt
physical properties (friability, free drainage, good structure) Papakauri soils, and
the better Kiripaka and Ohaeawai soils, are well suited to growing a wide range
of crops, provided irrigation is available during summer.
The moderately to strongly leached soils of the younger basalt landscapes
(Kenkeri, Ruatangata, Waiotu, Waimate North) have a high content of iron oxides
and gibbSite. Allophane is now o nly a minor component and kaolin-rype clays
dominate the profiles, which can have horizons with up to 80 percent clay content.
Des pite their high clay content these soils are still remarkably friable, probably
because of their high content of iron and aluminium oxides; consequently they
are classed as 'friable' volcanic clays as distinct from 'compact' (see p. 95). The
Clay Hills and Kauri
87
Plate 5.5
The Papakauri soils occur on the slopes
of the small basalt scoria cones of the
Auckland isthmus, Whanga rei, Kaikohe
and Kerikeri districts. Although most of
these soils are devoted to intensive
pastoral farming, they are also highly
suited to vegetabl e cropping and
horticultu re.
Waimate North soil is a red soil which, like Papakauri. owes its colour to the
iron oxide, haematite. The Kerikeri soil (Plate 5.7) is brown and is more typical
of the moderately leached soils found on the flat to easy rolling lava-flow topography
in the Kerikeri-Kaikohe districts. It is an important soil for the horticultural industry
in the Kerikeri district where over 1000 ha are now planted in citrus. kiwifruit
and tamarillos (Plate 5.8).
The strongly to very strongly leached friable volcanic clays are found on older
basalt sheets, particularly around the Bay of Islands. They are unique among New
Zealand's soils for they come closest to exhibiting the features of the highly
weathered red-brown laterite soils of the humid tropicS - high acidity, high content
of iron/aluminium oxides (often as nodules), and accumulation of kao lin. These
soils (Okaihau and Taraire) are known throughout Northland as 'ironstone soils'
(Plate 5.9). Indeed, the Taraire soils have such a high content of the aluminium
oxide, gibbsite, that they can be classed almost as bauxite, the ore from which
aluminium is smelted.
Soils from Older Volcanic Rocks
The most widespread volcanic soils are those developed in the older, deeply
weathered andesites, flood basalts and sheet lava flows of Tertiary and Upper
Cretaceous age (Fig. 5. 1). These are the compact volcanic clays (see p. 95) which
cover around 250000 ha of mainly hill country on Coromandel Peninsula
(Plate 5. 10 and Fig. 1.6a). The volcanic clays differ widely in their properties
according to the extent of their leaching, or the vegetation under which the soil
developed. Typical examples of moderately to strongly leached volcanic clays which
developed under podocarp forest (with some kaUri) are: the Waimatenui soils
(Plate 5. 1 I) on the deeply weathered, shattered dolerites; the Waitakere soils o n
the andesitic breccias; and the Tutamoe and Waipoua soils of the upland andesitic
lava fl ots of the Tutamoe Range and Waipoua Forest. Because of their low natural
fertility and tendency to dry out badly in summer, they are difficult to maintain
in satisfactory pastures for dairying.
Where kauri once dominated the forest, the compact volcanic clay soils are
very strongly leached. Many of these deforested Rangiuru and Aranga soils were
degraded through sheet and slip erosion and largely reverted to manuka shrubland
and rushes. For a long time farmers and agricu ltural scientists were puzzled over
the failure of these soils to respond to superp hosphate, lime and potash fertilisers
that had been so successful on strongly leached soils elsewhere in Northland. The
application of molybdenum has subsequently brought about a considerable
improvement in the use of this marginal land, although liming still seems necessary.
Pla te 5.6
The Kiripaka soils are weakly to
moderately leached volcanic loams on
the young basalt lava flows where
scoria and ash is ins ignificant.
Although generally stony, they are
well-structured soils, with kaolin and
gibbsite the predominant clay minerals.
Th e Kiripaka soils have much less
allophane than the closely related
Papakauri and Ohaeawai soi ls, but they
seem to have a higher iron oxide
conte nt , so their phosphate retention is
sti ll very high. The brown colour (cf.
the red Papakauri) is due to goethite
(rather th an haematite) being the
dominant oxide of iron.
88
The Living Mantle
Plate 5.8
Kerikeri, in the Bay of Islands, is one of
New Zealand's important horti cultural
centres. Th e orchard s are mainly
established on Okaihau and Kerikeri
soils (Plates 5.9 and 5.71 and a
characteristic feature of the landscape
is the euca lypt shelterbelts and Hakea
hedges . Generally the orchards are
small by New Zealand sta ndard s, but
the land is well utilised. Most orc hards
produce a mixture of citrus (oranges,
grapefruit, mandarins, lemons and
tangelos) and other subtropical fruits
(tamarillos, kiwifruit, feijoas). The
establishment of an irrigatio n scheme
has rectified the acute shortage o f soil
moisture suffered by these soi ls during
dry summer periods
The Okaihau soils represent the most
strong ly leached and weathered group
of friable volca nic clays. The most
distinctive f eature of th ese 'i ronston e
soils' is the many nodul es of iron and
al uminium oxides in th ei r subsoils; up
to 25 percent of certain horizons can
consist of iron oxides. Okaihau soi ls
developed under kauri /podoca rp f orest
but most have now been co nve rted t o
exotic forest plantations or pastu res.
Because of their low natural fe rtility
they have always requ ired high inputs
of f ertiliser; as a consequence some
so il s are now bei ng converted to
ho rticulture since t he intensive use of
f ertiliser (a nd irrig ation water ) is
thereby economica lly ju stified.
The Kerikeri soils are moderately to
strongly leached friabl e volcanic clays,
on older lava flows than are the
Kiripaka (Pla te 5.6 ) and Ohaeawai soils.
Compared with the Kiripaka soi ls, the
content of kaolin, gibbsite, and iron
oxides is even high er, accounting for
the strongly developed fine nut to
coarse granular structures of the
topsoi ls - properties which contribute
towards the va lue of the soil for
intensive horticulture.
Clay Hills and Kauri
89
Plate 5.10
The spine of the heavily forested
Coromandel Range, looking east across
the community of Tapu from above the
Firth of Thames. Most of the soils of
the Coromandel Range are steep land
soils developed in the andesitic parent
rock. Where slopes are gentler, the
soils (Waitake re, Mangonui and
Rangiuru soils) are moderately to very
strongly leached compact volcanic
clays similar to the Waimatenui soils
(Plate 5.11) of Northland.
Plate 5.11
The Waimatenui soils, from deeply
weathered shattered dole rites in the
volcanic uplands south -west of
Kaikohe, are typical of the moderately
to strongly leached compact volcanic
clays. The thin grey-brown topsoil
overlies 50cm of firm, very tightly
packed B horizon of moderate to strong
nut structure and very high clay
content (up to 90 percent). Almost all
this clay is kaolin, accounting for the
stickiness of the soil.
The soil is high in iron oxides (15
percent) throughout, including the red weathered C horizon below 65 cm
(where haematite produces the red
colour). Another feature of the
Wa imatenui soi l is the presence of
many small soft black iron/manganese
concretions in the Band C horizons.
The Puhoi soils are weakly leached
brown clays which have developed in
strongly weathered banded sandstone.
They are shallower than the more
leached brown clays (e.g. Whangaripo
soi ls, Plate 5.14) and the mottled,
weathered, soft sandstone can be seen
at the base of the B horizon around
90cm depth. It is not red-weathered.
The Puhoi soils contain relatively high
levels of magnesium, derived from the
parent sandstone. The strongly
developed nut structure, grey colour
mottled with brown, and the tightly
packed peds are features of the Puhoi
soil. Vermiculite is the dominant clay,
along with kaolin and smectite.
90
The Living Mantle
Plate 5 . 13
Puhoi soils occur on the hilly land of
the Warkworth -Wellsford district north
of Auckland. Despite thei r strong
structure they are slow drain ing and
have a tendency to poach in wet
winters. Nutrient levels are hi gh,
especially in the subsoil, and they are
capable of supporting excellent dairy
pastures with on ly moderate
topdressings of lime and
superphosphate
Soils from Sedimentary Rocks
Brown clays
The sedimentary rocks of Northland (Fig. 5.1 ) vary widely in their age, mineral
composition and resistance to weathering. Nevertheless, the intensity of chemical
weathering in this warm, humid environme nt has been sufficient to reduce them
to deep, clay-rich soils with yellow-brown subsoils. These are the brown clays (see
p. 96) which cover about 500 000 ha in the Northland·Auckland-Coromandel region
(Fig. I .6a) and lie at th e more weathered end of the sequence of soils developed
on sedimentary parent materials throughout New Zealand. Like their counterparts
fro m volcanic rocks, brow n clays are most easily described in terms of leaching
sequences.
The weakly leached brown clays, such as the Puhoi soils (Plate 5. 12), occupy
small areas of hilly land in the Warkworth-Wellsford district (Plate 5.13) where
they have developed in banded sa ndstones. The moderately to strongly leached
brown clays (such as the Whangaripo soils (Plate 5.14) on banded sandstones) are
muc h more extens ive on the different sedimentary rocks.
Where podocarps were a Significant element of the forest composition, the
brown clays are stro ngly leached and there is often red weathering of the parent
sedimentary rock. One example re lated to the Whangaripo soil is the Warkworth
soil on banded sandstone. These strongly leached brown clays have poorer drainage,
greyer subsoils (with mottles), higher acidity, and a higher proportion of halloysitic
clay.
The end members of this leaching sequence of brown clays are amo ng the
most difficult, degraded soils in No rthland. In the past they have been considered
as podzolised soils but they lack the hallmark of true podzols - a bleached, silicarich E horizon, or B horizons with accumulations of humus or iron/aluminium
oxides. In their morphology and chemistry they show many of the features of
gley soils (see Chapter 3); the differe nce is that in Northland they occur on gently
sloping, old landscapes (which once carried kauri forest), in contrast to the more
youthful landscapes elsewhere (see Chapter 3).
Typical examples of these surface-water gleys of Northland are the Waikare
soils (Plate 5.15) from siliceous mudstone, and the Hukerenui soils - the latter
being a large soil mapping unit covering all the major sedimentary parent rocks.
Indeed, the influence of the parent rocks is now relatively minor since they have
all been strongly weathered to deep clay-rich soils whose profiles are gleyed because
of the overriding influence of their rather impermeable subsoils.
Clay Hills and Kauri
Plate 5.14
Plate 5.15
The Whangaripo soils are moderately t o
strongly leached brown clays which
have developed in strong ly weathered
ba nded sandstone. Li ke the Puhoi soils.
the subsoils have strongly developed
(but coa rser) nut structures. The nuts
are tightly packed bu t are easily
separated when disturbed. The subsoil
colour. however. is more yellowishbrown refl ecting the presence of
around 5 percent iron oxides. Kao lin is
the co-dominant clay mineral w it h
vermiculite.
The Wa ika re soi ls are brown clays wi th
surface-water gley f eatures. covering
about 18 000 ha of the flat and rolling
landscapes on the si liceous mudstone
parent rocks between Hefensville and
W hanga rei. The subsoi l structure is
distinctly prismatic. breaking to coa rse
blocks. The grey colours and disti nct
mottling indicate reducing co nd itions
which . coupled with the low subsoil pH
(less than 5). are favourable for the
f o rmation of smectite which makes up
60 - 70 percent of the clay. Kaolin does
not form under these conditions
91
92
The Living Mantle
The Waikare and Hukerenui soils are mainly used for dairying but they pug
easily and severe pasture damage can be caused if high stock numbers are carried
during winter. Stock wintering pads or barns are almost essential. The variation
in summer rainfall is another problem; under ideal summer moisture conditions
these soils (if adequately fertilised) can produce 30 kg dry matterllla daily but this
drops to around 10 kglha during prolonged dry periods. High rates of lime and
superphosphate are necessary for pasture establishment but maintenance requirements of lime, phosphorus. potassium and molybdenum fertilisers are only moderate.
Soils of the coastal sand country and estuaries
The most extensive of the coastal sand soils (see Chapter 6) which extend along
the western coastline of Northland are the weakly to moderately leached Pinaki
soils and the moderately to strongly leached Houhora and Red Hill soils, together
covering 60 000 ha. There has been mixed success in establishing pastoral farming
on these soils because they are drought prone and the pastures are subject to
attack by pests. Consequently exotic forests have been planted during the last 20
years, notably three state forests - Aupouri, Woodhill, and Pouto. The establishment
of Aupouri State Forest, in particular, has given a Significant socio-economic stimulus
to the Far North. Nevertheless fears have been expressed at the widespread loss
of important indigenous scrubland communities and wi ldlife habitats which are
found only in the sand dunes and swamps of the Aupouri Peninsula and north
head of the Kaipara Harbour.
Whereas coastal sands are deposited by wind winnowing the mineral grains
of beaches. the tidal waters of estuaries sort and deposit the finer-textured sediments.
These estuarine muds are gradually stabilised by the growth of salt-to lerant
mangroves and salt-marsh vegetation (Plate 5.2). Because they are influenced by
both salt and a high water table they are termed saline gley soils (see Chapter 3).
Saline gley soils are Widespread around the estuaries of Northland and some have
been developed into good dairy pastures after drainage and topdreSSing. The Kaipara
soils (Plate 5.16) bordering the Wairoa River near Dargaville are one such example
of successfu l agricu ltural development for dairying.
Kauri podzols
The kauri podzols of Northland have exc ited interest among soil scientists overseas
as well as in New Zealand. and the kauri tree has gained the reputation of being
a classic podzolising vegetation. Indigenous forests containing kauri once covered
over 1 million ha of Northland and Coromandel. Most of these soils have now
been cleared of kauri and today only 20 000 ha of state forest with a high proportion
of kauri remain. But the legacy of kauri vegetation remains in the 300 000 ha
of kauri podzol soils (see p. 97 and Fig. 1.6a). These are the 'gumland soils' of
Northland w hich were exploited for their kauri resin once the forest was gone.
The kauri podzols pose some of the most difficult agricultural challenges in
Northland. The most widespread are the Wharekohe soils (Plate 5.17) which cover
about 60 000 ha of former and present gumlands on sandstone and mudstone
between Kaikohe and Whangarei. Large areas of Wharekohe soils have now been
brought into production through the application of agricu ltural research findings.
Although their nutrient leve ls are very low (requiring lime, phosphate. potassium,
copper and molybdenum), once developed, their phosphate requirements are more
moderate. Stocking as high as 15 -20 unitslha can be carried with careful wintering.
However, the greatest limitation to agricu ltural development is their slow spring
growth due to wetness and nitrogen deficiency. Some of the most intractable
Wharekohe soils have now been successfully reclaimed for exotic forestry.
Nevertheless the cost of developing the gumland soils has been enormous. With
hindSight it is now easy to see that the kauri forest should never have been
clearfelled. The kauri co-existed with these very infertile soils and could have
sustained a very valuable high-quali ty timber yield today.
Clay Hills and Kouri
If the Wharekohe soils are the best example of a kauri podzol on silty/clayey
parent materials, the Te Kopuru soils (Plate 5.18) on sandy parent materi als express
th e full podzol morpho logy in a more striking way. The Te Kopuru soils are the
o ldest and most podzolised of a wide range of soils which have developed in the
sands along the western coastline and on the lo ng sandspit of Aupouri Peninsula
behind Ninety Mile Beach. T hey have a dark, cemented Bhs ho rizo n which limits
root penetration and co nsequently causes pastures to be susceptible to drought
in summer and waterloggi ng in winter.
The kauri podzol is the rangatira of podzols in New Zealand, a soil so striking
in its morphology that it is a worthy companion for the 'children o f Tane' - the
enormous forest trees that still remain in the area. Although there are many other
podzols in the cooler, wetter regions of south-west New Zealand (see Chapters 9
and I I), nowhere else do they show such a close relationship with vegetation and
parent material. At its best in sandy parent materials, the albic horizon that has
developed is pure white, deep, and shaped like an egg-cup. Wheth er the cause
was the powerful organic acids or polyphenolic compounds in the leafllitter
leachates, or just the concentration of canopy-intercepted rainfall as a flow down
the trunk of the kauri and into the soil at its base, the infertile kauri podzol is,
iro nically, the one New Zealand soH that is sure to be illustrated in soil science
tex tbooks throughout the world.
93
Plate 5.16
Looking south across the broad sweep
of the Wairoa River below Dargaville,
where it dra ins into the Kaipara
Harbour. The gleyed Kaipara soils on
the w ide flats bordering the estuary
have been drained and developed to
dairy farms. Beyond , the T e Kopuru
(Plate 5.18) and Tangitiki soil s have
developed in the old sands between
the estua ry and the Tasman Sea in the
distance.
94
The Livins MamIe
Plate 5.17
Plate 5.18
The Wharekohe soils of the Northland gumlands are the classic
exa mple of the kauri podzol formed in deeply weathered
claystone and sandstone parent rocks. The deep mar-type
humus overlies the bleached E horizon (10-30 cm depth)
wh ich is relatively structureless, and consists la rgely of silt sized grains of quartz remaining after most clays and nutrie nts
have been leached to greater depths. The dark Bth and Bts
horizons between 30 and 60 cm depth are clay textured and
ri ch in translocated humic materials and precipitated iron and
aluminium. The pale mottled grey clay below 60 cm is deeply
weathered parent mud stone. The clay in the subsoil consists of
equa l proportions of smectite and ka ol in (the proportion of
halloysite in the latter increasing wi th depth).
Th e Te Kopuru soils of the old coastal sands of Northland
express well the mature profile morphology of kauri podzols
The black topsoil passes fairly abru ptly into the white, massive
E horizon. consist ing of 30 cm of quartz sand virtu ally devoid
of any organic matter or mineral nutrients. Immediately below
is a rigid. black Bhs horizon w here the sand grains have been
cemented together by translocated humic and aluminium
~~g~p(~~~~S~.~h~/~y) ~~~ i~~~s~t~e~~ ~;;e~t~~eh~~ir~~e~ ~~ ~~~t
penetration.
Liming of the Te Kopuru soils is necessary for good pasture
production but. since they are very poorly buffered. care is
required to avoid overiiming and depression of pasture growth.
Phosphate retention is very low - in fact zero f or the top
70 cm - and it is hardly surprising that phosphate is leached
from these soils. Nevertheless, maintenance levels of
phosphate are quite low once satis factory fertility levels have
been achieved (including periodic applications of mol ybdenum.
cobalt and, sometimes. selenium).
Clay Hills and Kauri
Distinguishing features of volcanic clays
WELL-DEVELOPED STRUCTURE structure; subsoils blocky to
BU LK DENSITY low «O.8 T /m 3) in topso ils; medium (0.8 - 1.2 T /m 3) in subso il s.
MODERATE TO HIGH PLANT-AVAILABLE WATER CAPAC ITY in the topsoils (20-25% of
soil volume); lower capacity in subsoils (14 -1 6% of soil volume).
FREE-DRAINING TOPSO ILS (macroporosity 12 -2 0%) but some subsoils slow drain ing
(macroporosity < 5%).
CLAY M INERALS
HI GH PHOSPHATE RETENTION (60-90%) - largely due to high iron oxide contem.
NO SIGN IFICANT TRACE ELEMENT DEFICIE NCIES, although molybdenum can be marginal
in strongly leached soil s.
USES OF VOLCANIC CLAYS
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96
The Living Mantle
Distinguishing features of the brown clays
PARENT MATERIALS AND LOCATION - brown clay is a simple term for the clay-rich so il s
developed o n silica- rich
rocks. The group includes those soils of
Northl andlA uc kl and classified as
yellow-brow n earths (see Appe ndix) as well as a
small number of clay-rich central yellow-brown earths like the Taita and Mapua soils (Chapter 8).
PROFILE C H ARACT ERI STICS
o somewhat variable; profiles deep, often wi th di ffuse boundaries between horizons;
o
to grey ropsoil s; subsoil s dominantly yellOWi sh-brown but often grey and
indicating gleying;
o deeply weathered parent material, usually yellow ish·brown
TEXTURES - clays, both topsoil and subsoil; day content high, 45 - 8 5%, most horizons 55 - 75%.
CONS ISTENCE - topsoils friab le
to
firm; subsoil s moderately to very fir m. Sticky.
STRUCTURES - poorly developed nut aggregates in A horizons; B horizo ns blocky, increaSingly
aggregated imo prisms with increase in leaching; dark brown c utans (clay-hu mic collOids) o n
sides of prisms
BULK DENSITIES - medium (0.9 -1. 2 T/m\ higher (1.3 - 1.7 T/mJ) in soils with surface-water
gley features (such as Waikare)
PLANT-AVAILABLE WAT ER CAPAC ITY - moderate to high in topSOils (20 - 29% of soil vol ume);
low in subso il s (8 - I 2% of soil volume)
POORLY DRAINED - subsoils (macroporosity < 5%); many topsoils freer draining (macroporosiry
8 - 10%) but gleyed soils are very poorly draining (macroporosity of only I - 3% throughout
the profile).
CLAY M IN ERALS
o kaolin and verm iculite are the main clays, with the kaolin proportion increas ing with
SO IL C HEMISTRY - acidic topSOils; low nutrient con ten t, particularly in subso il s; low inorgan ic
phosphorus levels in strongly leached an d gleyed soils. Phosphate retention moderate (30 -60%).
BIOLOG ICAL ACTI VITY GENERALLY HIGH - despite poor phys ical properties; most soil
o rganisms confined to topsoils w hic h have large, active populations, probably because of high
organic matte r contents and wanner te mperatures; biological activity of gleyed soils is markedly
less.
USES OF BROWN CLAYS
Most weakly leached brown cl ays on rolling country a re capable of supporting good pastu res
with only mode rate topdressings of lime and superphosph ate. Strongly leachedlgleyed soil s are
poorly drained and readily poach in vvinter; in summer they dry out badly. They need large amounts
o f lime to counte ract low pH and heavy dreSSings of su perphosphate (and application of
mo lybdenum) to m aintain satisfactory pastures. Consequently, much of the steeper hill country
has reverted ro shrubland. On the easier hill country se mi·intens ive sheep farming can be
maintained, but production is generally low. Most of these steeper lands are better suited to forestry.
Clay Hills and Kauri
Distinguishing features of podzols
PARENT MATERIALS AND LOCATION - soils vary in the extent to which they are podzolised.
Some show incipient podzolisation only in their soil chemistry; others, particularly in coarse
siliceous parent materials at stable sites which have been covered in mor-forming vegetation
in a humid climate for hundreds of years or more, show the full podzol morphology.
In terms of (Opography, climate and vegetation, three broad, inter-related categories of
podzolised soils can be distinguished-
o
humid to superhumid lowlands of the
STRUCTURELESS, massive E and B horizons
BULK DENSITIES
o low (0.3 -0.7 T/m 3) in some Ah horizons of podzols and gley podzols especially under intact
forest;
o moderate to high (1.0 - 1.8 T/m 3) in E and Bh horizons;
o high (1.8 - 2.3 T/m 3) in lower E and Bms horizons of fine-textured gley podzols (e.g. Okari(o
soils) and sandy kauri podzols.
VERY POORLY DRAINED SOILS, probably more because of the physical properties of the upper
horizons than the iron pans at depth; macroporosites generally < 4%.
CLAY MI N ERALS - quartz makes up most of the clay fraction in the topsoils and E horizons
of the more weathered podzols; kaolinite, gibbSite and smectite are also common constituents
of subsoils.
PHOSPHATE RETENTIONS - very low (0-20%) in topsoils and E horizons, high (85 -95%) in
subsoil Bh and Bms horizons.
USES OF PODZOLS
Podzo ls are among the most infertile and phYSically limiting soils for productive uses. They are
extremely acid, have very high C/N ratios, are lacking in most plant nutrients and are very poorly
drained. Most of their available nutrients were contained in their H and upper A horizons prior
to forest removal. Most podzols have been severely degraded through forest removal.
In both Northland and Westland, agriculture and exotic forestry have been established on
podzols which were once considered quite intractable - but at enormous costs. It has literally
been a mopping up job after the better soils have been developed. The only podzols which have
probably given a reasonable return for the effort invested are the sandy kauri podzols where the
necessary fertilisers (phosphate, potash, cobalt, molybdenum) have high availability because of
the low clay content of the soils.
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