CHAPTER TWO
Pumice Lands
T ~Tc!~~~~~~~~~: ~:i~~~~~~~~;~hn~S~~~~ ;:~e~~S~hi~~ :~~:n~~n~~~~~
to the Waikato basin and the Bay of Plenty. These are the 'pumice lands', so-called
because their soils are derived almost entirely from pumice, the popular term for
the distinctive soft, greyish-white, frothy, volcanic rock which mantles most of
the land. On a geological time scale, these pumice landscapes are very young (only
a few thousand years old) and the underlying volcanic rocks are generally less
than a million years old.
Most of the volcanic activity has been confined to a depress'ion that cuts
diagonally across the region for 250 km from White Island in the north-east to
Mt Ruapehu in the south-west. This depreSSion, the Taupo Volcanic Zone (Fig. 2.1),
is flanked by ranges consisting of much o lder greywacke and argillite rocks of
non-volcanic origin. These stand above the volcanic landfonns as the densely forested
Kaweka and Huiarau Ranges in the east and the Hauhungaroa and Rangitoto Ranges
of the King Country in the west.
Within the Taupo Volcanic Zone itself, the most widespread landforms of
volcanic origin are the huge plateaux formed by the eruption of volcanic gas and
molten rhyolite 300 000 - 750 000 years ago. This volcanic debris erupted at
temperatures sufficiently high to weld into a relatively soft but coherent rock
(ignimbrite) upon cooling. In the eastern sector of the volcanic zone an ignimbrite
sheet makes up the vast, flat Kaingaroa Plateau which slopes gently down from
an altitude of 700 m near Lake Taupo to 300 m at Matahina. In the west the
smaller ignimbrite plateaux - Mamaku, Tokoroa and West Taupo - are less
uniform, but rhey too slope gently down to the north. Within this central upland
rise the great rivers of the North Island - Waikato, Rangitaiki, Whakatane, Mohaka,
Ngaururoro, Rangitikei and Wanganui - to flow down to the agriculturally
important lowlands of the Waikato, Bay of Plenty, Hawke's Bay and WanganuiManawatu.
Development of the Pumice Lands
The pumice lands have presented something of an enigma to land developers since
the earliest days of colonisation. Access from the coastal lowlands was difficult,
but this was a blesSing in some respects for it ensured th~t the dense podocarp
forests of the King Country and western Urewera were not rapidly felled and
burned to make way for agriculture as happened in the lowlands of the Waikato,
Hawke's Bay and Manawatu. Indeed, the scenic importance of Lake Taupo and
the geothermal centres for tourism was appreciated at an early stage. The three
volcanic peaks of Tongariro, Ngauruhoe (Plate 2. 1) and Ruapehu dominate the
southern part of the region. Gifted to the Crown by the Ngati Tuwharetoa people
as long ago as 1886, they form the nucleus of T ongariro National Park, the winter
playground of the North Island.
Initially there were hopes for extensive pastoral farming with Merino sheep
on rhe open tussock plains and shrublands of the Kaingaroa Plateau and upper
basins of the Waikato River, but attempts to achieve this were plagued by a puzzling
stock malady termed 'bush sickness'. Nutrient deficiencies in the soils derived from
the pumice were the main cause and soil surveys during the mid-1930s eventually
33
Plate 2.1 (opposite)
Mt Ngauruhoe in eruption, 26 January
1974. The column of tephra reached
c.2000 m above the summit and hot
avalanches of andesitic ejecta can be
seen flowing down the northern and
western slopes. Ngauruhoe is the most
active vent in the Tongariro Volcanic
Centre, its classical, youthful, 900-m
cone having been built up over the past
2500 years. Over 60 eruptive episodes
have been recorded over the 140 years
of European settlement.
34
The Living Mantle
Fig. 2.1
The Taupo Volcanic Zone extends from
White Island to Mt Auapehu with
volcan ic activity concentrated in the
three main vo lcanic cen tres of
Tongariro, Taupo and Okataina (near
Rotorua).
60km
L-_~_~_---'!
established the association of bush sickness with soils derived from the Kaharoa
and Taupo pumice eruptions. These soil parent materials were deficient in a number
of trace elements important for animal health, chiefly cobalt, selenium, and copper.
Subsequently a major land-use debate ensued, leading government and private
industry to establish large areas of the pumice lands in exotic forest (principally
Pinus radiata), particularly during the depression years. By the late I 930s, however,
the widespread use of cobaltised superphosphate had successfully controlled bush
sickness and opened the way for the rapid pastoral development of the Ngakuru
area and, later Reporoa and the Galatea basin.
With hindsight, problems with agricultural development of the pumice lands
were probably for the best, for these exotic plantings during the depression were
the basis of New Zealand's future exotic timber industry. Today, exotic forests
cover 400 000 ha of the Rotorua-Taupo basin pumice lands, an important forest
resource making up 40 percent of the entire exotic forest estate in New Zealand.
Pumice Lands
There is an increasing public call for an end to the milling of most of the remaining
indigenous forests of the central North Island pumice lands, and it is therefore
timely to contemplate how difficult it would be to reserve these indigenous forests
if the SUitability of the pumice lands for exotic trees had not been recognised
and acted upon.
Soil-forming Tephras
To understand the soil pattern of the pumice lands it is necessary to appreciate
the age, composition, and distribution of the material erupted from the Taupo
Volcanic Zone. The largest and most devastating of these eruptions within recorded
history was the Taupo eruption variously estimated at around A.D. 130 (by carbon
dating), or around A.D. 186 (from historic Roman and Chinese records). From a
vent close to the White Cliffs in the north-eastern corner of present-day Lake Taupo
(Plate 2.2), 60 km 3 of rhyolitiC pumice and ignimbrite were erupted, incinerating
the forest and entirely remoulding the landscape. This was over 60 times the volume
of the debris from the well-documented Mt St Helens erur.tion in North America
in 1980, yet it is only a small fraction of the 15 000 km 3 of volcanic rock and
ash erupted since volcanism commenced in earnest within the zone about 1 million
years ago.
Most of the erupted material is ignimbrite which solidified long ago and has
been subsequently mantled with successive eruptions of less consolidated airfall
volcanic material collectively termed 'tephra' (ranging in size from fine particles
3
of ash, through gravels Qapilli) to large boulders). Although the volume (300 km )
of tephras erupted during the last 40 000 years is much smaller than the earlier
landform-generating ignimbrites, it is the tephras which are important as the soil
parent materials.
Scientific and engineering interest in these tephra showers is such that their
distributions have now been mapped and their pedological properties better
appreciated. To illustrate their widespread distribution, the 12 major tephras erupted
during the past 40 000 years are shown on the North Island map in Fig. 2.2. The
legend to Fig. 2.2 indicates that most of the tephras are rhyolitic in composition
and were ejected in cataclysmic eruptions from the central part of the volcanic
zone - Rotorua caldera, and the Okataina and Taupo Volcanic Centres (Fig. 2.1).
35
Plate 2 .2
The north-eastern shores of Lake
Taupo. looking north from above
Hatepe . The prominent finely dissected
scrub-covered Ouaha Ridge inland of
the White Cliffs consists of great
depths of Taupo Pumice. which was
probably erupted from a deep offshore
basin within the Horomatangi Reef
Elsewhere this characteristic pattern of
rill erosion in Taupo Pumice has been
largely masked by the growth of exotic
forest plantations. A good example of a
beach remnant can be seen in the
foreground. cutting across Ouaha
Ridge. It marks the level to which Lake
Taupo rose (over 30 m above its
present level) when the outlet was
blocked after the Taupo eruption.
36
The Living Mantle
LITHOLOGY
AGE (YEARS)
RotomahanaMud
Hydrothermal
100 }
TaraweraAsh& Lapilli
Basaltic
100
TEPHRA FORMATI ON
VOLCANIC CENTRE
VOLUME (km 3 )
1.3
Okataina
Rhyolitic
Taupo Pumice
Taupo
Rhyolitic
WaimihiaLapilii
Taupo
Rhyolitic
19
Okataina
Rhyolitic
10
okataina
Rhyolitic
Okataina
Rhyolitic
Okataina
Rhyolitic
RotomaAsh
7350
13500
RotoruaAsh
Okataina
Rhyolitic
AokautereAsh
Taupo
Rhyolitic
(Hydrothermal)
19900
16
okataina
Dacitic
30100
66
'MangaoniLapilii
'RotoehuAsh
'I metre depth
Rhyolitic
Pumice Lands
37
TE NGAE SECTION
Fig. 2.2 (opposite)
The map shows the distribution of the
twelve principal tephras (listed in the
legend ) erupted during the last 40 000
~~~~~~r~~e~t~e:~ih~ ~~~o~~~taina
_
_
-
-
_ _ _ _ _ _ _ _ _ _ _ _ _,
eruptions from the Taupo and Okataina
Centres were usually extremely violent
and separated by long periods of
volcanic inactivity during which soil
development could take place on the
tephra surface. In contrast the
andesitic tephras from the Tongariro
Centre were probably erupted
intermittently with little time for topsoi l
development on each relatively small
shower. With this lack of paleosols to
act as markers they are consequently
more difficul t to separate into l"~'"--',". ~----leruptions.
Tephfas
Ohakune
1
Taupo
1
Reporoa
1
Kawerau
D
Andesitic (Tongarifo)
D
AhyolitiC(Taupo)
D
Rhyolitic (plus minor basaltic)
(AotofualOkataina)
1
White Island
38
The Livinn Manrle
However, much smaller amou nts of the more basic andesitic tephras have been
erupted intermittently and less violently from the volcanic zone, interestingly enough
from the northern (White Island. Whale Island and Mt Edgecumbe) and southern
(fongariro Volcanic Centre) ends.
Yet the imprint upon our soils of volcanism in the Taupo Volcanic Zone extends
far beyo nd the pumice lands. Older, fi ner tephras erupted from this zone cover
much of the lowlands of the Waikato and Bay of Plenty. The major rhyolitic eruption
that gave rise to the Kawakawa Tephra 20 000 years ago deposited several
centimetres of fine ash as far away from its Taupo sou rce as Christchurch and
the Chatham Islands; shards of volcanic glass found in Antarctica are considered
to have been part of this same Kawakawa Tephra. Yet tephra from relatively minor
andes itic eruptions can be carried great distances; during the I950s as h from an
eruption of Mt Ngauruhoe was co llected from car and ho use roofs in Well ington
and Hamilton.
An obvious question is: 'How can all these different tephras be recognised?'
The answer is complex, but most tephras have distinctive co lo urs, or a developed
topsoil (subsequently buried and now called a 'paleosol). or a distinctive particle
size or mineralogy. The tephras have built up like layers in a huge volcanic landscape
cake, with interfingering of the tephras from the three volcanic centres - Tongariro,
Taupo and Okataina. This is ill ustrated by cross-section and photographs in Fig. 2.3
for 24 tephras erupted over the last 20 000 years along the axis of the Taupo
Volcanic Zone.
Only the upper. most recent members of the tephra layers lie within the rooting
zone of plants. and only five of them - Ngauru hoe Ashes. the Tarawera Formation
(Tarawera Ash and Lap illi . and Rotomahana Mud). Kah aroa Ash. Taupo Pumice.
and the o lder andesitic tephras collectively called 'Tongariro Ashes' - are entirely
soil-forming (Fig. 2.4). There are sufficient differences in the physical and chemical
properties of these soil-forming tephras to have a Significant effect on land use
in the pumice lands.
The Soil Pattern of the Pumice Lands
The volcan ic soils of the pumice lands have a number of features that distingu ish
them from the more typ ical soils formed on non-volcanic paren t materials in other
parts of New Zealand. They are gene rally deep. weakly weathered. coarse-textured
and susceptible to erosion because of their low cohesion. Most of these properties
reflect their youthfulness. for most of the soil parent tephras post-date the Waimihia
eruption (3400 years ago). These young volcanic soils fall into two main groups
shown in Fig. 2.4:
o pumice soils (see p. 52) formed from rhyolitic pumice from the Taupo and Kaharoa
eruptions; these pumice soils are widespread, covering aro und I 500 000 ha
(Fig. 1.6(a). p. 28);
o raw volca nic soils (see p. 52) w hich are very young soils formed in tephra
(Fig. 1.6a). These very coarse. localised tephras (Ngauruhoe Ashes) are from
the Tarawe ra eruption of 1886 and the pe riodic minor eruptions of Mts
Ngauruhoe and Ruapehu since the Taupo eruptio n.
Both groups are shown in Fig. I .6(a). This figure also shows that there are relatively
small. but sign ificant. areas within the Taupo Volcanic Zo ne (e.g. the Mamaku
Plateau and the upl ands of the south around Ohakune and Waiouru) where the
soils are of volcanic origin but are much o lder and the tep hras have weathered
to volcanic loam s (described on p. -53). These soils are widespread in the loarnlands
of the Waikato. Bay of Plenty. and Taranaki (Chapters 3 and 4).
Airfall tephras were depos ited on both flat and hilly land; gu llying of the fl at
land led to redeposition of the tephra as alluvial terraces. while teph ras eroded
from the hill slo pes built up deep deposits of pumice colluvium. In othe r places.
such as the Galatea basin , fans spilt out of the ranges and onto the river terraces
and valley floors. With this redistribution within the landscape, the soil parent
tephras were modified (particularly in terms of their particle size and mixture
with weathered non-volcanic rocks). Consequently the volcanic soils in the region
differ accord ing to their position in the landscape and the nature and degree. of
development of the soil-forming tephras; the total range is immense. (The detailed
pattern can be found in the soil survey publications listed in the bibliography.)
Pumice Lands
SURFACETEPHRAS
DNgauruhoeTephra
D
RotomahanaMud
DTaraweraAshandlapilii
DKaharoaAsh(Ka)
DTauPoPumice(Tp)
Burrell Tephra (B)
DStratfordTePhra(S)
UNOERlYINGTEPHRAS
Waimihia Formation (Wm)
WhakataneAsh(Wk)
MamakuAsh (Ma)
RotoruaAsh(Rr)
OkarekaAsh{Ok)
RotoehuAsh(Re)
UNOIFFERENTIATEOTEPHRAS
DTonllariroAsheS(TII)
DEgmontAshes(E)
D
HamiltonandKauroaAshes
DWaikatorhYOlitittephras
o
Fig. 2.4
Map of soil-forming tephras in central
North Island and T aranaki. T he teph ras
contributing to the top metre of soil are
shown by both colour (su rface tephra)
and symbol (for significant underlying
tephras).
Pureora to Lake Waikaremoana
The variation of soils with changes in topography and climate can be appreciated
by traversing the central part of the pumice lands, from Pureora in the west to
Lake Waikaremoana in the east (Fig. 2.5). Taupo Pumice is the dominant soil·
forming tephra in the west and central part of the traverse. The imprint of
differences in climate and vegetation upon the airfall part of this tephra is well
illustrated by the develo pment sequence of the widespread Taupo, Oruanui and
Tiho i soils on th e rolli ng and hilly land west of the Waikato River.
North Taranaki andesi!it
tephras
39
40
The Liv;nn Mancle
Plates 2.3 - 2.5
The sequen~e of Tau~o, - Orua~ui a~~ Ti~oi soils shows a .trend of incre.asing soil development with an increase in leaching and
differences In vegetation. Their position In the landscape IS shown in Fig. 2.5.
Plate 2 .3
Plate 2.4
Plate 2.5
The Taupo soils are the least
developed; they are very widespread in
the Taupo-upper Waikato basin at
elevations of 200 - 450 m and rainfall
of 1100 - 1200 mm. A black A horizon
has developed under an indigenous
vegetation of manuka, tutu, bracken
and grasses, the characteristic
shrub/ fernland induced when the Taupo
eruption destroyed the earlier
podoca rp/hardwood forest. The 8
horizon is only 10 - 15 cm thick and
there is usually a coarse-textured layer
of Taupo Lapilli in the pale yellow brown C horizon.
At higher elevations of 450 - 600 m,
the Oruanui soils show more signs of
soil development. Rainfall is now
1200 - 1500 mm and the Bs horizon is
much thicker (up to 50 em) and is
redder in hue because of the
accumulation of iron oxides and humus
under the stronger leaching conditions.
The A horizon is deeper (15 - 20 cm)
and also black due to bracken fern
which replaced much of the forest .
The end member of the sequence is
the Tihoi soil which is widespread,
particularly under podocarp/hardwood
forest, on the flanks of the Rangitoto
and Hauhungaroa Ranges above 550 m
and under a rainfall of
1 500 - 2000 mm. A somewhat
bleached E horizon (indicating
eluviation of Fe and AI oxides) is found
(particularly under rimu trees) and there
is a thick (20 em) 8s horizon (indicating
deposition of Fe and Al oxides) below
this E horizon.
Plate 2.6
The Poronui soils are alluvial soils
developed from water-depos ited Taupo
Pumice in the Taupo district. Their
natural vegetation is indigenous
sh rubland which can tolerate their
coarse-textu red character (Plate 2.7)
The layers of pumice alluvium in the
subsoil indicate the great depth of
Taupo Pumice that has been deposited
and its susceptibi lity to erosion by
wate r (Plate 2.7).
Pumice Lands
41
Fig. 2.5
Rainfall and soil pattern along landform
transect from Pureora to Lake
Waikaremoana.
With increasing altitude, rainfall also increases and the Taupo, Oruanui and
Tihoi soils become progressively leached (Fig. 2.5). Although the colour and depth
of their horizons reflect this increase in leaching and the influence of different
vegetation (plates 2.3 - 2.5), the three soils are too young and unweathered to exhibit
significant differences in chemical (e.g. exchangeable nutrients) and physical (e.g.
clay content, water-holding capacity) properties.
Both the Taupo and Oruanui soils have been largely cleared of fern, shrubland
and indigenous forest and have been developed to pasture and exotic forest. The
Tihoi soils still support areas of high-quality podocarpihardwood forest on the flanks
of the Hauhungaroa and Rangitoto Ranges although here also large areas have
been converted to pasture and exotic forest.
Further west, on the uplands of the King Country, the effect of the Taupo
eruption was probably not as severe. Here Taupo Pumice is thinner (usually less
than 50 cm) overlying large areas of older tephras. On hilly and steep slopes it
may be absent altogether. These soils have composite properties, a pumice topsoil
and a volcanic loam subsoil, and are mapped as a comp lex of Maroa, Tihia,
Ngaroma. Piropiro and Waione soils. They are generally better for farming than
the true pumice soils since plant roots can extract nutrients and moisture from
the underlying finer-textured brown tephra.
After the Taupo eruption huge volumes of pumice dammed Lake Taupo and
some of the rivers in the catchment. When these pumice dams were later eroded
away water levels dropped, exposing stranded shorelines (Plate 2.2) and large areas
of water-sorted pumice. The Whenuaroa soils in the Reporoa-Broadlands section
of the Waikato catchment, or the Otamatea and Poronui soils further south. are
typical of these alluvial pumice soils. They are susceptible to drought since the
finer material has been washed out during deposition leaving coarser sands
(Plate 2.6). Generally less weathered, they are low in available magnesium and
(along with the flow tephra soils) have usually supported only a shrub and tussock
vegetation rather than forest (Plate 2.7).
DUring the Taupo e ruption glOwing tephra avalanches ('nuees ardentes) swept
across the landscape as incandescent mud flows. incinerating the forest and flOwing
into valleys and depressions, filling them to a considerable depth with pumice.
These 'flow tephras' can be recognised by their poor sorting and their compactness.
Angular fragments of pumice are interlocked with sharp, unweathered mineral
shards (Plate 2.8) to such an extent that they can act as a Significant barrier to
root penetration (see Plate J 4.4).
42
The Living Mantle
Plate 2.7
A view north across the headwaters of
the Ri pia and Rangitaik i Rivers to the
exotic forests of the southern
Kaingaroa Plateau. Here the airfall and
flow Taupo Pumice has been
redeposited as thi ck deposits of
allu vium and colluvium. The fireresistant manuka/monoao/bristle
tussock vegetation in the foreground is
typical of the relatively small areas of
soils developed from Taupo Pumice
now remaining in an unmodified state.
The pattern of land development in the
distance indicates appropriate uses of
these coarse, deep soi ls once nutrient
and erosion limitations are recognised.
Note the deep gully erosion in the
foreground.
Plate 2.8
Kaingaroa soils are pumice soils
developed on deep flow tephra from
the Taupo eruption. They cover a la rge
part of the southern and western
sectors of the Kaingaroa Plains
(Fig . 2.5 ). The dark A horizon is
sha llow and there is only a very weakly
developed shallow B horizon. Most of
the profile in the photograph consists
of a deep , compact C horizon of
angular pumice fragments held tightly
in a matrix of sharp, unweathered
shards of volcanic glass . This subsoil
can be a significant barrier to tree -root
penetration (Plate 14.4 ).
Devastating Taupo flow tephras swept over flat areas like the Kaingaroa Plateau,
and across the Hauhungaroa Range to the west and into the King Country. Flow
tephras infilled the upper Ngaururoro catchment in the northern Kaimanawa Range
and even extended as far south as the T aruarau River and Ngamatea Swamp
(Plate 2.9), just north of the highest plateau on the Taihape-Napier road. A common
feature of the flow tephras is the presence of charred logs and pieces of charcoal
such as that shown in the Taruarau soil at Ngamatea (Plate 2.10).
At lower elevations Atiamuri soils are found on the flow tephra, but the most
widespread are the Kaingaroa soils (Plate 2.8) which cover large areas of the more
elevated, moister Kaingaroa Plateau (Fig. 2.5). Both soils have very shallow A
horizons, weakly developed shallow B hOrizons, and compact C horizons which
restrict plant growth; as such. they are quite distinct from the more friable soils
like Taupo and Oruanui. Despite the physical limitations of the flow tephra pumice
soils, their best use is for exotic forestry (Plate 2.11), espeCially with the modern
practice of deep-ripping to loosen their compact subsoil.
Towards the eastern margin of the Kaingaroa Plateau, Kaharoa Ash (which
is typically whiter. harder and less vesicular than th e softer Taupo Pumice) begins
to dominate the upper part of the soil profile. These Te Rere and Pekepeke soils
are more like the Taupo and Oruanui soils in their properties and they both support
large areas of the huge Kaingaroa exotic forest plantations which extend as a sombre
green swath e from the upper Rangitaiki to the Tarawera River.
Further east is the valley of the Whirinaki River, a major tributary of the
Rangitaiki, at the junction of the ignimbrite sheets underlying the Kaingaroa Plateau
and th e older greywackes of the Huiarau Range (Fig. 2.5). Here lie some of the
most impressive podocarp forests remaining in the North Island and it is interesting
to speculate upon the role of volcanic eruptions. especially the Taupo eruption,
in determining such a forest pattern (Plate 2.12). It is likely that the Taupo eruption
destroyed the forest on the Kaingaroa Plateau but the Whirinaki valley may have
been far enough away from the source for sufficient seed trees to have survived
on the steeper slopes. The eruptions, in rejuvenating the soils of the valley floor
and the roBing country on the northern and western margins, tended to reverse
the successional trend towards hardwood forests (dominated by tawa, kamahi and
rewarewa) which are found on the more developed hill soils. Today tall, dense
podocarp forest dominates these easier slopes and can be considered to represent
only an intermediate stage in the post-eruption vegetation recolonisation.
Pumice Lands
43
Plate 2.9
Th e rolling uplands of the Ngamatea
Plateau (c. 1 000 m) are now largely
established in pasture. The cutting has
exposed the profile of the Ngamatea
soil (thin, ai rfall Taupo Pumice over
Tongari ro Ash). The Taruarau soil
(thick, fl ow tephra Taupo Pumice over
Tongariro Ash!. shown in Plate 2.10, is
found at a sli ghtly lower altitude
(around the pine trees in the distance)
where the nueeardente swept th rough
from the southern Kaimanawa Range.
Plate 2.10
Th e Taruarau soil is developed in thick
flow Taupo Pumice overlying Tongariro
Ash. This profile on the Ngamatea
Plateau shows t he white C horizon in
Taupo Pumice perched above the
weathered Tongariro Ashes (Plate 2.9),
Th e wel l-developed, black A horizon is
a characteristic of this soil, and other
pumice soils developed from Taupo
Pumice. Th e carbonised remains of
trees (oriented in a north-west/southeast direction) can be found at the
base of the T aupo Pumice , indicating
the f orce of the nuee ardente which
incinerated them.
Plate 2.11
Young pine trees established in
Kaingaroa soils (Plate 2.8) at the
southern end of the Kaingaroa Plateau.
Despite the harsh climate of this upland
area, and the coa rse, compact nature
of the subsoi l, the t rees appear to be
growing vigorously and , as yet, show
no sign of nutrient deficiencies.
44
The Ljvjng Mantle
In the ranges to the east of the Whirinaki River the mantle of tephra is generally
much thinner. It consists of Kaharoa Ash and Taupo Pumice over Whakatane Ash
in the Urewera, and Taupo Pumice over Waimihia Ash in the southern Huiarau
and Ahimanawa Ranges (Fig. 2.4). These Urewera steepland soils support the vast
upland podocarp/hardwood forests of the western part of Urewera National Park.
They have not been investigated or mapped in any detail , as their overriding
importance for soil, flora and fauna conservation has never been questioned.
The soils in the easternmost part of the Urewera uplands lie on a complex
jumble of softer calcareous sandstones and siltstones of Miocene age (Fig. 2.5).
Here slopes are more gentle and much of the deposited tephra has resisted erosion.
Rainfall in these cool uplands around Lake Waikaremoana (Plate 2.1 3) is up to
2500 mm per annum, with very heavy falls from occasional storms from the south
and south-east. The risk of erosion is very high if this dense vegetation is disturbed
and it is essential that these fragile soils be protected to avoid the enormous loss
of biota and soil productivity which occurred through forest removal along the
length of the eastern slopes of these axial ranges (Chapter 7).
Plate 2.12
Rotorua-Okataina Volcanic Centres
In the Whirinaki valley, on the southwest margin of the Huiarua Range ,
Taupo Pum ice and Waimihia Ash
overlie varying depth s of the andesitic
Tongariro Ashes; Taupo Pumice
deposits infilled the valley floors and
provided an excellent rooting medium
for the recolonisation of the podocarp
forest, such as these dense stands of
rimu, matai , totara , miro and kahikatea.
The soil landscape pattern traversed across the Taupo basin is generally repeated
in the Rotorua area; most soil parent materials are of rhyolitic tephra, with Kaharoa
Ash particularly prevalent north and south-east of the Rotorua Lakes (Fig. 2.4).
Taupo and Oruanui soils occur on similar landforms and similar soil development
and leaching sequences occur (Plate 2.14). The coarse-textured Oropi and Oturoa
soils are similar to Taupo and Oruanui soils and are found on Kaharoa Ash and
Taupo Pumice around the northern margins of Lake Rotorua, at an altitude of
320 m and an annual rainfall of 1600 ~2000 mm (Plate 2_15). The Oropi soils
are very susceptible to drought because of their coarse texture and, like the Oturoa
soils, they have a low natural nutrient status w hich is readily rectified by fertilisers.
Although the Oropi soils have been developed to good pasture land they are probably
better suited to forestry. The Oturoa soils are more versatile (only slightly susceptible
to drought) and have a horticultural potential (berry fruit and pip fruit orchards)
in addition to their use for dairy and beef cattle.
Pumice Lands
45
Plate 2.13
Lakes Waikareiti (foreground) and
Waikaremoana nestle among the
tephra-smoothed uplands on the
eastern side of the Huiarau Range
within Urewera National Park. These
Ruakituri and Matawai soils (Fig. 2 .5)
are podzolised pumice soils developed
on Kaharoa Ash and Taupo Pumice
overlying Waimihia Lapilli. The strata of
the underlying Miocene-age sediments
can be seen in the cliffs of Panekiri
Bluff above Lake Waikaremoana in the
middle distance . In this cool, moist
environment around Lake Waikareiti,
the forest is a mixture of red and silver
beech, the altitudinal limit of rimu
forest being reached somewhat lower
around the shores of Lake
Waikaremoana at gOO-m altitude
Plate 2.14
View south along the escarpment of
the Horohoro rhyolite dome towards
Ngakuru and the hills of the upper
Waikato River around lake Ohakuri
The Horohoro rhyolite dome marks the
apex of the ignimbrite flows that make
up the Mamaku Plateau to the north.
Taupo and Oruanui soils (from Taupo
Pumice) are found under the pasture at
the foot of the escarpment while the
summit plateau is a complex of
Mamaku soi ls (podzolised volcanic
loams)
Plate 2 . 15
The striking caldera of lake Rotorua,
looking east across the northern end
towards Lake Rotoiti. The formation of
the Rotorua caldera was probably
associated with the eruption of the
Mamaku Ignimbrite around 170 000
years ago. Below the prominent
esca rpment, Dturoa soils have
developed on old lake terraces and
slopes that were covered by a larger
lake which was once 100 m higher
than the present Lake Rotorua. Beyond
the escarpment, sandy Dropi soils have
developed on the rolling tephra-covered
land which can be seen sloping down
towards the Bay of Plenty (Chapter 3)
46
The LivinS Mantle
Plate 2 .1 6
The summit of the Mamaku Plateau has
been cleared of large areas of
indigenous podocarp forest and
established in pasture. A striking
feature of the landscape is the large
number of ignimbrite tors and coni ca l
mounds . These are the eroded
remnants of a jointed ignimbrite surface
standing on a more resistant welded
ignimbrite base. The landscape is
mantled with Mamaku, Rotoma and
Rotorua tephras, and the predominant
soils are the podzolised Mamaku soils
which developed under the indigenous
~~e~~a~~~ni; th is cooler, moist
To the west of the Rotorua caldera much of this rhyolitic pumice has been
eroded away, allowing the underlying o lder Mamaku, Rotoma, Waiohau and
Rotorua tephras to dominate the soil profile (Fig. 2.4). The climb of 250 m in
altitude along State Highway 5 from the shoreli ne of Lake Rotorua at Ngongotaha
to the broad summit of the Mamaku Plateau traverses a zone of Significant soil
development where these older tephras make up most of the soil parent material.
The annual rainfall increases from 1600 mm to 2400 mm and the soils (Ngakuru,
Ngongotaha, Waiteti and Mamaku) correspondingly exhibit increasing podzolisation
as the effect of leaching intensifies with the transition into a high rainfall, lower
temperature, and mor-forming podocarp forest environment. Iron has accumulated
in the B horizon of the Ngongotaha and Waiteti soils while the strongly podzolised
Mamaku soils have the typical bleached E horizon of podzols and have some
similarities to the Tihoi soils (Plate 2.5). These soils are volcanic loarns which have
deep brown, friable subsoils and a characteristic greasy feel because of their high
content of allophanic clays, typical products of the weathering of these older rhyolitic
tephras in this wetter environment over the past 7000 -13 000 years.
The broad summit of much of the Mamaku Plateau is dotted with small, conical
tors of more resistant ignimbrite (Plate 2.16). Although the higher altitude and
cooler climate of these Marnaku soils means a shorter growing season, they still
have a potential carrying capacity of I 5 stock unitslha. Nevertheless, any further
forest clearance on the Mamaku Plateau is a matter of considerable public
controversy (Plate 2.17). This is because there is now greater recognition of the
importance for water and soil conservation and wildlife habitat of the large arc
of upland indigenous forest stretching from the Kaimai Range through the Mamaku
Plateau to its southern termination in the striking Horohoro Escarpment (plate 2.14).
Pumice Lands
In sharp contrast to these more developed soils in the west of the Rotorua
Basin, the soils to the east are generally much younger. Here the landscape still
bears the imprint of the only major eruption within the history of European
settlement - the Mt T arawera eruption of 10 June 1886. The ejecta from the
eruption was of two types:
D previously erupted rhyolitic material around the Rotomahana and Waimangu
craters. This was intensely altered by hydrothennal activity and violently ejected
to blanket a wide area on the eastern side of Lake Rotorua with up to 2 m
of a pale olive grey sandy deposit, the Rotomahana Mud (Figs. 2.2 and 2.4);
D a basaltic tephra, the Tarawera Ash and Lapilli. This erupted from the main
fissure on Mt Tarawera and swept in an arc to the north and east across
large part of the Bay of Plenty (Figs. 2.2 and 2.4).
The Rotomahana soils (Plate 2.18) cover around 13 000 ha of rolling and hilly
land between Lakes Rotorua and Tarawera. Because of the sandy and silty nature
of their parent material (Rotomahana Mud), the hmy topography and the destruction
of vegetation during the eruption, the Rotomahana soils have developed on a highly
erodible landscape (Plate 2.19). Cultivation has smoothed out many of these rills
and most of the Rotomahana soils have now been established in excellent pastures,
particularly for dairying (Plate 2.20). Because of their hydrothermal origin,
Rotomahana soils have a number of Significant differences in their soil chemistry
when compared with most pumice soils. Their high levels of available molybdenum
have been implicated in inducing copper deficiency (and consequent ill-thrift) in
dairy cows. The soils are also deficient in boron (for root crops) and manganese
(for celery and some fruit trees). The cobalt content of these soils is, however,
generally considered sufficient for grazing animals, unlike that of the pumice soils
associated with the Taupo and Kaharoa tephras.
In contrast to the Rotomahana soils, the Tarawera soils are very gravelly because
of their coarse unweathered basaltic scoria. They are consequently drought prone
and in summer the black scoria absorbs heat, thus further decreaSing the moisture
status of the soil.
Pasture establishment has been difficult on the Tarawera soils and they are
better suited to deep.rooted crops Oucerne or tree plantations) or retained in their
indigenous vegetation of manuka or (where fire has been controlled) the mixed
kanuka/tawa forest of the hill slopes.
47
Plate 2.18
Profile of a Rotomahana soil near
Waimangu . A depth of 60 cm of grey
Rotomahana Mud overlies Taupo
Pumice with its characteristic dark
topsoil . In contrast, the present topsoil
is very shallow and there is little other
profile development . The coarser nature
of the Taupo Pumice subsoil is
apparent and the gravel band below
the buried topsoil is Taupo Lapilli
The Rotomahana Mud consist ed of
lake bottom sands and silts which had
already been partially weathered.
Consequently, these Rotomahana soils
have much higher clay contents (up to
20 - 30 percent) than other raw
volcanic soits. These clays consist
largely of the more crystalline forms,
such as mica, smectite and kaolin, as
well as the ubiquitous amorphous clay
of weathered volcanic soil s allophane.
Plate 2.17
On its western side where it slopes
down to the Waikato basin, the
Mamaku Plateau shows a striking
pattern of parallel gullies which have
been eroded in the underlying
ignimbrite. The soils (Ngaroma) on the
flat interfluves have developed in thin
Taupo Pumice over older rhyolitic and
andesitic tephras. Most of this
landscape has now been converted to
exotic forests after a long -standing
controversy centring on the high value
~:t~r~ f~~:~r~~~i;~~ous
forest for
48
The Living Mantle
Plate 2.19
View of the Waimangu regio n, looking
north-east to Mt Tarawera, soon after
the eruption of the Rotomahana Mud in
1886. Severe gullying has already
commenced in these soft,
unconsolidated lake-bottom sediments.
The photograph shows the new Lake
Rotomahana beginning to form; it has
risen 24' m since the eruption. Mt
Tarawera in the background was the
source of the rhyolitic Kaharoa Ash of
c.660 years ago and of the basaltic
Tarawera Ash and Lapilli showers
which immediately preceded the
hydrothermal explosions from Lake
Rotomahana. The Rotomahana Mud fell
over an area of about 13 000 ha and
buried the villages of Te Ariki, Moura
and T e Wairoa, where over 150
persons died.
Plate 2.20
The Rotomahana Mud landscape today,
in contrast to the immediate posteruption landscape in Plate 2.19 . The
rills in the soft material are still a
distinctive feature of the landscape,
where not obscured by trees.
Pumice Lands
49
Tongariro Volcanic Centre
At the southern extremity of the Taupo Volcanic Zone, the most dramatic volcanic
landfo rms in New Zealand dominate the landscape. Whereas the soils of the
TaupolRotorua area have mainly rhyolitic parent materials, the soils around the
Tongariro Volcanic Centre (Fig. 2.1) have developed on andesitic tephras (some
of it relatively young) in an upland and mountainous environment characterised
by cold temperatures and high rainfalls (except for a significant triangular rainshadow
area wedged between Mt Ruapehu and the arc of the Kaimanawa, Kaweka and
north-west Ruahine Ranges). The dominant soil-forming tephras (Fig. 2.4) are the
still-accumulating Ngauruhoe Ash (0 - 1800 years ago), Taupo Pumice (1800 years
ago) and the so-called 'Tongariro Ashes', which consist of a series of andesitic tephras
erupted from the Tongariro Volcanic Centre over the period 1800 -14000 years
ago.
In the immediate vicinity of the T ongariro-Ruapehu volcanic massifs, Ngauruhoe
and Waimarino soils are developed on Ngauruhoe Ashes over Taupo Pumice and
T ongariro Ashes. They are coarse-textured, highly erodible raw volcanic soils of
no commercial use because of their poor physical properties and the cold climate;
however, they support a most attractive subalpine flora protected within the
Tongariro National Park. These Ngauruhoe soils are best viewed where the Desert
Road traverses the eastern edge of the so-called Rangipo Desert (a misnomer since
these subalpine gravel fields have a relatively high annual rainfall of 1200 -1600 mm;
nevertheless, this is only half of the precipitation at this altitude on the western
slopes of Mt Ruapehu).
Because of the prevailing westerly winds, neither the Ngauruhoe Ashes nor
Taupo Pumice have accumulated to any great extent to the south-west of the
Ruapehu massif; here the Tongariro Ashes are the important soil parent materials
giving rise to deep, volcanic loarns such as the moderately leached Ohakune soils.
Despite the high altitude and cooler environment, the excellent physical properties
of the O hakune soils and the plentiful rainfall make them highly suited to market
gardening for vegetables such as carrots (Plate 2.21). The wetter, strongly leached
Pokaka soils at the foot of the ringplain on the western side of Ruapehu support
good pasture but are not as versatile as the Ohakune soils. The climate and mor~
broken topography dictate the retention of most of the Pokaka soils in indigenous
forest.
To the east of Ruapehu, the Ngauruhoe soils merge into Waiouru and Ngamatea
soils (Plate 2.9). Around Waiouru the landscape changes dramatically as the
smoother landforms give way to the sharp-edged plateaux and escarpments that
characterise the uplifted sedimentary rocks around the Rangitikei, Whangaehu
and Mangawhero valleys. Yet these sandstones, siltstones and limestones have little
effect on the soils, for the ash mantle is still thick enough to encompass the entire
soil profile. The result is one of the most amaz ing and least known landscapes
in the North Island - the only such area truly resembling parts of the South Island.
These are the red tussock uplands (altitude 1000 - 1200 m) stretching across the
vast open spaces of the upper Moawhango catchment (plate 2.22) to the Ngamatea
Plateau at the foot of the rugged Kaweka Range and the Mangaohane Plateau
wedged between the northern Ruahine Range and the canyon of the Rangitikei
River (Plate 2.23).
Most of the volcanic soils lying on these plateaux have the typical volcanic
loam attributes of depth, friable consistence, loamy texture and high moistureholding capacity, their only limitations being climatic in that the growing season
is much shorter. The most extensive soils of agricultural importance are the
Moawhango soils. These lie at the limit of the Ngauruhoe Ashes and have only
a thin covering of airfall Taupo Pumice over deep Tongariro Ashes sitting on
sedimentary rocks (Plate 2.24). However, thick depOSits of Taupo Pumice from
nuees ardentes which poured through the Kaimanawa Range and down the Taruarau
Valley cover much of the Ngamatea Plateau. This is the only Significant area of
pumice soils (Taruarau soils) in the southern part of the pumice lands (Plate 2.10).
Where developed to pasture the Taruarau soils need careful management to keep
a close vegetative cover in order to avoid frost-heave and subsequent erosion by
wind and water.
Plate 2.21
Old andesitic Tongariro Ashes mantle
the ringplains of the volcanoes of the
Tongariro Volcanic Centre but have
generally been covered by the younger,
coarser tephra from the Taupo eruption
and the recent eruptions of Mt
Ngauruhoe (Plate 2.1). The Ohakune
region in the south-west is in the
shadow of Mt Ruapehu and here the
T ongariro Ashes are still soil-forming.
The deep, friable Ohakune soils are
typica l volcanic loams (see Plate 2.24)
which are suitable for the production of
vegetables such as these carrots despite the relatively high altitude
1600 mi.
50
The [ivins Ma ntle
Pl ate 2.22
Raw volcanic soils
The volcan ic massif of Mt Ruapehu
(left) and the active cone of Mt
Ngau ruhoe (right) dominate the
landscape in the upper Moawhango
catch ment near Waiouru. Waiouru and
Moawhango soils (Plate 2.24) have
developed on the tephra-mantled
surface of uplifted, faulted and
dissected sed imenta ry rocks in the
foreground. Red tussock, with the
occasional patch of beech fore st which
ha s survived historic fires, covers this
su rvi ving remnant of the North Island's
indigenou s tussock grassland s. Most of
these tussock lands have been
converted to pasture (Plate 2.9). Those
in this landscape have a precarious
future as a milita ry train ing ground,
part of w hich is being inexorably
Raw volcan ic soils are a very heterogeneous group of soils, their common attribute
being their volcanic origins and extreme youthfulness. T hey are of o nly local
distribution around North Island volcanic centres that have been active in historic
times (generally the last 800 years), e.g.:
o the Ngauruh oe so ils around Mts Ruapehu and Ngauruhoe;
o th e Tarawera and Rotomahana soils of the Rotorua area;
o the Burrell soils of the upper eastern slopes of Mt Taranaki;
o the Rangitoto soils of Rangitoto Island in the Hauraki Gulf.
Their pare nt materials are generally andesitic tephras or basaltic lavas. They
exhibit no B horizon development and generally only a shallow A horizon. Textures
are sandy, clay contents very low, structures weak, bulk density is low and their
water-holding capacity is generally so low that they are droughty and support limited
floras. Many of these soils are protected within national parks or reserves.
The Rotomahana soils (Plate 2.(8) are an exceptio n, exhibi ting more
development because of the hydrothermal nature of the Rotomahana Mud.
At the southern limit of the pumice lands, the landscape becomes more angular
and dissected, characterised by the terraces and hill country of the mid-Rangitikei
River. The Tongariro Ashes are, however, still the dom inant soil-forming materials
on the flat terrace surfaces arou nd Taihape (Ohakune soils) although the landforms
are quite different (Chapter 6). Other major tephra showers, such as the Kawakawa
Formation and the Taupo Pumice, were thinly distributed much further south.
Most of these tephra deposits were subsequently eroded away and the underlying
sedimentary rocks, and loess derived from both sources, are the dominant soil
parent materials of the Rangitikei and Manawatu (see Chapter 6).
Our journey down from the volcanic pumice plateaux is not southwards,
howeve r. Rathe r, it begins high on the northern rim of the plateau, looking out
across th e fertile loamlands of the Waikato and Bay of Plenty.
~~~t~i:t~~ by the exotic tree,
Pinus
Pumice Lands
51
Plate 2.23
At the southernmost corner of the
Mangaohane Plateau, Tongariro Ashes
still dominate the soil profile. Here
Titapu soils occur in a higher, moister
environment than the Moawhango soils
(Plate 2.24) and support a vegetative
cover of red tussock and subalpine
herbs. A unique mountain cedar forest,
fringed with mountain toatoa, has
survived the historic fires which are
thought to have induced the red
tussock which dominates most of
these plateaux.
Plate 2.24
The Moawhango soils are widespread
in the tephra-covered uplands (below
1100 m) to the south-west of Mt
Ruapehu. They have a trace of Taupo
Pumice in the topsoil but T ongariro
Ashes dominate all A, Band C horizons
to a depth of nearly 2 m. Below the
sharp break at 2 m depth is the
underly ing, li ghter -coloured, weathered
sedimentary basement rock of Pliocene
age. In their properties the Moawhango
soils are similar to the volcanic toams
of the Waikato and Taranaki.
52
The Livina Mantle
Distinguishing features of pumice soils
and raw volcanic soils
PARENT MATERJ AL AN D DISTRJBUTION - rhyolitiC tephra, with at least top 50 em of profil e
developed in tephras which are betwee n 660 and 3 500 years old. Predominant soil of volcanic
plateau of ce ntral North Island; also scattered thro ughout Bay of Ple nty and Hawke's Bay
PROFILE C HARACTERI STICS - the thic kness and darkness of the A horizon is closely related
to the vege tative cover, e.g. deep and black under bracken fe rn, shallow and grey under manuka
scrub, brown unde r podocarplhardwood fore st; the B ho rizon becomes more reddish-brown
as the pumice soils become more leached (see Taupo-O ruanui-Tihoi soil leaching seque nce,
p. 4 1). Strongly leac hed and podzolised profiles may show E, Bh , and Bs horizon s.
TEXTURES - ge ne rally coarse, shOWing a gradation from gravelly sand near the e ruption source
to silty sand near the periphery of the ash showe r. Weakly weathe red; altho ugh weathe ring
rapidly in a mode rate weathering zone, their youthfulness a nd the he te rogeneou s locatio n
of the weathering produc ts (as a skin around the grain s o f pumice) give the soils an overall
weakly weathe red charac ter.
CLAY CONTENT LO W - gene rally
< 10%,
consisting mainly of allo ph ane.
STRU CTURE WEAKLY DEVE LOPED - crumb o r gra nular.
FRJABLE TO LOOSE CONSISTENCE - lack of weathe ring produc ts suc h as iron oxides whic h
te nd to give a soil co hesion. Conseque ntly, very su sceptible to e rosion if vegetative cover
incomple te.
BULK D ENSITY LO W - 0.6..0.8 Tim'
PLANT-AVAILABLE MO ISTURE HIGH - highly porous ye t readily avai lable moisture ra nges
fro m 22 - 30% of soil volume: owing to the highly vesicular nature of the pumice , sufficient
mo isture is retained for the plant root to exploit while the free-draining prope rties confe r
advantages of soil ae ration . Whe re pumice soils are deep these physical prope rties allow deeprooting plants like lucerne and Pinus radiata to tap large reserves of soil nutrie nts and moistu re.
SOIL AN IMAL POPULATIONS LOW - most soil anim als and micro-organisms are concentrated
in the topsoil ; the coarse textures and droughtiness of some of the pumice soils may be
respo nSible for the diffic ulty of establishing earthwo rms in some areas.
IMPO RTANT N UTRJ ENTS LAC KI NG - relative to othe r soil pare nt mate rials, the T aupo a nd
Kaha roa rhyolitiC te phras are low in the major ele me nts potassium , m agnesium, calcium ,
phosphorus and sulphur as well as some important trace elements such as copper, cobalt and
sele nium. Molybde num is generally high and has bee n implicated in copper deficien cy in cattle
grazing pasture on the Rotomahana soils.
USES OF PUMICE SOILS
Pumice soil s respond we ll to fertiliser additions of phospho ru s, sulphur, potassium. magneS ium
and trace elements: becau se of their low clay contents and weakly weathe red nature it is easier
to shift their nutrient balance by fe rtiliser manageme nt th an it is with more weathered soil s in
othe r pa rts of New Zealand .
Pumice Lands
Distinguishing features of volcanic loams
PARENT MATERIAL AND DISTRIBUTION - airfall tephras (both rhyolitiC and andesitic).
generally betwee n 3500 and 50000 yea rs old ; also from alluvium or loess containing a high
proportion of this tephric material. W idespread in Bay of Plenty. Waikato, King Country and
Taranaki.
PROFILE C HARACTERl STI CS - At Band C horizo ns.
o A horizo ns moderately deep (15 -25 em), black to brown in colour;
o B horizons quite deep (20 -60 em), commonly yellow-brown in colour but some red-brown
in higher rainfall areas.
TEXTURES - generally sa ndy loam or fin er (cf. coa rser textures of pumice SOils).
C LAY MINERALS - cl ay co ntents generally 10 - 25%. through weathering o f volcanic glasses.
Allophane dominates the clay fraction and its conte nt increases (and the halloysite content
decreases) with inc reasing rainfall and leaching of the soil (see Table 3. 1). The content of silicon
in so il solution appea rs to be quite important in govern ing whether alloph ane or halloysite
is the predominant clay form. When wet the soils fee l slippery rather than sticky. a nd there
is generally no evide nce of clay moveme nt down the so il profile.
STRUCTURES - are strongly deve loped nut in th e A horizon, but B horizo ns have weaker block
struc ture (breaking to fin e crumb or gra nul ar). They are particul a rly res istant to puddling
by the passage of farm ve hicles or animals.
VERY FRJABLE CO SISTENCE - of both topsoil and subsoil horizons.
BULK DENSITY LOW - generally less than 0.85 T/ml.
MOISTURE RETE NTION HIGH - although mu ch of the water held is not read ily available to
plants. The readily-ava ilabl e water retained by th ese soils (15 - 25% of so il volume), although
reasonable, is not as high as that retained by the pumice soil s. Good drainage is e nsured by
the highly porous Band C horizons which contain an exceptionally high conte nt of large pores.
ORGANIC MATIER - stable mineral/o rganic complexes are a feature, particularly of topsoils.
The carbo n content is high (7 - 14%) and their relatively high carbon to nitrogen ratios indicate
the res istance of this organic matter to bio logical breakdown
PHOSPH ATE RETENTION VERY HIGH - particu larly in the subsoil, indicating the effect of
topsoil organic matte r in decreasing the ability of amorphous clays to retain phosphate. The
large capacity of the volcanic loams to adsorb phosphate is one of their most important
c haracteristics and an understanding of the so rptio n be haviour of allophane in the soils is
critical for predi cti ng fe rti lise r (superphosphate) mainte nance requireme nts.
NO SIGN IFICANT TRACE ELEMENT DEFI C IENC IES - although cobalt is marginal in strongly
leached volcanic loams such as Mai roa and Ma maku SO il s; low reserves of magneSium and
potass ium; sulphur deficiencies un likely; responsive to liming where so il pH < 5.9.
HIGH POPULATIONS OF SOIL ORGANISMS - particularly in A horizon; earthworm populations
are similar to those in soils from sedi mentary parent rocks. Microbial biomass is generally
high, grass grub is a problem in drought-pron e pastures, and nematodes are a problem in
both pastoral farmin g and horticulture.
USES OF VOLCANIC LOAMS
Volcanic loams are ge ne rally of high value for food prod uction because they are deep a nd have
excelle nt phYSical properties (free-drainage, good structure, high plant-available moisture retention).
T hey require regular mainte na nce dreSSings of phosphatic fertilisers but, nevert heless, they are
among our most ve rsat ile so ils for pasture growth (16 - 18 tonnes dry matte rlh a in Wa ikato and
Taranaki), cropping (maize in Waikato) and hortic ulture (such as kiwi fruit, berry fr uit, pip and
sto ne fruit, citrus, asparagus, feijoas a nd tamarillos, espeCially in the Bay of Plenty).
53