Download Chapter 8 - New Zealand Society of Soil Sciences

CHAPTER EIGHT
Rugged Axis
T ~;tir~t~~~n~o;h~r:~~~. ~~i~~::~:~~P~e;~g~oe~C~:b;~I:t:v:l~u;ea~1 I~:~s::~
(Plate 8.1) - hill country with steep slopes, c1iffed coastlines, and very limited
areas of flat land (except for the Waimea and Motueka floodpl ains within the
Moutere Depression of central Nelson). As the region lies along the western margin
of the boundary between the Pacific and Indian-Australian Plates, it is intensely
folded and faulted along the same north-eastlsouth-west trend as Hawke's Bay and
the Wairarapa (Chapter 7). The Tararua, Rimutaka and Richmond Ranges form
the eastern backbone of the region. As these ranges have been uplifted the country
to the west has been tilted downwards leaving a series of basins (Hutt Valley, PoriruaPauatahanui inlet, Moutere depression). The landscape is therefore largely made
up of a series of tilted blocks with parallel fault lines acting as hinge lines.
The region has a number of other unifying features. It lacks the soft Tertiary
sedimentary rocks so characteristic of the eastern North Island and Marlborough
lowlands (Chapter 7). Instead the hill country consists of much older, harder rocks
- Mesozoic greywacke and argillite in Wellington, and complex bands of schist,
sandstone and ultramafic rocks of Mesozoic and late Paleozoic age in the
Marlborough Sounds and east Nelson. Much of this hill country is comparatively
stable, with erosion mainly confined to areas associated with rapid uplift, as in
the Rimutaka Range (Plate 8.2).
Climatically the region is equable, at least in terms of rainfall (1200 - I 600 mm
even ly distributed throughout the year) and temperatures (warm summers, mild
winters). Winds, up to gale force from the north-west and west, however, frequently
batter Wellington and the Marlborough Sounds. In contrast, Nelson is sunny and
relatively sheltered. Before European settlement almost all of the region was densely
forested, with podocarplhardwood forest at lower altitudes and beech forest on
the ranges and hills; unlike the soil landscape regions discussed so far, the
Wellington-Nelson region is still dominated by forest and shrubland landscapes.
Soil Landscapes of the Wellington Area
The remnants of an ancient 'peneplain' can be recognised in much of the Wellington
area (Plate 8.3). The former greywacke mountains were worn down by weathering
and erosion over tens of millions of years; today this peneplain is deeply dissected,
with the interfluves and hollows covered by loess or colluvium and many of the
former valleys infiIIed with debris from earlier erosion periods to form fossil gullies.
The soils in these gently rolling hollows (Judgeford SOils) and smooth ridge crests
(Belmont soUs, Plate 8.4) are friable with yellow-brown subsoils and very welldeveloped nut aggregates. On the steeper gully slopes most of this silty loess or
colluvium has been eroded away and the soils (Korokoro and Makara) are much
shallower over weathered greywacke bedrock (Plate 8.5). Belmont and Judgeford
soils are typical brown earths (seep.l3 7), a large and important group of soils found
on parent materials derived from sedimentary rocks under a climate which is
sufficiently moist for the soil not to dry out during any month of the year.
127
Plate 8.' (opposite)
Looking north-east down the Pelorus
River in the heavily forested northern
Richmond Range towards the ridge
crests of the Marlborough Sounds on
the skyline. Th is rugged landscape of
east Nelson is typical of the hilly nature
of the Wellington-Nelson soil landscape
region.
128
The [ivins Mantle
Plate 8.2
View south-west down the
Orongorongo River and the crest of the
Rimutaka Range to Cook Strait and the
Seaward Kaikoura Mountains of the
South Island. The Rimutaka Range at
the southern end of the North Island
axial ranges is very young. It has been
uplifted only within the last 1 - 2 million
years and is currently rising at the
relatively rapid rate of 4 mm a year
Consequently most of the soil on the
upper slopes IRimutaka steepland soils)
has been stripped away, leading to
aggradation of the river bed.
Plate 8.3
Looking north -east from above
Paremata across the ancient peneplain
of the Wellington uplands which slope
up to the summit of the Tararua
Ranges 11500 mI. The grain of the
landscape is quite apparent with the
ridges and valleys aligned parallel to a
series of north -south splinter faults
associated with the main Ohariu and
Wellington Faults
On gentler slopes, Judgeford and
Belmont soils (Plate 8.4) have
developed in the silty loess and
colluvium; on the gully sides this drift
material has been largely eroded away
and the shallower Kotokoro and Makara
soils (Plate 8.5) occur over greywacke
bedrock. With increasing altitude
towards the distant Akatarawa and
Tararua Ranges, the rainfall increases
sharply and the soils (still under
indigenous forest) are strongly leached
fRuahine and Rimutaka steepland soils,
Plate 8.6) and podzolised (Renata
soils) . These catchments ate important
to Wellington for both high quality
water supply and outdoor recreation.
Plate 8.4
Plate 8.5
Belmont soils are typical brown earths
with their friable brown topsoils, firm
yellow-brown subsoils and well developed fine nut structure. They
have developed in the greywacke loess
and soli f luction debris (plus minor
amounts of airfall tephra) which covers
the rounded hill-tops of Wellington; the
associated Judgeford soils occur on the
gently rolling slopes. Both soils are well
drained and will maintain high quality
pastures if topdressed with phosphate,
lime and molybdenum
Korokoro hill soils and Makara
steep land soils occur on t he slopes
where only a thin (or non -existent)
layer of greywacke drift material still
remains over weathered greywacke
They are related to deeper Judgeford
and Belmont soils and they share the
~:~s~:~~:~~ti:~o~~o~~Od
drainage and
130
The Livina Mantle
Plate 8.6
Ruahine steepland soils extend along
the flanks of the Ruahine, Tararua,
Akatarawa and Rimutaka Ranges at
altitudes of 300 - 600 m and under
rainfalls of 1500 - 3000 mm. They are
generally covered by
podocarp/hardwood forests (rimu, rata ,
kamahi, hinau) with some silver and
hard beech. Their topsoils are a da rk
reddish-brow n mar built up from the
decaying indigenous forest litter. The
underlying fractured greywacke is only
weakly to moderately wea th ered,
indicating the instability of the forested
slopes. They are very leached acidic
soils which can be developed for exotic
forestry with care but are mostly
retained in indigenous forest for wate r
supp ly an d outdoo r recreation.
On some of the hill country around the Pauatahanui arm of the Porirua Harbou r,
and east of Wellington Harbour and the Hutt Valley, the soils undergo a marked
change. Whereas Belmo nt/Ju dgeford soils are friable silt loams (with o nly 20 - 30
percent of clay), the Paremata and Taita soils (Fig. 1.1) of th e eastern hills are
deep, compact and clay-textured (40 - 70 percent clay). They have poor phys ical
properties ~ow plant-available wate r, poo r drainage, high bulk density) and very
low fertility (high acidity, low phosp hate and molybdenum).
Because initial fertiliser inpu ts were not sufficiently high to overcome the
nutrient deficiencies, pastoral development of these soils generally failed. Most
of th em reverted to gorse and manuka - the fire-scarred hills that formed the
eastern backdrop of the Hutt Valley for so many yea rs (Plate 1.3 and Fig. 1. 1).
With better fire control, second-gro wth indigenous forest is gradually emerging
through th e gorse and the soils have proven to be very suitable for exotic forestry
and residential development (Plates 1.3 and 14.12).
The dee ply weathered nature of the Taita and Paremata soils is probably the
result of the ir landscapes not being uplifted and eroded to the extent of the other
uplands in the Wellington area. Deep road, railway and quarry cuttings often expose
exceptional thicknesses of kaolin-rich red-weathered greywacke - indicating th e
ancient nature of this soil landscape. In their properties the soils show close affinities
with the brown clays of No rthl and (see Chapter 5).
Al though Taita hill soils are co nsidered to be relat ively stable, this landscape
was severely affected by the rainstorm of 20 Decembe r 19 76. In 10 hours ove r
200 mm of rain fell in a concentrated band across the Hutt Valley, causing over
900 landslides and widespread floo ding. The in teresting feature of this erosio n
was th e highly selective impact of the deluge upo n th e landscape; the fossil gullies
Rugged Axis
13 1
were the sites of the debris avalanches and mud flows, as the water scoured out
the sodden debris deposited in the gullies during the cold phases of the late
Quaternary Period. It made little djfference whether such soils were covered with
und isturbed indigenous forest, exotic forest, pasture or houses. Rainfall of such
duration and intensity occurs in the Wellington region only every 100 or so years,
so this process of re-excavation of fossil gullies is very infrequent. The landscape
that is evolving is therefore primarily the res ult of occasional devastating eve nts
rather than a gradual wearing away of the land surface.
No rth and east of the Taita hill-soil landscape. the land steps up to the steep.
forest-covered slopes of the Rimutaka and Tararua Ranges (Plates 8.2 and 8 .3).
As altitude and rainfall increase, the hill soils and steepland soils become browner
in colour and contain less clay. They are a complex of Ruahine steepland, Rimutaka
steepland. and Renata soils. all strongly leached (rainfall now 2000 - 4000 mm)
and with very acidic. humus-rich topsoils (Plate 8.6). Although Ruahine steepland
soils are well suited to exotic forestry, the Renata soils and Rimutaka steepland
soils at higher altitudes have been retained in their dense beechlkamahi forest,
such is their importance for water supply and outdoor rec reation.
Hill Country of the Marlborough Sounds and East Nelson
Pla t e B.7
Although Cook Strait is a very significant topographic and communication barrier,
Wellington and the Marlborough Sounds are nevertheless closely related in terms
of th eir climate, vegetation and soils. The Marlborough Sounds lie at the northern
end of the Richmond and Bryant Ranges. a block of rugged hill country (Plate 8. 1)
which lies between the Wairau Fault and the Moutere depressio n and has bee n
tilted north-eas t_ Consequently the sea flooded the valleys at the northern end
of these ranges giving rise to the well·known 'drowned valley' landscape of the
Sounds (Plate 8_ 7).
The pare nt rocks of the Marlborough Sounds and east Nelson hill country
are a complex mixture of bands of greywacke, schist and ultramafic rocks. The
degree of weathering of these rocks varies with altitude . Below 200 m the soil
parent materials are strongly weathered and are probably remnants of an older
landscape which was deeply weathered during the warmer periods between the
Look ing south-west across Tawero
Point and Pelorus Sound to the hill
country inland of the Marlborough
Sounds (see Plate 8.1 for view in
opposite d irection). These old
greywacke rocks have been strong ly
wea thered at altitudes below about
200 m and have formed clay-ri ch soils
(e .g. Opouri soils, Pl ate B.8)'
Indi genous forest ha s been removed
from much of this steepland landscape
but pastoral esta bli shm ent met with
mixed success; the t end ency for
reversion to scrub is evident.
Consequently large areas of the Sounds
are now being esta bli shed in exotic
forest plantations
132
The Liv;nB MontIe
glacial stages of th e late Q uaternary Period. Above 200 m, the soil parent materials
are weakly weathered deposits of angular gravels (including some fossil screes over
10m thick). T hese deposits we re probably fo rmed by periglacial weathering of
exposed rock during the colde r glacial stages of the late Q uatern ary Period.
Rainfall increases markedly with both an increase in altitude and the transition
from the 'outer so unds' (1 200 mm) to the 'inner sounds' (2000 mm). With the
increase in rainfall the soils become mo re leached of nutrients, and acidity increases;
the processes of podzolisatio n and gleying become increasingly impo rtant and are
reflected in soil properties. Some of the forest soils which have developed under
an annu al rainfall of more than 2000 mm have surface ho ri zo ns of raw forest
litter ove r greyish o r pale-colo ured eluvial hori zo ns characteristic of podzo lised
soils. The O po uri soils are typical examples; they contain 40 -60 percent clay (from
strongly weathered greywacke), are very low in nutrients, and have appreciable
amounts of amo rphous mate rial (allophane and mineral-organ ic comp lexes) in their
subsoils (Plate 8 .8). No t all of the forest soils in this high·leaching environment
have pale-colo ured eluvial horizo ns, but most have subsoils wi th relatively high
levels of amo rpho us mate ri als and organiC matte r (usually 3 - 7 percent carbon).
T hese are probably pod zolisation featu res. whereby topSOil o rganiC matter is being
mobilised and leached down the profile.
The present land-use pattern of the Marlbo rough Soun ds is a mixture of:
o
o
o
o
Plate 8.8
The Opouri soi ls of the wetter, forested
hill country of the Ma rlborough Sou nds
are hill and steepland soils w hich are
very strong ly weathered and leached.
This profile, under beech/podoca rp
f orest nea r sea level in Tennyson Inlet,
shows evidence o f podzolisation .
Forest litter (5 cm) overlies a thin
(0 - 7 cm ), pale-grey si lt loa m textured
E hori zon, ove r 80 cm of yell ow Bw
and Bh horizons wh ich are 50 - 60
pe rcent clay. Th is clay is mai nl y
kaolin it e and vermiculi te bu t t he
presence of min or am oun ts of
amorphous clay s gives the subsoil a
high capacity f o r t he ret enti on of
phos phat e.
In t heir mo rphology m ost Opouri soils
are very simila r to other brow n c lays,
such as t he Tai t a soi ls o f Wellin gt on
(Fig. 1. 1 b) and the W hanga ripo soils of
Northla nd (Plate 5 . 14).
co nservation of natu ral landscapes (both terrestrial and marine) fo r o utdoo r
recreation, natu re conservation, wate r and soil conservatio n and tourism. The
most im portant natu ral areas are protected as reserves withi n the Mar lborough
Sounds Ma ritime Park;
sheep and cattle (beef) fa rmin g;
exotic forestry;
marine farming.
O ver two-thirds of the ind ige no us fo rest cover of th e Sounds was cleared during
th e 1880 - 1920 period in an attempt to establish pasto ral fa rming. But most of
the fa rming failed through a combination of poo r road access, steep to pography.
insufficient supply of fres h water, and infertile, clayey soils. Because of inadequ ate
fe rtiliser programmes, many of the pastures reverted to shrubland, 5000 ha of
w hich have bee n established in exotic fo rest plan tations since 1920 . T he po tential
of these soils fo r exotic affo restatio n is relatively high with estimates of up to
40 000 ha be ing sui table witho ut havi ng to impinge upo n any existing indige nous
forest landscapes. Yet these proposals fo r majo r afforestation within the Marlborough
Sounds need ca reful eval uatio n as they have implicati o ns for o the r existing uses.
The timbe r harvesting may require transport of the logs across the wate rs of th e
Sounds, thereby interfe ring with tourism and water-based recreatio n. Furthenn o re
the waters of the Sounds are now subject to mo re than 300 mussel-fanning licences
and th e success of mussel farming is very dependent upo n high wate r qu ality w hich could be jeopard ised through excessive sediment fro m forest harvesting
or contaminatio n fro m effluents associated with holiday dwellings o r large-scale
co mmercial enterprises.
Most of the hill country of east Nelso n (the Bryant and Richmo nd Ranges)
still carries heavy indige no us forest and a large proportio n of it is conse rved withi n
Richmo nd State Forest Park (Plate 8.1). The eastern slopes of the Bryant Range
are important fo r water supply to Nelson city and the Waimea Plains. Much of
this land reverted to bracken fer n and gorse after the infe rtile. shallow soils (Lee.
Whangamoa and Pelo rus steepland soils) on th e greywacke and sandsto ne slopes
were cleared of their indigenous forest. Large areas have subseque ntly been
established in exotic fo rest.
An in teresting geological feature of the east Nelson hill country is a band of
ultramafic rock (the 'Nelson mi ne ral belt) which exte nds fro m O'Urville Islan d
to Tophouse near Lake Rotoiti. Ultramafic rocks (such as peridotite and serpentinite)
have an unusual compositio n in th at they are even 'mo re basic' th an basal t; their
silica co ntent is relatively low and they contain higher conte nts of iron and
magneSium-rich minerals. The term mineral belt refers to the association of metallic
o res (usually of chrom ium, nickel and cobalt) with ultramafic rocks. The bestknown ultram afic landmark is Dun Mountain (so named because of the du ll red
colour of th e weathered rocks) behind Nelson city, where chro rnite o re was once
Rugged Axis
133
quarried. Another very striking landscape in this mineral belt is the once-glaciated
headwaters of the south branch of the Motueka River near the Red Hills at the
southern end of the Ri chmond Range (Plate 8.9).
The fertility of the Dun soils is very low; they are slightly alkaline, phosphorus
and potassium are very low and magnesium is very high - probably high enough
to be toxic to most plants. Levels of trace elements are also quite unusual;
molybdenum is very low while cobalt is very high, an d high levels of nickel and
chromium have been suggested as the reason why only a very restricted range
of ind igenous herbs and shrubs can survive on these soils. As an association of
rocks, soils and plants they are a scientific curiosity and will continue to excite
the interest of naturalists and mineral prospectors, but they are o f no productive
use to the farmer or forester.
The Mou tere Depression - Forestry and Horticulture
The Nelson district. with its sunshine and calm weather and long-established rural
landscapes and crafts, is a climatic and cultural contrast to the rugged hill country
of Wellington and the Marlborough Sounds. Nelson city stands at the northern
end of the 25 km-wide Moutere depreSS ion which slopes down from its southern
apex near Nelson Lakes National Park. Like the Marlborough Sounds, the depreSSion
has been til ted and flooded at its northern end to form shallow Tasman Bay.
Most of the Mourere depreSSion is filled with deeply weathered, clay-bound
gravels (Moutere Gravels) which were deposited as a sheet in th e late Tertiary/early
Quaternary and remained largely unaffected by subsequent glacial advances. These
Moutere Gravels cover around 100000 ha and exhibit a very characteristic pattern
of fin e-textured dissection with regu larly spaced spurs and drainage channels that
confer a 'herringbone' pattern on the landscape (Plate 8.10). Their soils are brown
earths (with stony subsoils) that d iffer in the ir extent of weathering and leaching.
In th e higher and wetter areas (500 - 600 m altitude; 1400 mm annual rainfall)
th e soils are moderately podzolised under the prevailing hard beech forest (Hope
soils). At lower altitudes, the sto ny, low-fertility soils (Spooner hill and Korere
hill soils, Plate 8. I I) were difficult to establish in pasture but have subsequently
produced high-quality exotic forests, largely Pinus radiata and Douglas fir with
mino r areas of larch and eucalyptus species. Golden Dow ns, at 40 000 ha, is the
second largest state exotic forest in New Zealand and is established almost entirely
on soils developed in the Mo utere Gravels (Plate 8. 10).
Pla te 8.9
Looking north into the glacier-smoothed
headwaters of the south branch of the
Motueka River in the Red Hill s at the
southern end of the Richmond Range.
Thi s largely treeless landscape is due
to the influence of the ultramafic rocks
of the Nelso n mineral belt; elsewhere in
this locality the beech forest treeline is
around 1300 m altitude - the height
of the tarn -covered plateau in the
foreground . The sparse vegetation
consists of a limited number of species
of herbs, grasses and scattered shrubs
which are able to tolerate the very high
leve ls of magnesium and toxic trace
elements in these shallow Dun soils.
Plate 8.10
The Moutere Gravels landscape,
looking north-east across the Motupiko
River to the exotic forests of Golden
Downs. In places the Moutere Gravels
are up to 300 m thick and the regular
height of the ridge crests indicates
their origin as a sheet of ancient
gravels which were subsequently
eroded to give the cha racteristic
'herringbone' pattern of ridges and
gullies.
Closer to the coast, between Upper Moutere and Mapua, the gravels are more
strongly weathered than in the Spooner soil landscape, and the topography is more
subdued (Plate 8.12). Interestingly, despite the lower annual rainfall (1000 mm),
the Mapua soils in this landscape are more leached and have a deeper, more clayrich subsoil than the Spooner hill soils. This deep weathering is considered to
be another example of a landscape Qike the eastern Hutt Valley and parts of the
Marlborough Sounds) that has survived the rejuvenating effect of erosion during
the Quaternary Ice Ages, and the strongly weathered soils have many of the
properties of the brown clays so rypical of Northland (see Chapter 5).
The Mapua soils have traditionally been the basis of the apple and pear orchards
of Nelson, not because of their suitability for pip·fruit trees but largely because
ofland speculation and patterns ofland ownership at the beginning of this century.
Despite some major nutrient deficiencies, the Mapua soils do have the advantage
of high mOisture-holding capacity in their clayey subsoils and in droughty periods
fruit trees and other deep-rooted crops survive better than similar crops on other
soils in the district. For success, pastoral farming also requires the addition of
superp hosphate, lime, potash, and the trace elements molybdenum, copper, and
cobalt. With the benefit of modern scientific information, however, it is easy to
see that the Mapua soils are not ideal for a pip-fruit industry. Consequently most
of the recent horticultural expansion in the Nelson district has been on the alluvial
soils associated with the Waimea and Motueka Rivers which flank the eastern
and western sides of the Moutere Gravels.
On the Waimea and Motueka Plains, the three recent alluvial soils of most
horticultural importance are the Riwaka soils (4000 hal on the low river terraces
in the Motueka district, the Waimea soUs (3000 hal on the floodplain of the Waimea
Ruaned Axis
River below Brightwater (Plate 8.13). and the Motupiko soils (8000 hal of the
Motupiko. T admor and WaHti valleys. (For description of recent alluvial soils.
see Chapter 7). The Riwaka and Waimea soils are of moderate to high natural
fertility; the Motupiko soils, in alluvium mainly derived from the Moutere Gravels,
are of low fertility but have good loamy textures. The other alluvial soils of
horticultural importance are the Ranzau soils (Plate 8.14) which are stony terrace
soils of the intermediate river terraces (Plate 8.1 3 and Chapter 12).
Deep. friable Riwaka soils are particularly valued for horticulture. especially
the production of hops and tobacco, two specialist crops grown only in the Nelson
district. The area of both crops (hops 160 ha. tobacco 500 hal has declined in
recent years as berry fruit and kiwifruit have increased in popularity. The Nelson
d istrict now produces a quarter of New Zealand's berry fruit including half the
boysenberries and raspberries (400 hal; boysenberries grow on a wide range of
soils while raspberries are suited to the loamy Motupiko soils (provided boron
Plate 8.12 (below)
Plate 8.11 (right )
The Mapua soils occur on the deeply
weathered. rolling landscape at the
Tasman Bay end of the Moutere
Gravels. Over 1200 ha of these soils
are devoted to pip fruit orchards and
for many years the industry was
plagued with soil -related problems; like
the other soils on the gravels they are
strongly acidic and very infertile.
lacking both magnesium and boron.
important nutrients for pip fruit.
Research showed that the large 'corky'
pits and inferior quality of the apples
could be rectified with applications of
borax and dolomite. Mapua soils also
suffered a considerable amount of
sheet erosion through cultivation of
their weakly structured topsoils.
Spooner hill soils are the most
extensive soils on the Moutere Gravels.
occurring on 30 000 ha of the steeper
slopes in this landscape (Plate 8 . 10).
They are moderately acidic soils with
low fertility - low levels of
phosphorus. potassium and calcium
and the trace elements molybdenum.
selenium . copper and cobalt. Because
of their steepness and infertility they
have not been developed in pasture;
instead. they have proved very suitable
for exotic forestry. Although weathered
stones and gravels tightly packed
within a matrix of sand and clay occur
throughout the profile. the tree roots
seem to be able to penetrate to
sufficient depth.
135
136
The Livina Mantle
Plate 8.13
Looking west across the floodplain and
terraces of the Wairoa River near
Brightwater. A band of fertile Waimea
soils flanks the river with stony Ranzau
soils (Plate 8.14) occupying the
intermediate river terraces in the
foreground and middle distance.
Beyond the floodplain is a wedge of
rolling pastureland on the strongly
weathered Mapua soils. The exotic
forest-covered hill country in the
distance is the main sheet of Moutere
Gravels with moderately weathered
Spooner soils (Plate 8.11)
and magnesium deficiencies are rectified). Although Nelson has very high sunshine
hours, winter temperatures are too low for most citrus except on the eastern
foothills of the Waimea Plains; only those subtropicals which can withstand winter
frosts (e.g. feijoas and kiwifruit) thrive in the district. The area of kiwifruit has
grown rapidly from 8 ha in 1975 to around 1000 ha. most of it on the alluvial
soils of the depression. However, apples still dominate the wide range of horticultural
crops grown in the Nelson district; pip fruit cover 2000 ha of the 5000 ha devoted
to horticulture, making Nelson almost as important as Hawke's Bay for pip fruit
production for expo rt.
RUBBed Axis
137
Distinguishing features of brown earths
PARENT MATERlAlS AND LOCATION ~ Brown earths are formed from silica-rich. non-caJcareous
parent rocks which are rypically of sedimentary (grc)"vacke, argillite, schist, sandstone, siltstone)
or igneous (granite, gneiss) o rigin. They are formed in such rocks weathered in situ. or in colluvium,
alluvium, loess or moraine derived from (and often mixed with) these rocks.
Brown earths (and associated upland and hill soils) cover around 3 500 000 ha. mainly in east
Taranaki-Wanganui-Rangitikei, the east coast of the North Island, Wellington, Marlborough
Sou nds, Nelso n-Bulle r. South land and the SoUlh Island high count ry.
C LIMATE - Brown earths have developed under a wide range of climatic condit ion s. the only
requirement being sufficie nt rainfall to maimain soil moisture between fi eld capacity and wilting
point for all months of the year. Most (2 500000 hal occur at lower altitudes - below 500 m
in the so uthern North Island and 300 m in the South Island. This subgroup is often referred
to as IOlllland brollln eOrlhs (Fig. 1.6). These lowlands have a mOist, cool to mild temperate climate,
w ith mean annual temperatures of 11°C and annual rai nfalls of 1000 - 1800 mm (well disuibuted
throughout the year)
Upland brollln earths. the othe r subgroup. occur in I million ha of the South Island high country
(300 - \ 000 m altitude). These uplands have a cold to cool temperate climate, with mean annual
te mperatures of 6°C and ann ual rainfalls of 700 - I 500 rnm (although mist and dew also
contribute a lot of moisture at these altitudes). Almost all brown earths probably developed
under podocarplhardwood and beech fore st, mostly destroyed by fires during moa-humer times,
and the upland brown earths now occur main ly under tussock grassland. With increases in altitude
and rainfall these brown earths grade into podzols (see Chapter 5).
~m
PROFILE CHARACTERISTICS
o dark brown or greyish brm... n A horizon mergi ng into yellow-brown B ho rizon;
o considerable mixing of A and B horizons by earthwonns;
o no fragipan or iro npan in the subsoil
TEXTURES - dominantly silt loam, but occasionally sandy loam; clay content moderate (20 - 35%),
but lower in upland brown earths (15 -200/0); little difference in clay content between horizons.
STRU CTURES - usually well-developed nut in A horizon, tending to granu lar under grass; nut
and blocky in B horizon tending to prismatic in strongly leached soils; weake r structure in upland
brown earths.
FRJABLE TOPSO ILS - fri able to firm subsoils.
BULK DENSITY - med ium (0.9 - 1.1 TIm) in topsoils, higher (1.1 - 1.4 T Im) in subsoils; bulk
densities of upland brown earths are generally lower.
PLANT-AVAILABLE WATER CAPACITI - high in topSOils (23-300/0 of soil volu me); low in subsoils
( 10 - 14% of soil volume). Upl and brown earth s have a higher storage capacity ( 16-24% of so il
volume) in their subsoils.
fREE-DRA IN ING SOILS - with moderate macroporosity ( 10 - 14%).
C LA Y M IN ERALS - dominated by vermiculite, indicating a moderate degree of weathe ring. Many
up land brown earths contain appreciable quantities of amorphous iron and aluminium in their
subsoils; consequently. they have a moderate retemion of phosphate (30 -60%) in their topsoils,
but high retention (60 -90%) in the subsoils
SOIL CHEM ISTRY - varies w ith the parent rock. vegetation and degree of leaching; where leach ing
is high (e.g. most upland brown earths) nutrie nt co ntents are low and acidity is high. Organic
matte r levels are quite high (carbon contents of 6 - 8%) and CIN ratios are moderate (12 -1 5)
LARGE AND ACfIVE POPULATIONS OF SOIL ORGAN ISMS - partic ularly earthworms.
USES OF BROWN EARTHS
Brown earths are very su itable for both pastoral and forestry uses because of their properties of
good structure and free drainage.
The upland brown earths of the Canterbury and Otago tussocklands have traditionally been
used for extensive grazing. Many of these grasslands and soils have been serio usly depleted through
burning. overgrazing, and induced erosion; sustained use depends upon the mainte nance of a dense
vegetative cover of climatically suitable grasses and legumes through techniques like aerial oversowing
and topdressing and rotational grazinglspelling to allow recovery of the he rbage. Significant growth
rates of climatically suitable exot ic trees (such as Corsican pine and Douglas fir) can be obtained
on these soils
Brown earths are the basis of the intensive pastoral use of the Southland Plain s where stocking
rates of 20-24 un itslha can be maintained. They are also important forestry soils in WellingtonMarlborough Sounds. Nelson, north Westland and easte rn Otago.
The Ranzau soils cover about 3000 ha
of the intermediate terraces and gently
sloping fans on the eastern side of the
Waimea River between Stoke and
Brightwater. Although Ranzau soils are
stony and have low moistu re retention,
WIth spray irrigation they have proven
very successful for market gardening,
kiwifruit, necta rines and apples.
Th e profile ill ustrated is found on the
terraces of the Wairoa River in the
foreground of Plate 8.13. Stones and
gravels occu r throughout the profile,
and fertility is low to mod e rate.
Nitrogen , phosphorus and potassium
fe rtilisers are required fo r horticultural
crops and boron levels are marginal for
apples.