CHAPTER FIVE

Clay Hills and Kauri

THE 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.

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 oldest 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 leaching 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 The younger basaltic volcanoes. however. are the most recognisable. These Geological sketch map of Northland- basaltic eruptions began over a million years ago in the Whangarei and Bay of Auckland-Coromandel. 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 rough. 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 lan2 - 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. day-rich 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

(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.

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 l l soils which have developed in the deeply weathered banded sandstone

Plate 5.3

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 scoria 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, ferrihydrite ) 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 boulders which now remain as the characteristic basalt dry stone walls around paddocks. Because of their excellent 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 avai lable 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. Despite 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

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 on the andesitic breccias; and the Tutamoe and Waipoua soils of the upland andesitic lava flots 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 agricultural scientists were puzzled over the failure of these soils to respond to superphosphate, 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.

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, Whangarei, Kaikohe and Kerikeri districts. Although most of these soils are devoted to intensive pastoral farming, they are also highly suited to vegetable cropping and horticulture.

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 insignificant. Although generally stony, they are well-structured soils, with kaolin and gibbsite the predominant clay minerals. The Kiripaka soils have much less allophane than the closely related Papakauri and Ohaeawai soi ls, but they seem to have a higher iron oxide content, so their phosphate retention is sti ll very high. The brown colour (cf. the red Papakauri) is due to goethite (rather than 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 horticultural centres. The 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 standards, but the land is well utilised. Most orchards produce a mixture of citrus (oranges, grapefruit , mandarins, lemons and tangelos) and other subtropical fruits (tamarillos, kiwifruit, feijoas). The establishment of an irrigation scheme has rectified the acute shortage o f soil moisture suffered by these soi ls during dry summer periods

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 higher, 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.

The Okaihau soils represent the most strong ly leached and weathered group of friable volcanic clays. The most distinctive feature of these 'i ronstone soils' is the many nodules of iron and aluminium oxides in thei r subsoils; up to 25 percent of certain horizons can consist of iron oxides. Okaihau soi ls developed under kauri /podocarp forest but most have now been converted to exotic forest plantations or pastures. Because of their low natural fe rtility they have always requ ired high inputs of fertiliser; as a consequence some so ils are now being converted to horticulture since t he intensive use of fertiliser (and irrigation water) is thereby economically justified.

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 f irm, 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 Waimatenui soi l is the presence of many small soft black iron/manganese concretions in the Band C horizons.

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 (Waitakere, Mangonui and Rangiuru soils) are moderately to very strongly leached compact volcanic clays similar to the Waimatenui soils (Plate 5.11) of Northland.

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 draining and have a tendency to poach in wet winters. Nutrient levels are high, especially in the subsoil, and they are capable of supporting excellent dairy pastures with only 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 the more weathered end of the sequence of soils developed on sedimentary parent materials throughout New Zealand. Like their counterparts from volcanic rocks, brown 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 sandstones. The moderately to strongly leached brown clays (such as the Whangaripo soils (Plate 5.14) on banded sandstones) are muc h more extensive on the different sedimentary rocks.

Where podocarps were a Significant element of the forest composition, the brown clays are strongly leached and there is often red weathering of the parent sedimentary rock. One example related 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 among the most difficult, degraded soils in Northland. In the past they have been considered as podzolised soils but they lack the hallmark of true podzols - a bleached, silica-rich 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 difference 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.

Plate 5.14

The Whangaripo soils are moderately to strongly leached brown clays which have developed in strong ly weathered banded sandstone. Like the Puhoi soils. the subsoils have strongly developed (but coarser) nut structures. The nuts are tightly packed bu t are easily separated when disturbed. The subsoil colour. however. is more yellowish-brown refl ecting the presence of around 5 percent iron oxides. Kaolin is the co-dominant clay mineral w it h vermiculite.

Clay Hills and Kauri 9 1

Plate 5.15

The Waikare 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 hangarei. The subsoi l structure is distinctly prismatic. breaking to coarse blocks. The grey colours and distinct mottling indicate reducing conditions which. coupled with the low subsoil pH (less than 5). are favourable for the formation of smectite which makes up 60 - 70 percent of the clay. Kaolin does not form under these conditions

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 require-ments 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 agricultural development for dairying.

Kauri podzols The kauri podzols of Northland have excited 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 which 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 agricultural research findings. Although their nutrient levels 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 agricultural 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.

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 materials express the full podzol morphology 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 long sandspit of Aupouri Peninsula behind Ninety Mile Beach. They have a dark, cemented Bhs ho rizon 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 of 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. Whether 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, ironically, the one New Zealand soH that is sure to be illustrated in soil science tex tbooks throughout the world.

Clay Hills and Kouri 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 wide flats bordering the estuary have been drained and developed to dairy farms. Beyond , the Te Kopuru (Plate 5.18) and Tangitiki soils have developed in the old sands between the estuary and the Tasman Sea in the distance.

94 The Livins MamIe

Plate 5.17

The Wharekohe soils of the Northland gumlands are the classic example 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 largely of silt -sized grains of quartz remaining after most clays and nutrients 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 mudstone. The clay in the subsoil consists of equal proportions of smectite and kaol in (the proportion of halloysite in the latter increasing wi th depth).

Plate 5.18

The Te Kopuru soils of the old coastal sands of Northland express well the mature profile morphology of kauri podzols The black topsoil passes fairly abruptly into the white, massive E horizon. consist ing of 30 cm of quartz sand virtually 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 for 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 molybdenum. cobalt and, sometimes. selenium).

Distinguishing features of volcanic clays

WELL-DEVELOPED STRUCTURE -structure; subsoils blocky to

BULK 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 CAPACITY in the topsoils (20-25% of soil volume); lower capacity in subsoils (14 -1 6% of soil volume).

FREE-DRAINING TOPSOILS (macroporosity 12 -20%) but some subsoils slow drain ing (macroporosity < 5%).

CLAY M INERALS

HIGH PHOSPHATE RETENTION (60-90%) - largely due to high iron oxide contem.

NO SIGNIFICANT TRACE ELEMENT DEFICIE NCIES, although molybdenum can be marginal in strongly leached soil s.

USES OF VOLCANIC CLAYS

Clay Hills and Kauri 95

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 uckl and classified as yellow-brown earths (see Appendix) as well as a small number of clay-rich central yellow-brown earths like the Taita and Mapua soils (Chapter 8).

PROFILE C HARACT ERISTICS o somewhat variable; profiles deep, often wi th di ffuse boundaries between horizons; o to grey ropsoil s; subsoil s dominantly yellOWish-brown but often grey and

indicating gleying; o deeply weathered parent material, usually yellowish·brown

TEXTURES - clays, both topsoil and subsoil; day content high, 45 -8 5%, most horizons 55 - 75%.

CONSISTENCE - topsoils friab le to firm; subsoil s moderately to very firm. Sticky.

STRUCTURES - poorly developed nut aggregates in A horizons; B horizo ns blocky, increaSingly aggregated imo prisms with increase in leaching; dark brown cutans (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 CAPACITY - 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 MIN ERALS o kaolin and vermiculite are the main clays, with the kaolin proportion increas ing with

SOIL CHEMISTRY - 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%).

BIOLOGICAL ACTI VITY GENERALLY HIGH - despite poor phys ical properties; most soil o rganisms confined to topsoils which have large, active populations, probably because of high organic matte r contents and wanner temperatures; biological activity of gleyed soils is markedly less.

USES OF BROWN CLAYS

Most weakly leached brown clays on rolling country a re capable of supporting good pastu res with only moderate topdressings of lime and superphosphate. 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 molybdenum) to m aintain satisfactory pastures. Consequently, much of the steeper hill country has reverted ro shrubland. On the easier hill country semi·intens ive sheep farming can be maintained, but production is generally low. Most of these steeper lands are better suited to forestry.

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.

Clay Hills and Kauri 97

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