The Soils of Aotearoa New Zealand

New Zealand’s soils are diverse, reflecting the varied and dynamic landscape. Located on the boundary between the Pacific and Australian tectonic plates, in the southern mid-latitudes, and surrounded by oceans, New Zealand consequently has: active tectonism, high mountains, and generally fast rates of erosion; active volcanism; a varying but generally temperate, mainly moist climate; and unique flora and fauna. The soils of New Zealand were first cultivated about 750 years ago by Māori with more intensive agricultural development commencing around 1840 following the arrival of Europeans. About two-thirds of New Zealand soils have now been developed for food production or other human use including plantation forestry, with much of the remainder in native vegetation, often in protected areas. Soils have an essential role in the New Zealand economy, underpinning food, wool, and forestry production as well as providing a substrate for urban development, filtering of water, and recycling of nutrients and wastes. Without agricultural inputs New Zealand’s soils are generally of moderate or low fertility. The use of fertilisers has greatly increased soil productive capacity. The New Zealand Soil Classification identifies 15 Soil orders: Allophanic, Anthropic, Brown, Gley, Granular, Melanic, Organic, Oxidic, Pallic, Podzol, Pumice, Raw, Recent, Semiarid, and Ultic Soils.

Allophanic Soils cover 5% of New Zealand, predominantly in the Taranaki, Waikato, and western Bay of Plenty regions. Allophanic Soils are usually formed in deep accumulations of tephra (volcanic ash) and other volcanic debris, or occasionally on rapidly weathering rocks, usually with good drainage and relatively high rainfall. Thus they occur in the North Island around Mt Taranaki and in areas peripheral to the Pumice Soils where tephras from the central North Island volcanoes and Mt Taranaki have accumulated, typically millimetre by millimetre, over millennia. Most Allophanic Soils originated by developmental and/or retardant upbuilding pedogenesis. Many are ≥20,000 years old. Allophanic Soils provide an excellent medium for plant growth because of their friability, low bulk density, fine structure, high water-storage capacity, and ability to support deep-rooting plants. Allophanic Soils contain significant amounts of a globally uncommon clay mineral, allophane, which is formed mainly from the dissolution products of volcanic glass. Allophane, an Al-rich aluminosilicate, comprises tiny spherules with a huge surface area which strongly adsorbs anions, notably of phosphorus and sulphur. Allophane will adsorb a proportion of the phosphorus applied as fertiliser, making it unavailable for plant uptake, but can be used to advantage to adsorb phosphorus from irrigated effluent.

Anthropic Soils comprise soils constructed by people to achieve engineering, recreational, agricultural, land-rehabilitation, or waste-management goals. The original soil may be recontoured, stripped (leaving a truncated soil) and relocated, severely mixed, have inorganic or organic spoil or refuse added (≥30 cm in thickness), reformed into sports fields, or utilised in urban amenity landscaping, rain gardens, or roof gardens. The original character of the soil may be considerably altered, thus there is no typical Anthropic Soil. Some are formed as a result of careful planning and management for a particular purpose and so may be an ‘improvement’ on the original soil at the site. However, other Anthropic Soils are an inadvertent side effect of human activities and represent land that has been altered sufficiently for much, or all, of the original soil character to be lost. Relatively minor modifications (such as mixing an A-horizon by ploughing) do not normally qualify the soil as an Anthropic Soil. Four groups are recognised within Anthropic Soils: Truncated, Refuse, Mixed, and Fill, reflecting the mechanisms by which the soils were constructed.

Brown Soils are New Zealand’s most extensive soil order covering about 43% of the country. Brown Soils occur on the moister hill and mountain lands throughout New Zealand except where tephra deposits predominate. Brown Soils also occur on flat to rolling lands in wetter regions such as the Southland lowlands. Most Brown Soils occur in areas with mean annual rainfall greater than about 800 mm in the South Island, and 1000 mm in the North Island. Sandy Brown Soils and stony Brown Soils may form in areas with lower rainfalls in materials with low (<75 mm) water holding capacity. Brown Soils have grey-brown topsoils, brown to yellowish brown subsoils, and profiles with A–B–C horizonation. They form from a wide range of parent materials, predominantly siliceous quartzo-feldspathic rocks (e.g. greywacke, mudstone, schist, rhyolite) and their derivatives (e.g. sand dunes, colluvium, alluvium, and loess). Brown Soils are moderately weathered with moderate to strong leaching. The distinctive brown colours are due to thin coatings of iron and aluminium hydrous oxides, or aluminosilicates, potentially including allophane and ferrihydrite, on particle surfaces. The Brown Soils underlie much of New Zealand’s sheep and beef production along with dairying, cropping, plantation forestry, and extensive areas of public conservation land.

Gley Soils form where soils are saturated for long periods due to high or perched water tables, or slow drainage. Gley Soils occur throughout New Zealand but are more common in areas of high rainfall such as the West Coast of the South Island. Distinctive pale blue-grey colours in Gley Soils indicate a lack of oxygen and associated reducing conditions. When water saturates the soil, microbes use up the available oxygen leading to anaerobic (or reducing) conditions where the iron in rust-coloured iron oxides (containing Fe³⁺) is reduced to a colourless, soluble form (containing Fe²⁺). Manganese oxides are similarly reduced from black or brown compounds (containing Mn⁴⁺) to a pale, soluble form (containing Mn²⁺). The soluble, reduced cations may migrate to places where oxygen is available, such as along root channels or at profile texture changes. There they may be oxidised back to Fe³⁺ forming rust-coloured (or Mn⁴⁺ giving brown/black) mottles, concretions, or iron pans. Many Gley Soils occur on fertile, flat, flood plains, and terraces. Where drainage is installed they can be highly productive for pastoral and cropping land uses. Careful management is needed to ensure drains are maintained and to prevent pugging and compaction during wet periods.

Granular Soils form on old (≫c. 45,000 years), strongly weathered, tephra mainly on rolling hills and terraces in the northern and western Waikato and South Auckland areas, and on mafic volcanic rocks in Northland. Granular Soils develop on the older tephras where young tephra layers are thin or absent. The soils are dark reddish brown, moderately acidic, sticky, and clay rich. The clays dry to form strongly developed aggregates that give the soils their distinctive granular structure, and their name. Granular Soils comprise just 1% of New Zealand soils and are important for vegetable production. ‘Pukekohe onions’ are a well-known product of these soils along with potatoes, salad greens, and brassicas. Granular Soils around Pukekohe provide about one-third of New Zealand’s fresh vegetable production. Some Granular Soils have been used continuously for horticulture for over 70 years. Granular Soils are extraordinarily resilient to frequent ploughing, although erosion needs to be managed. They can have reduced workability after heavy rain and are naturally low in nutrients. The Granular Soils of Pukekohe and Bombay benefit from a frost-free climate, allowing vegetable production in late winter. There is concern that the productive Granular Soils in the Pukekohe/Bombay area are being lost to urban development.

Melanic Soils make up about 1% of New Zealand’s soils and are scattered throughout the country where soils are developed on calcium- or magnesium-rich rocks. Melanic Soils form on limestone, marble, calcareous sandstone, and mafic to ultramafic rocks. Melanic Soils occur on limestones that outcrop throughout New Zealand, but particularly along the east coast in Hawke’s Bay, Wairarapa, Marlborough, and Otago. They also form on ultramafic rocks in northwest Nelson. The most extensive areas of Melanic Soils are in coastal North Otago on limestone and mafic volcanic rocks (basalts and tuffaceous greywackes). Topsoils are black (giving the soil order its name), deep, and rich in soil organic matter, and smectite clays, that bind the soil together forming strong stable aggregates. The upper subsoil is often relatively thin grading into the underlying parent rocks. Melanic Soils are rich in calcium and/or magnesium which contribute to high natural fertility and near neutral to slightly alkaline soil pHs. The underlying rock forms a physical barrier, limiting root penetration, but water may drain through the cracks. The clayey materials of Melanic Soils are used for cricket pitches because of their strong shrink-swell character. Truffles can form a niche crop on some alkaline Melanic Soils.

Organic Soils form from partly decomposed remains of plants in peat-forming wetlands, or in deep accumulations of forest litter. Organic Soils comprise about 1% of New Zealand with the largest areas in the Waikato, on the high rainfall South Island West Coast, and in some moist mountainous areas where soils are often saturated. Organic Soils are acidic (pH ≤  ~4), have a low dry bulk density, high C:N ratios, high shrinkage potential, and exceptionally high soil water holding capacity. Organic Soils have been developed mainly for pasture, maize, potato, and blueberry production, and are also mined to extract peat. Organic Soils have naturally low nutrient contents so need inputs of fertiliser and lime, along with drainage, for productive farming. Such inputs hasten the biodegradation of Organic Soils. To slow the rate of degradation, water tables need to be kept high to prevent oxygen from entering the soil. Although about 70% of Organic Soils have been heavily impacted by drainage and development for farming, or by peat mining, some natural areas remain in reserves. The Whangamarino wetland and the rainfed Kopouatai and Moanatuatua bogs are recognised as wetlands of international importance with unique indigenous vegetation, including the tall cane rush (Sporodanthus ferrugineus).

Oxidic Soils occupy less than 1% of New Zealand, occurring in small areas throughout Northland, but concentrated near Kerikeri. New Zealand’s Oxidic Soils represent a southern limit of the globally extensive Oxisols which occur mainly in the tropics and subtropics. Oxidic Soils form in New Zealand where conditions favour the most intense weathering. Thus Oxidic Soils occur mainly on readily weatherable, iron-rich, basaltic to andesitic volcanic or mafic/ultramafic igneous rocks and scoriaceous deposits on sites that have been stable, potentially for millions of years, in the warm, humid environment of Northland. The parent materials of Oxidic Soils have been almost totally transformed to clay minerals, oxides, and oxyhydroxides, including gibbsite, goethite, haematite, and ferrihydrite. The soils have a friable, fine polyhedral, structure although some may include horizons comprising over 50% iron- or aluminium-rich nodules giving a gravelly texture. The soils are highly leached and low in nutrients, but with good fertiliser management are used to successfully grow citrus and other subtropical crops. The first grape vines in New Zealand were probably planted on Oxidic Soils by an associate of Samuel Marsden at Kerikeri in 1817, and the first British-style farm was established on Oxidic Soils inland from Kerikeri in 1830.

Pallic Soils are prominent on the rolling downlands of the lower North Island and eastern South Island. Pallic Soils, the third-most extensive soil order, covering 12% of New Zealand, occur in regions where there is normally a dry summer with a mean annual rainfall of about 500–1000 mm in the South Island and <~800 mm in the North Island. Pallic Soils are often formed in loess, a uniform silt-dominated material that is blown off river floodplains and deposited onto the surrounding landscape. A dense (soil dry bulk density ≥~1.5 t m⁻³) fragipan often forms in the subsoil which can obstruct root penetration and drainage. Thus Pallic Soils often suffer winter wetness, as water perches above the fragipan, and are also prone to summer drought. Subsoils are pale-coloured (“pallic”) because of their low contents of iron oxides and, in some cases, gleying. Pallic Soils are most suitable for pastoral grazing, maize, and other summer crops. Tree crops may be limited by winter saturation and rooting depth. Subsoil drainage can be effective in preventing winter or spring saturation. Some Pallic Soils form in alluvium or slope debris and are friable, without a strong fragipan, hence are deep rooting and versatile.

Podzol Soils are formed in association with podocarp and beech forests in areas with good drainage and rainfall ≥~1200 mm yr⁻¹, which provides an environment of acid leaching. Podzol Soils cover 13% of New Zealand and are most widespread on the South Island West Coast, the high country of the North Island, and in Northland. Distinctive Perch-gley Podzol Soils occur under high rainfall on the South Island West Coast. The Podzol’s most distinctive feature is a pale grey, or white, E horizon beneath the topsoil. The E horizon is pale because iron, aluminium, and silicon-bearing materials, and organic compounds, have been moved downwards and redeposited beneath the E horizon as dark-coloured podzolic-B horizons. The podzolic-B horizons usually comprise an organic-rich (Bh) horizon over a reddish iron-mineral-rich (Bs) horizon. Podzol Soils are defined by the podzolic-B horizon or a humus- or ortstein-pan. Podzols are extremely acidic with pH often as low as 4. The acidity, and E-Bh-Bs horizonation, are caused by acidified rainwater seeping downwards, mobilising and leaching or translocating organic materials, iron, aluminium, and silicon compounds, and other nutrients down the profile. Enlerached materials subsequently accumulate in lower horizons where increased pH, and microbial activity, support adsorption, and precipitation.

Pumice Soils cover about 16% of North Island, formed mainly in unconsolidated rhyolitic (silica-rich) pumice deposited by the Taupo eruption (AD 232 ± 10) from the huge caldera now occupied by Lake Taupo, and the Kaharoa eruption (AD 1314 ± 12) from Mt Tarawera. Other areas of Pumice Soils are formed in 3500-year-old Waimihia pumice deposits and on fluvially reworked pumice alongside rivers. Pumice Soils comprise sandy to gravelly pumice materials. They are weakly weathered with a low clay content and have A, and weakly developed Bw, horizons within pumice deposits that are at least 25 cm thick. Pumice Soils have low reserves of major nutrients and are likely to be deficient in trace elements including cobalt, copper, molybdenum, boron, iodine, and selenium. For agricultural development, it is necessary to add fertiliser and trace elements, particularly cobalt as its deficiency causes ‘bush sickness’ (vitamin B₁₂ deficiency) which leads to death of sheep and cattle. Pinus radiata trees have been successfully grown, supporting a major plantation forestry industry. However, in recent decades, many plantations have been converted to dairying. Pumice Soils are readily eroded by water, and if not protected by vegetation, severe gully erosion can form in a single rainstorm.

Raw Soils have minimal evidence of soil development, usually because of a short time since the parent materials were deposited or exposed at the land surface. The Raw Soils are associated with some of New Zealand’s most spectacular scenic landscapes. Raw Soils are scattered throughout New Zealand, particularly in association with high mountains (eroding alpine rock areas and active screes), braided rivers, beaches and tidal estuaries, non-stabilised sand dunes, recently emplaced lavas or tephras, and active geothermal areas. They cover about 3% of New Zealand (about 700,000 ha). Raw Soils lack a distinct topsoil or the topsoil is very thin (<5 cm) and they have no B horizon . The soil profile properties, therefore, correspond largely with those of the parent materials.

Recent Soils cover about 6% of New Zealand and are developed where material has been deposited, or eroded, and soil development has recommenced on new parent materials. Recent Soils occur where repeated deposits are thin enough, or there has been enough time, for a topsoil ≥5 cm thick to form. They may also have a weakly developed B horizon. Many Recent Soils occur on the flat alluvial flood plains of our major river systems. Such Fluvial Recent Soils are productive and widely used for dairy, and other pastoral, farming as well as for cropping and horticulture. However, many Recent Soils remain at risk of flooding, and extensive flood protection works have been installed to safeguard Recent Soils in both urban and rural areas in many regions. A number of towns, including Whangarei, Paeroa, Edgecumbe, Whakatane, Gisborne, Wairoa, Palmerston North, Whanganui, Westport, Hokitika, and Mosgiel, are built, in part, on Recent Soils, and have been subject to flooding. Recent Soils also occur on eroded hill and mountain country, older relatively stable sand dunes, and surfaces mantled with young tephra deposits. Sandy Recent Soils are particularly prone to wind erosion, hence maintenance of vegetation cover to prevent damage is important.

Semiarid Soils form in the inland basins of Central Otago and South Canterbury under rainfalls of less than about 500 mm yr⁻¹. The soils have an annual moisture deficit with hot summers and cold winters. Parent materials are predominantly alluvium, colluvium, and loess derived from greywacke and schist. Semiarid Soils range from immature gravelly sandy soils, on low terraces, to moderately weathered soils with clay accumulation on older, higher, terraces, fans, and hills. Accumulations of salts and illuvial clays can occur in subsoils. Pockets of Solonetzic Semiarid Soils occur with high pH (>8.5) and sufficient sodium to preclude the growth of plants, except for a few specially adapted salt-tolerant plant species. With irrigation, Semiarid Soils can be developed for productive agriculture and horticulture. Stone fruit, including apricots and cherries targeting the Northern Hemisphere Christmas market, and grape growing for wine production are important industries in the region, along with merino fine wool. Although there has been concern about the potential for irrigation to cause salinisation, such fears have proven largely unfounded as the plentiful use of high-quality water, from the major rivers that flow through the region, along with good drainage, has tended to wash any salts from the soil.

Ultic Soils are strongly weathered, clay-rich, soils that form on stable land surfaces, mainly on quartz- and feldspar-rich parent materials, predominantly in the northern half of the North Island. Scattered occurrences also occur on extremely old surfaces in other regions of New Zealand including Wellington, Marlborough/Nelson, Otago, and Southland. Some Ultic Soils developed on iron-rich volcanogenic, calcite-bearing sandstones of Tertiary age in the Auckland-Northland region have distinct relict reddening in lower subsoils. One subgroup of the Yellow Ultic Soils in the northern Waikato region is formed on halloysitic tephra deposits over a buried clay-rich paleosol. Ultic Soils generally have slow permeability and are prone to wetness in winter with attendant risks of compaction or pugging, and are droughty in summer. They are strongly leached and naturally acidic (pHs <5.5) with illuvial clay-rich subsoils and pale-coloured upper subsoils. They have low soil fertility. Thus substantial additions of lime and fertiliser are needed for productive use. Ultic Soils are best suited to pastoral agriculture and forestry. They are an option for urban development that avoids the more productive soils, provided the engineering design takes into account the potential for mud-sliding on the Tertiary sedimentary rocks (Waitemata Group) in the Auckland-Northland regions.

The Ross Sea region of Antarctica contains the largest ice-free areas in Antarctica (about 6700 km²). Soils in the Ross Sea region have a mean annual temperature of about −15 to −25 °C and so all contain permafrost. Antarctic soils generally comprise: a stony desert pavement from which fine material has been removed, predominantly by wind erosion; an active layer that freezes and thaws each year; and underlying permafrost in which temperatures remain below 0 °C. The McMurdo Dry Valleys, described as a ‘cold desert’, are Antarctica’s largest continuous ice-free area, formed where evaporation exceeds precipitation. In some of the driest areas, there is not enough soil moisture to form ice-cement and the soils are described as having ‘dry permafrost’. However, in coastal areas, and areas where there is some input of snow or meltwater, the permafrost is ice-cemented, patterned ground features form at the land surface, and soils may contain lenses or wedges of ice. Soils range from having minimal weathering and soil development on active surfaces, such as wind-blown sand dunes and sites of fluvial activity, through to ancient soils on stable, older, higher altitude surfaces, with highly weathered desert pavements, and salt-rich layers in the soil.

Soils in Aotearoa New Zealand are diverse.  Soils of all 12 orders in Soil Taxonomy occur in New Zealand, which is unusual for a relatively small land area. Also, all the soil orders of the New Zealand Soil Classification can be identified elsewhere in the world. However, some features of New Zealand soils are globally uncommon including the large area of Pumice Soils; the world’s oldest Allophanic Soils; Pallic Soils formed in loess with minimal calcium carbonate and often with fragipans; Ultic Soils with podzol-like morphologies including distinctive ‘egg-cup’ E horizons and densipans; Perch-gley Podzol Soils on the West Coast of South Island; extensive Brown Soils that mainly lack subsoil illuvial clay; relatively high soil carbon contents in topsoils, and yet few soils that have deep (>25 cm) dark topsoils; and the ‘dry permafrost’ soils of the McMurdo Dry Valleys in Antarctica. Ten methods used in New Zealand to evaluate soils and assist in land management decisions are described, ranging from traditional knowledge, through digital soil mapping, concepts of high-class soils, soil versatility, and soil vulnerability, to complex technical evaluation systems including ecosystem service modelling. Aotearoa’s diverse and productive soils are a life-supporting taonga/treasure which all need to treat with kaitiakitanga/care.