The New Zealand Soil Classification (the system used to identify and name soils) defines soil as any natural material on the land’s surface that has the potential to support life, even if that is only bacteria. Soil scientists point out the difference between soil and dirt. Soil is a natural material in its proper place, but dirt is soil in the wrong place, such as on one’s shoes or car.
The soil is a natural body. It is not just a jumble of rotten rock, water, air and decayed plants – it shows characteristics of order. It shows order at the microscopic level of clay minerals and organic humus, and at the larger level in soil layers seen in a road cutting, or in patterns of soil types across the landscape. Soils link natural systems above ground – plants, animals, landforms, surface water and the atmosphere – with natural systems below ground, such as rocks and groundwater.
All life on earth depends on soil. Soil performs vital functions in our ecosystems. The soil stores reserves of water for plants, controls water seepage into streams and groundwater, and reduces rapid runoff that could cause erosion and floods. The soil stores carbon (in New Zealand, it stores about four times more than plants), so it helps minimise the release of carbon dioxide into the air. Soil microbes help absorb nitrogen from the air and feed it to plants. Nutrients are released as the rock fragments decay. Soil filters water, and helps absorb and break down toxins.
We are most familiar with topsoil – the dark layer (also called the horizon) at the ground’s surface. Digging deeper yields surprising colours and structures. The underlying subsoil gives clues to the history of the soil and past environmental conditions. It also shows the conditions that plant roots must explore, and how this affects land use choices.
Weather, micro-organisms (or microbes) and chemicals all act to break down gravels, sands and silts into clay. However this is balanced in many soils by gradual accumulation of layers of loess (fine, wind-blown soil), volcanic ash, pumice and alluvium (clay, silt and gravel left behind by a stream or river). Ash and pumice ejected from volcanoes cover the central North Island. Loess covers large areas of the South Island and the south and east of the North Island.
The material from which soil is formed is called parent material.
Soil colour is caused by its age, wetness, the climate in which it formed, and its parent material. In topsoils, the true colour is masked by dark organic matter, but it can be seen clearly in subsoils.
Some brightly-coloured soils have been used to make paints.
Soil texture depends on the proportion of clay, silt, sand, stones (or gravel) and boulders. Clay particles are the smallest, then silt, then sand. Clay surfaces have an electrical charge and attract nutrient atoms and molecules such as potassium and calcium. Sand’s comparatively larger particles create spaces in the soil which drain water and allow air to reach plant roots.
A mixture of sand, silt and clay is called loam. A loam can be named after the type of soil particle that dominates, for example silt loam, clay loam or sandy loam.
When a spade full of soil is dropped on the ground it shatters into aggregates – clusters of soil particles, also called peds. The shape, size and strength of aggregates show how porous the soil is. This controls water storage and drainage, air circulation to plant roots, and how deep plant roots can penetrate.
For a small country, New Zealand has a wide diversity of soils. There are 15 main types of soil (soil orders) and 1,914 subdivisions of soil orders (soil series).
Changes in soil across a landscape are caused by differing rocks, climate (especially rainfall), vegetation, groundwater, topography and age. Age is the factor that controls all the others. In a stable site free from erosion or the depositing of sediment, the soil can become established and grow old. However, if the soil is disturbed by erosion, trees falling so their roots wrench and mix the soil, or falling volcanic ash, loess or alluvium, ageing will be set back. The soil will be rejuvenated or destroyed and has to start forming again. Soil also varies depending on topography – for instance, it tends to erode from the top of a slope and build up at the bottom.
These controlling factors are the basis of the 15 soil orders in the New Zealand Soil Classification. This classification is used to identify soils, make soil maps, search soil databases, and plan and guide land management. New Zealand soils have a number of features that reflect the country’s unique environment and history.
Most New Zealand soils are similar to those elsewhere in the world, but some soils and their properties are unique. New Zealand soils are predominantly acidic. This may be because there is relatively little natural lime present. Native forest species also have an acidifying effect. This acidity is neutralised for farming by adding large quantities of lime and nutrients as fertiliser.
New Zealand soils have relatively large amounts of organic matter. This may be a result of:
Collectively, Brown, Pallic, Podzol and Semi-arid soils cover more than 69% of New Zealand. They form in different climates, although rainfall has the most effect on which type develops.
Brown, Pallic and Semi-arid soils may form beneath a range of vegetation types, but Podzols are restricted mainly to native forest. All four types occur mainly on quartz-rich parent rock, commonly greywacke, schist and granite.
Brown soils cover 43% of New Zealand. They extend from the country’s mountainous and hilly backbone down to the moist lowlands, where summer droughts are uncommon and soils remain damp throughout the year (except in some sandy or very stony soils, which may experience drought). Annual rainfall is usually more than 800 millimetres (south) or 1,000 millimetres (north). The relatively wet climate causes leaching of nutrients, which are washed into drainage waters and eventually into streams and rivers. This makes the soils acid, with limited fertility. Fertilised Brown soils, such as those on the Southland plains, make good land for sheep, beef and dairy farming.
When early European explorers saw New Zealand’s extensive conifer–broadleaf, beech and kauri forests, they concluded that the land was richly fertile. They gave glowing reports of the potential for agriculture. But in some areas, this was not to be. New Zealand’s native forests, unlike the northern hemisphere’s deciduous forests, can grow well on acidic and low-fertility soils.
Podzolised soils cover 13% of New Zealand. The name podzol comes from a Russian word for wood ash, describing the white subsoil – digging through the thin black topsoil exposes an almost white horizon (layer). Digging deeper reveals a bright ginger brown. These colours have been caused by strong acid leaching from native trees (especially conifers, beech and kauri) and high rainfall (more than 1,300 millimetres per year). The rain water carries acids from the trees, stripping aluminium and iron from the upper part of the soil and leaving it bleached white. It deposits the elements along with coloured organic matter, creating the ginger-brown layer. Sometimes this takes the form of an iron ‘pan’ or impervious layer in the subsoil. Podzols have low fertility and extreme acidity.
Pallic soils cover 12% of New Zealand. These soils are dry for part of the growing season, especially summer to mid-autumn. They form where annual rainfall is between 500 and 1,000 millimetres. They are called pallic because of the subsoils’ pale straw colour. New Zealand Pallic soils have hard, brittle fragipans (compact, deep subsoil layers), and are probably unique (one Russian soil scientist described them as ‘extremely peculiar’). These soils are formed in loess – wind-blown dust. In other countries, loess contains lime, but in New Zealand it does not. This is because most of the loess is formed from the lime-poor greywacke that makes up the main mountain range.
Most Pallic soils have limited uses (mostly sheep grazing) because the subsoil is dense. Roots cannot penetrate to moisture deep down, so the soil becomes even drier. Although dry in summer, the soil can be wet in winter or spring. Some younger Pallic soils are not so dense and have a range of agricultural uses.
Semi-arid soils, which cover 1% of New Zealand, are dry for most of the annual growing season. They are found in the inland basins of Otago and Canterbury, where the annual rainfall is less than 500 millimetres. There is too little rain to wash out the sodium, calcium and other elements released from the slow weathering of the parent material. Consequently, lime-rich and saline deposits accumulate in the subsoil. Irrigation gives the soil a good wash, flushing out the salts and transporting them into lower-lying areas. Nutrient levels are relatively high, but the soil must be irrigated to produce a crop.
Extensive areas of land from the Coromandel Peninsula to Northland have escaped erosion or volcanic ash deposition for more than 50,000 years. Smaller areas occur in Wellington, Marlborough and Nelson. Although old for New Zealand, these soils are not old compared to the ancient soils of Australia, and are young compared to the rocks on which they are formed. Because these soils have been weathering for a long time they have a high clay content. They are acidic and of low fertility.
Ultic soils, covering 3% of New Zealand, are weathered from quartz-rich sedimentary rocks. These yellow clay soils, hard in summer and wet in winter, are well known to Auckland gardeners. Lime can correct acidity, and compost can balance the heavy clay content.
Granular soils, covering 1% of the country, and Oxidic soils, covering less than 1%, are weathered from ancient, dark volcanic rocks or ash. Iron oxide weathered from the volcanic parent materials causes a brown colouration. The term ‘granular’ refers to the soil structure. Despite the high clay content, stickiness when wet, and low natural fertility, large areas are being used for horticulture at Pukekohe, demonstrating the productivity of these soils when they are well managed.
Oxidic soils are the most strongly weathered of New Zealand’s soils. They contain iron and aluminium oxides. They retain nutrients poorly, but can be productive when well managed, as, for example, in the orchards of the Bay of Islands.
Young soils are present on new or disturbed areas of land – where there is erosion or sediments being deposited, or on steep slopes, rocky outcrops, coastal sand dunes, flood plains, near active volcanoes, in tidal estuaries, and in cities.
Raw soils cover 3% of New Zealand. These are infant soils that will probably never grow old because they occur where there is constant erosion or sediment build-up. The topsoil is missing or very thin. These soils are not farmed, as the frequent disturbance prevents plant growth, and fertility is limited by the lack of sufficient organic matter to produce nitrogen and other nutrients.
Recent soils cover 6% of New Zealand. They occur where erosion and sediment build-up are low enough to allow well-developed topsoils to form. However, subsoils usually show very little development. Many of New Zealand’s most versatile soils are Recent soils. They are usually fertile and allow plant roots to penetrate deeply, unless rock or dense clay is present.
Anthropic soils cover less than 1% of the country. They are created when people deposit rubbish in land fills, strip away the natural soil, or carry out earth works. Most extensive in urban areas, these soils are also common anywhere the land has been mined.
Prolonged wetness affects the colour of soils, the types of plants that grow in them, and the rate of plant growth. In New Zealand there were extensive wetlands before European settlement, but large areas of Gley and Organic soils have now been drained for farming.
Gley soils, which cover 3% of New Zealand, are saturated with water for most of the year. Their subsoils are grey with rust-coloured spots or mottles. Gley soils are usually on low-lying land where there are high groundwater tables, or on hill slopes where there are seepages. They can be very productive when drained.
Organic soils, covering 1% of the country, are peat soils that are wet throughout the year. Located in wetlands, they formed from the decomposed remains of wetland plants (peat) or forest litter. The peat or litter accumulated because the processes that decompose fresh organic matter happened very slowly.
Organic soils can be productive when drained and fertilised. However, their use is not likely to be sustainable in the long term because drainage dries out the organic matter. It then decomposes more rapidly and the soils begin to shrink.
Soil parent materials, whether they are loess, ash, alluvium or weathered rock, are influenced by the nature of the original rock from which they formed. The parent rock influences the release of nutrients (for example calcium), the formation of clay minerals, and the amount of iron oxides, stones and clay.
Allophanic soils cover 5% of the country and are found mostly in the central North Island. They are predominantly made up of volcanic ash. The ash contains natural volcanic glass, which weathers in the soil to become allophane. Allophane is a stiff, jelly-like mineral that produces a porous, low-density soil. Allophanic soils have a wide range of uses.
Pumice soils cover 7% of New Zealand. They are dominated by pumice, or pumice sand high in volcanic glass. These soils are found mainly in the central North Island, where most of the pumice originated from the powerful Taupō eruption, 1,800 years ago. Many Pumice soils are used in commercial forestry. They were unsuitable for animal farming until they were found to be deficient in cobalt. Now that their deficiencies have been corrected by fertilisers, they are very productive.
Melanic soils, which cover 1% of the country, come from rocks that are rich in calcium (including limestone) or magnesium and iron (from dark volcanic rocks). Calcium and magnesium stabilise organic matter and promote the development of firm soil aggregates. Consequently, Melanic soils are fertile, with organic-rich black topsoils.
Soil types form patterns in the landscape that are related to changes in environment, geology and landscape history. The patterns are predictable and can be used to make soil maps and guide land management.
If we could look at the soil over a broad area from a high-flying aircraft we would see variations, mainly due to changes in climate and geology. For example, dry valleys may contain Pallic soils that give way to Brown soils on wet mountain slopes.
For more detail, we would need to fly lower. From here, changes in soil relate to topography, the age of the land surface and the type of parent material. For example, we would see Recent soils in alluvium beside streams, mature Brown soils on a flat river terrace and in rock debris on a hillside, Raw soils and Recent soils in erosion scars, and Gley soils in small wet gullies. To see even more detail we could examine soils in a paddock and find variation in texture, thickness of layers and wetness. These changes can cause variation in pasture and crop yield.
Catenas are common in hilly countries like New Zealand. They are a sequence of soil types linked like a chain from hill crest to valley floor. A drainage catena is the transition from well-drained soils higher on a hill to successively wetter soils, with increasing mottling and greyness on a valley floor. Changes in soil depth and texture are also common. Catenas and other patterns help people precisely predict soil locations, understand the hydrology of water catchments, and learn the history of landforms.
Positioning technology is used to map variations in soil properties. These detailed maps enable farmers to adjust irrigation and fertilisation. This precision helps reduce the costs and adverse environmental effects of soil management.
Soil changes are plotted by soil surveyors and marked on soil maps. Soil profiles are taken by sampling to a depth of 1 metre across a range of landforms. Relationships between soils and landforms are used to predict the locations of main soil transitions. Analyses, descriptions, and maps of New Zealand’s soils are stored in a national soils database.
A versatile soil capable of many uses needs to be deep, fine-textured, moist, free-draining, loamy, and have an organic-rich topsoil. These properties best enable plant roots to take up nutrients, water and oxygen, and get enough support for rapid growth. Fertility is highest in soils young enough not to have been leached and old enough to have built up organic matter. They are also derived from parent rocks that are well supplied with essential nutrients.
New Zealand’s best soils are called ‘versatile’ or ‘high-class’. They supply the nutrients required for optimum plant growth, and are good for growing food. Their area is limited (about 5.5% of New Zealand). High-class soils are most common among the Recent and Allophanic soils. They are also found among the other soil orders, but least among Podzol, Raw, Anthropic, and Brown soils that are acidic, lie on steep hills, or where the earth is frequently disturbed.
Because high-class soils are rare in New Zealand, it has been argued that they should be reserved for horticulture and agriculture, and not used for towns.
The last century of farming in New Zealand has changed many soils that were originally poor. Acidity has been corrected by adding lime, low fertility by using fertilisers, drought by implementing irrigation, wetness by drainage, and poor drainage by breaking up subsoil ‘pans’ with deep-pronged equipment. These and other interventions have contributed to the country’s wealth, and an increasing intensity of land use. More intensive production, whether by horticulture, cropping or dairying, raises the issue of sustainability. Can we continue to use soils without losing quality or harming the environment? A knowledge of soils, and the opportunities and constraints they present, is essential for managing the land.
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