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Soil erosion and conservation

by  Paul Gregg

Farming has been the basis of New Zealand’s economy, and soil is the basis of farming. When soil erodes, it makes land less productive, and can lead to floods, slips and damage to infrastructure and property. Methods for controlling erosion include planting trees, stabilising soil and controlling rivers and streams.


Soil features and impact of erosion

New Zealand’s soil is a valuable resource. The country’s economy has always been based largely on the export of commodities produced from the land.

Micro-organisms

Soils may appear lifeless and inactive, covering the landscape but hidden by vegetation. However, they are seething with organisms. In one gram of soil there may be billions of micro-organisms, weighing up to 11–22 tonnes per hectare.

Soil types

Because of the wide variation in New Zealand’s climate, geology, topography and vegetation, there are at least 3,500 types of soil. Some are 50–60,000 years old; others are only a few years old.

Soil layers

A mature soil can be divided into three basic parts:

  • Topsoil – the top layer, usually dark and rich in organic matter. It contains nutrients and water required by plants for their growth.
  • Subsoil – under the topsoil, and usually paler. It helps to anchor plant roots, and provides water.
  • Parent material – from which the soil is formed.

Extent of erosion

In 1997 the Ministry for the Environment noted that:

  • 50% of the country was affected by moderate to slight erosion
  • 10% had severe to extreme erosion (eastern North Island, parts of Taranaki, and the South Island high country)
  • only 31% of the total area could sustain pastoral farming without significant control of erosion
  • a further 28% could support restricted livestock grazing combined with erosion control.

It stated that the erosion of agricultural soils in the North Island hill country and South Island high country was of major concern.

Eroding earth

At least 25%, and possibly much more, of the world's agricultural land now has moderate to severe erosion. As a result, about 30% of farmable land in the US has been abandoned. Land used for growing crops is the most susceptible because the soil is repeatedly cultivated and left without protective plant cover. About half the world's pasturelands are also subject to erosion from overgrazing.

Between 1997 and 2007 over 36,000 hectares of pasture were planted in trees as a means of reducing erosion, especially in the Gisborne, Manawatū and Waikato hill country.

Cost of erosion

Hill country erosion is estimated to cost between $100 million and $150 million annually. Part of this is through lost production and nutrients. A 1980s erosion study in the Wairarapa showed that on young soil slips (where topsoil and subsoil were eroded), pasture yield was only 20% of that on uneroded land.

Where does the soil go?

Water run-off or gravity carry soil to lowland or stream sites. These deposits may:

  • form new soils, as on plains that are regularly flooded
  • lie on farmland, reducing production
  • run into stream or river beds
  • be carried out to sea.

Impact on the environment

Deposits of eroded soil may cause infrastructure damage, such as blocking roads and drains. They may also damage houses, roads, fences, power and phone lines, waterways and aquatic habitats. The increased turbidity (cloudiness) of water reduces the amount of light that can penetrate. This adversely affects fish diversity, food supply and use of waterways for recreation.


Natural and human causes of erosion

Natural erosion

The landscape of New Zealand has been shaped by wind, water, volcanoes and glaciers over at least 25 million years. The forces that wear away at the land are considered part of natural erosion.

Accelerated erosion

The rate of erosion can be increased by unusual events such as earthquakes and cyclones, or by human activity such as cutting down forests. This is known as accelerated erosion.

When only some of the topsoil is affected, it is difficult to distinguish between normal and accelerated erosion. But it is quite clear-cut when all the topsoil and some subsoil has been lost from the original location.

North Island forests: protecting the soil

In the North Island, much of the land was once forested up to altitudes of 1,500 metres. The forest had developed over a long period, and trees gave stability through their deep, extensive root systems. Forest canopies lessened the erosive impact of rainfall, and tree litter recycled nutrients and protected the soil from wind and water.

Clearing the forest

From the early 19th century, settlers felled the trees for timber. They also cleared the land for farming, by logging trees and setting fire to the understorey. They sowed English grasses on the forest ash deposits, which provided an initial source of nutrients. But the nutrient value of the ash soon declined, causing a fall in pasture productivity.

From forest to pasture

Pasture has a limited root system and gives the soil less protection from rain than trees. This created a big change in how water interacted with the landscape. The water yields of forested catchments were equal to 15–20% of the rainfall, but under pasture this increased to 30–40% of the rainfall. The increased water run-off increased the risk of erosion, especially on steep land.

Green Acres

In the 1950s artist Avis Acres produced a weekly comic strip featuring two pōhutukawa fairies, Hutu and Kawa, who lived in the bush. Her stories showed an understanding of ecology and a strong conservationist ethos. In Hutu and Kawa find an island, she described the terrible impact of possums on native trees and birds.

South Island tussock

In the South Island, early colonists found land covered in both tussock and forest. To transform it into farmland, they mostly cleared the land by burning, and then hand-sowed pasture seeds.

Later, burning tussockland became a regular practice. The tussock ecosystem, which had developed in the absence of fire, became weakened. Its protective mantle deteriorated, particularly in the harsh environment of the high country. The erosive forces of wind, frost and water accelerated soil erosion where vegetation was sparse.

Impact of animals

Both domestic and wild livestock have played a part in accelerating soil erosion.

Rabbits were recorded for the first time in New Zealand in 1838, and were causing major problems by the 1890s. Overrunning many parts of the country, especially Central Otago, they stripped the pasture cover, which led to wind and water erosion.

Deer and possums, introduced in the mid-19th century, upset the balance of forest environments. Adding to the impact was overgrazing by sheep and cattle on tussock land in the South Island, and hill country in the North Island.

Mountain lands

In the South Island, grazing sheep damaged the plant cover on the fragile mountain lands. Deer, tahr and chamois, brought in for hunting, further exposed soil to erosion.

Cultivation

At first, cultivation of farmland was confined to terrain of easy contour, mainly on river terraces near the main towns. But as farming moved on to steeper land, the loss of soil during heavy rainstorms increased. Cultivated soils were also eroded in areas with strong prevailing westerly winds such as Canterbury, the Manawatū and the North Island’s East Coast.


Types of erosion

The main forms of erosion are:

  • surface erosion
  • fluvial erosion
  • mass-movement erosion
  • streambank erosion.

Surface erosion

When rain, wind or frost detach soil particles from the surface, the particles are washed or blown off the paddock.

Sheet erosion

This occurs when rain falls on bare or sparsely covered soil, loosening fine particles (silt, clay and humus) that are carried downhill in surface run-off. Sheet erosion lowers the fertility of the soil, because it removes the most productive layer, which has usually been enriched by fertiliser.

Wind erosion

The wind can remove the valuable fine soil on the land surface. Seasonally strong winds hit many areas, such as sand dunes, the Central Volcanic Plateau, Wairarapa, Hawke’s Bay, Canterbury and Otago. If covering vegetation has been grazed or disturbed, wind erosion can be severe.

Fluvial erosion

This occurs when running water gouges shallow channels or deep gullies into the soil.

Rill erosion

On sloping land, particularly if cultivated, water run-off may gather in small V-shaped channels or rills. These are particularly evident in pumice soils or those formed from loess (wind-blown dust), but can occur on all hill soils.

Gully erosion

Gully erosion occurs on unconsolidated subsoils. These are generally deep and generate a lot of sediment, which often feeds into rivers.

Gullies tend to erode at their head, eating back into the landscape. They are widespread throughout the country. In the 1950s, there was serious gully erosion in sandstone at Pohangina in the Manawatū. Severe gullying also developed in pumice soils on the Volcanic Plateau when pastures were first sown.

Tunnel gullying

When water enters the soil on a slope, usually down cracks after a dry period, it scours out the subsoil to form a tunnel. The soil often later collapses inwards, and an open gully forms.

Loess soils (fine dust) are very prone to this kind of erosion, especially on Banks Peninsula and in Marlborough. Wither Hills near Blenheim had many severe tunnel gullies in the mid-1950s. Once the surface was reshaped by bulldozers, some of this land has been used for growing grapes and olives. Although somewhat localised, tunnel gullies also occur in volcanic pumice and Northland’s weathered sandstones.

Mass-movement erosion

When gravity combines with heavy rain or earthquakes, whole slopes can slump, slip or slide.

Muddy waters

The Tarndale slip is the largest in the southern hemisphere. It lies on the mudstone banks of the Waipaoa River, inland from Gisborne, and adds a considerable amount of sediment to the river. Forest has been planted on nearby former farmland to slow the growth of the slip.

Slips

Slips are one of the main forms of erosion in the North Island. They are a direct result of the change from forest to pasture.

Slips occur when the soil (topsoil and subsoil) on slopes becomes saturated. Unless held by plant roots to the underlying surface, it slides downhill, exposing the underlying material.

Slips are common where the underlying rock is mudstone (papa) or siltstone – as in the hill country of Gisborne, Taihape, Whanganui, Taranaki and King Country. The Wairarapa hill country, where loess lies on north-facing slopes, is also susceptible.

In the South Island, soil slips are widespread throughout the hill country, but mostly north of Christchurch.

Awash with it

During a February 2004 storm in Manawatū, there were 62,000 landslides, totalling 1,000 million tonnes of soil. About 50% came from hill-country farms. Flooding caused $100 million worth of damage. Around 210 million tonnes of soil are washed into New Zealand rivers annually: about 10 times the global average. This is a mix of natural and accelerated erosion.

Earth flows

These can be spectacular, but are not widespread. Saturated soils move downwards en masse, and may include underlying rock. However, the surface vegetative mat stays intact, and forms humps and hollows.

Scree erosion

This occurs in steep or mountainous areas, where the underlying rocks are greywacke (in both the North and South islands) or schist (South Island). At high altitude, gravity pulls material downhill into a fan shape, which can be quite spectacular.

There is debate over whether this is accelerated or normal erosion. What is clear is that screes can continue to move if vegetative cover is reduced, or stock track across their slopes.

Streambank erosion

This is common throughout New Zealand, especially in rivers and watercourses with periodic flooding. Although not dramatic, it is significant because most of the valuable farmland lies beside main rivers, on alluvial terraces.


Surveys and research

Early surveys

The first major survey of the extent and severity of erosion in the South Island high country was carried out by J. D. Raeside and H. S. Gibbs in 1945.

They found that 25% of the land was extremely eroded, with less than half the topsoil remaining. Only 20% had minor or no erosion.

The first national erosion survey was made in 1973 as part of the New Zealand Land Resource Inventory.

Farmer versus government

The call for quantitative measurements was, in part, the result of debate between farmers and regulatory bodies about the impact of farming methods on soil erosion. A royal commission on sheep farming, which finally reported in 1949, saw farmers in deep conflict with conservationists over the use of private land.

Aerial topdressing

Aerial topdressing (applying fertiliser from small planes) was first trialled in 1949. The main aim was to restore the fertility of hill soils, establishing a vigorous pasture cover to reduce erosion. By this method, superphosphate and white clover could be spread onto relatively unproductive land. This resulted in the widespread pastoral development of hill country.

The increase in pasture cover led to a reduction in sheet erosion and associated gully erosion.

Never too late

Surface erosion can reduce up to 60% of crop yield, 80% of pasture growth and 90% of tussock biomass. Most of these losses can be stopped with soil conservation methods.

Catchment board research

Catchment boards, which managed river catchment areas, undertook research and demonstration projects on local erosion. The South Island focus was mainly on managing high-country tussock land. In the North Island, researchers developed ways to control gully erosion and slips.

Soil conservation farms

With the strong advocacy of soil conservator Doug Campbell, the Soil Conservation and Rivers Control Council purchased or leased a total of 24 farms or areas with erosion problems. These ranged from Wairākei in the North Island’s central plateau, to Tangōio near Napier, Makara near Wellington, Wither Hills near Blenheim, Moutere near Nelson, Adair near Timaru, Tara Hills near Ōmarama, and Mid-Dome in Southland.

These farms were run by the Department of Agriculture with the aim of researching and demonstrating erosion control measures appropriate to each region. All the farms were later sold.

Forest Service research

In 1956 the government Forest Service established the Forest and Range Experimental Station in the Craigieburn Range. Researchers investigated the land at high altitudes (above 1,000–1,200 metres) in problem areas. This was challenging work in a harsh climate. Research focused on understanding erosion processes and finding plant species to colonise and stabilise steep, eroded slopes.

Administration changes

As a result of the Water and Soil Conservation Act 1967, soil conservation work moved from the Department of Agriculture to the newly created Ministry of Works Water and Soil Division.

The Soil Conservation Centre was set up at Aokautere near Palmerston North, and investigated the impact of soil erosion on pasture productivity in the East Coast, and on Taranaki hill country. Research included testing poplar and willow species for controlling gully and slip erosion, and using remote sensing – such as satellite imaging and aerial photography – to assess the effects of erosion on the landscape.

In 1986 the Ministry of Works was disbanded. Soil erosion research is now carried out by Crown research institutes – principally Landcare, but also HortResearch, AgResearch, the National Institute of Water and Atmospheric Research (NIWA), and Scion.

Later surveys

Detailed assessments (at a scale of 1:63,000) were not conducted until the 1970s. In 1999 they were completed at a scale of 1:50,000. The extent and severity of the erosion problem is given in the Ministry for the Environment’s 1997 assessment of the environment.

Modern techniques such as remote sensing and geographical information systems (GIS) can be used to rapidly evaluate the degree, extent and type of erosion.


Legislation and catchment boards: 1940s–1950s

The Soil Conservation and Rivers Control Act

There had been some erosion surveys in the 1930s, along with growing concerns about erosion. In September 1941 the Soil Conservation and Rivers Control Act was passed, the Soil Conservation and Rivers Control Council was set up.

The aims of the council were to:

  • promote soil conservation
  • prevent and reduce erosion
  • prevent flood damage
  • use land in a way that would achieve these aims.

Catchment boards

The act set up catchment districts, and the Soil Conservation and Rivers Control Council established catchment boards (with elected and non-elected members). The main purpose of the boards was to minimise and prevent damage by floods and erosion.

By 1945, 11 boards had been formed, and by 1967 there were another six similar commissions or authorities. The entire New Zealand landscape was covered.

For the greater good

Spreading fertiliser, which encouraged pasture growth, was crucial to the success of soil conservation efforts on steep land. However, at first, soil erosion prevention was done for economic rather than environmental reasons.

Single subsidies for farmers

In 1946, the Soil Conservation and Rivers Control Council devised a subsidy system to help farmers conserve the soil.

Up to 1956, farmers were assisted with single methods such as retirement fencing, which fenced off areas not suitable for grazing. Windbreaks, debris dams, pasture furrows and tree-planting in gullies were also subsidised.

Whole-farm plans

Catchment boards soon realised that a whole-farm approach was needed when several methods were in use. The farmer had to adopt a mutually agreed ‘farm plan’ over a set period, normally five years.

A farm plan was based on a land capability survey. This divided farmland into eight classes – four arable (crop-growing) and four non-arable. The surveys were first done in 1952.

Engineers vs conservators

The Soil Conservation and Rivers Control Act 1941 linked erosion with flooding. Catchment board staff were river engineers and soil conservators, who often disagreed on how to control flooding.

Some engineers believed that works such as stop banks and stone gabions (rocks inside wire mesh) would control flooding, whereas soil conservators considered that the upper parts of a river catchment should be the focus. Eventually both viewpoints were included in flood control programmes.

Catchment control schemes

The first major catchment or river control scheme was begun by the South Canterbury Catchment Board, and approved by the Soil Council in 1951 for the Ōpihi catchment.

In 1973 conservationist Lance McCaskill noted:

[T]here has been an outstanding change in the main catchment, rabbits have disappeared, vegetation provides almost continuous cover, soil erosion is no longer a serious problem, wells and springs run throughout the year, the smaller streams are under control, and the hydraulic efficiency of the main river channels is steadily improving. 1

Other catchments then started similar programmes to control erosion and rivers.

Footnotes
  1. L. W. McCaskill. Hold this land: a history of soil conservation in New Zealand. Wellington: A. H. & A. W. Reed, 1973, p. 221. › Back

Legislation and subsidies: 1960s–1990s

Water and Soil Conservation Act 1967

After a review of New Zealand’s soil conservation in 1964, the Water and Soil Conservation Act was passed in 1967. A Water and Soil Division was set up within the Ministry of Works and Development. Under its control were the Soil Conservation and Rivers Control Council, the Water Pollution Control Council and the Water Allocation Council.

Importantly, at local level the control lay with each catchment authority, via the regional water boards.

Emphasis was placed on measuring the extent and types of erosion through land inventory surveys and land capability assessments.

The end of subsidies

The removal of agricultural subsidies in the late 1980s sparked the conversion of marginal farm land to plantation forestry, or back to native bush. These changes are now seen as beneficial in terms of reducing erosion. But the loss of subsidies halted many erosion control measures by catchment boards, as landowners were no longer keen, or could not afford to carry them out.

Resource Management Act 1991

The Resource Management Act 1991 is based on the sustainable management of resources (including soils) and encourages long-term planning.

Regional councils (essentially the former catchment boards) and territorial authorities (district and city councils) implement the act under the guidance of the Ministry for the Environment.

Responsibilities

The councils’ main responsibilities are:

  • to manage environmental, resource and transport planning, including the sustainable use of land, air and water
  • to protect communities against the effects of flooding.

Erosion damages water quality

A major focus for the councils is water quality. This is affected by soil run-off, which carries sediment and nutrients (mostly nitrogen and phosphorus) into waterways. Phosphorus enters waterways in dissolved form, and attached to soil particles. Excessive nutrients in the water can cause excessive growth of aquatic plants, which lowers the oxygen content and affects fish life.

Revived interest

Concern over water quality has renewed the interest in soil conservation. Many regional councils have identified erosion as a major issue. A Manawatū plan has been developed to encourage hill country farmers to plant 77,000 hectares in trees. It is proposed that farmers be offered a government subsidy, and money from local rates. In this way, the subsidy system, scrapped in the 1980s, has re-emerged.


Biological control of erosion

A range of measures have been used to control or prevent erosion on New Zealand’s farmland. Some were adopted from other countries such as the USA, some were learnt by trial and error, and others developed from research.

Biological control is usually cheaper, but more risky, than mechanical methods.

Trees

Trees reduce the rate of erosion by:

  • protecting the soil from the impact of rain
  • transpiring large amounts of water, which counteracts very wet soil
  • binding soil to sloping land with their roots.

Spaced planting

Trees, commonly poplars, are planted strategically on slip-prone hill country such as the area around Taihape. Thousands have also been planted throughout the North Island hill country.

They are planted as poles, and protected from grazing animals until established. Poplars are fast growing and their root systems develop rapidly. But they are deciduous and so do not transpire as much water in winter as evergreen species.

Where trees can be easily harvested, pines (Pinus radiata) may be planted at a site where both pasture and trees for timber are managed together. Known as agroforestry, this can be successful on more productive sites and less steep terrain, where trees generally grow faster.

Pair planting

This method is used to control gully erosion, for both permanent and ephemeral streams. Willows or poplars are planted about 2–3 metres apart on each side of the gully floor. In time their roots create a damming effect which slows the water flow, stabilises the gully sides, and allows other plants to regrow.

Stream bank planting

Willows are planted at the edge of streams and rivers to stem the erosion of the banks. Traps or small dams may also be used to catch material dislodged from the edge.

Riparian strip

This is a strip (10 or more metres wide) near a stream or river, which is fenced to exclude farm animals. It may be planted with a mix of trees, shrubs and pasture. The aim is to trap sediment, and nitrogen and phosphorus in water run-off, from nearby slopes before it enters waterways.

Plantation trees

Pines and cedars planted for forestry production (around 400 stems per hectare) can control slips and gully erosion on hill country, if they cover an area larger than the gully. Some 320 square kilometres were reforested through the East Coast forestry project, begun in 1992.

Shelter belts

Rows of trees, planted at right angles to the prevailing wind, help reduce wind erosion and protect livestock and crops. Radiata pine (Pinus radiata) and macrocarpa (Cupressus macrocarpa) are the most widely used species.

Shelter belts can have more than one row, and more than one species, and may be up to 10 metres wide. For best effect they should be as long as possible – at least 12 times as long as the height of the trees.

Protection forestry

Protection forestry is managed on land that has little production value, but has considerable value as erosion protection if fenced off from grazing animals. Often these areas are the headwaters of large watersheds. They may consist of regenerating forest or be planted in trees (exotic or native).

Saved by trees

Research in the Gisborne region by Landcare scientist Mike Marden showed that reforestation of gullies successfully controlled erosion in all but the largest of them. Unplanted gullies continue to erode.

Pasture

A dense and vigorous pasture will reduce sheet and wind erosion, but its shallow roots do not prevent slips, gullies and slumping. In the South Island high country, runholders are encouraged to retire the upper catchment areas from grazing and allow the snow tussock ecosystems to regenerate.

Animal pest control

Controlling animal pests is important in reducing erosion caused by farming. An example is Molesworth Station in Marlborough, once a sheep station ravaged by rabbits. Sheep were replaced by beef cattle, which do not graze as closely, so they expose the pasture to rabbit invasion. Controlling the rabbits helped in the revegetation of depleted hill country.


Mechanical control of erosion

Several mechanical methods are used to control and prevent erosion.

Flumes

Flumes are artificial channels that control the flow of water down a slope and release it into an area where its impact is reduced. They are often placed at the head of gullies to prevent the backward erosion of the headwall by water flowing over the top.

Dams

Debris dams are sited in the floor of gullies. Built of wood planks or tyres, they trap material moving down the gully floor. Often this technique is used in conjunction with pair planting.

Detention dams are small dams on farms or sites such as ephemeral waterways which, under heavy rainfall, can create erosion within the waterway. The dams are designed with a wide spillway that allows some storage of water, and in flood conditions allows a steady and slow release of water over the spillway.

Bulldozing

Where an earthflow has occurred, land smoothing is used to stabilise the soil. Bulldozers smooth the surface of the earthflow, so water will run off rather than pond and saturate the unstable soil. This technique is expensive.

Infilling can be used where tunnel gully erosion has occurred. The gully edges are pushed into the centre, which is compacted. The contour of the land is then shaped to spread run-off. This method was used successfully in the early 1960s at Wither Hills near Blenheim.

Furrows

Pasture furrows were introduced in the 1950s, notably in Canterbury’s cultivated downlands, to control run-off and prevent sheet and rill erosion.

In the pasture phase of crop rotation, small channels are ploughed about 10 metres apart across the slope. These divert run-off to grassed waterways, which then feed into natural streams and rivers.

A variant of pasture furrows are graded banks, which are much wider and further apart. These were used in Northland.

Cultivation techniques

Conservation tillage is where crop-growing soils are left, after harvest, covered in crop residues. This acts as a mulch, protecting the soil from wind erosion and raindrop impact.

With contour cultivation, all cultivation is done across the slope. This creates a series of mini-barriers to the downward flow of water.

Direct drilling is a method where pasture seeds or crops are drilled straight into the soil, under pasture. The advantage is that being unploughed, the soil is not vulnerable to erosion.


External links and sources

More suggestions and sources


How to cite this page: Paul Gregg, 'Soil erosion and conservation', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/soil-erosion-and-conservation/print (accessed 6 December 2019)

Story by Paul Gregg, published 24 Nov 2008