Land and Soil

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Soil Loss in Aotearoa New Zealand. Our Unique Soil and Geology

Aotearoa New Zealand's distinctive geological composition significantly contributes to its notable soil loss and elevated erosion rates, surpassing global averages. The country's geologically youthful and rugged terrain, coupled with its seismic activity, are pivotal drivers of this phenomenon.

Statistics NZ noted that in 2012, it was estimated that 192 million tonnes of eroded soil entered Aotearoa New Zealand’s rivers each year; of this, an estimated 84 million tonnes (44 percent) was from exotic grassland. The graph below shows the modelled estimates of sediment load to waterways per region.

Estimated average rate of soil erosion in New Zealand, by region, 2012

Source: Manaaki Whenua – Landcare Research

Of South Island regions, West Coast (49 million tonnes/year), Otago (18 million tonnes/year), and Canterbury (17 million tonnes/year) had the highest levels of sediment movement into waterways. In each of these three regions, sediment movement into waterways estimated to be from exotic grassland was: Canterbury (20 percent), Otago (10 percent), and West Coast (5 percent).

Of North Island regions, Gisborne (40 million tonnes/year), Northland (15 million tonnes/year), and Manawatu/Wanganui (13 million tonnes/year) had the highest levels of sediment movement into waterways. In each of these three regions, sediment movement into waterways estimated to be from exotic grassland was: Manawatu/Wanganui (86 percent), Northland (82 percent), and Gisborne (70 percent).

Particularly vulnerable is the East Coast of the North Island, marked by soft rock geology in the Gisborne area and unique soil types such as the silt and clay-rich mudstone of the Hawke's Bay region. These fertile soils, while conducive to agriculture, are notably susceptible to erosion, especially during intense rainfall. Adding to the challenge, the region's frequent encounters with cyclones heighten erosion risks.

Throughout the nation, diverse geological attributes play a role in erosion dynamics. For instance, the steeper slopes resulting from tectonic forces in the South Island's Canterbury region are associated with greater erosion potential. In the Waikato region, volcanic activity has created ash-rich soils that are both fertile and fragile, necessitating careful land management to prevent erosion.

The tectonic forces shaping Aotearoa New Zealand's landscape contribute to steep slopes and rapid rock weathering. Coupled with the nation's substantial rainfall, these factors accelerate erosion processes. In the East Coast, frequent heavy rains, combined with delicate soils, and vegetation clearance result in substantial soil runoff and erosion.

Sediment losses could increase by 233% by the end of the century due to climate change. Without mitigation, these losses will be greatest in Northland, Gisborne and Manawatū-Wanganui.

The consequences of erosion are far-reaching. Sediment-laden runoff infiltrates waterways, adversely affecting water quality and aquatic ecosystems. Moreover, topsoil loss diminishes productivity, constraining the land's capacity to support vegetation, provide jobs and build local economies.

To safeguard waterways and productive soil, Aotearoa New Zealand is embracing land use practices tailored to specific geological challenges across various regions. Robust soil conservation practices and vegetation management play pivotal roles in preserving landscape integrity, particularly in erosion-prone regions.

Long term soil erosion. All landcover types 2012.

Source: Stats NZ

Major Erosion Types

The major erosion types outlined in the Soil Conservation Technical Handbook are:

Mass movement erosion - which occurs when heavy rain or earthquakes cause whole slopes to slump, slip or landslide. Most hill slopes steeper than 15 degrees are susceptible to mass movement, and those steeper than 28 degrees generally have severe potential. Storms are the primary triggers. This is the most common form of erosion in the hill country.

Fluvial erosion - which occurs when running water gouges shallow channels or deeper gullies into the soil. On sloping land the gullies can cut deep into the subsoil or undermine surrounding soils.

Surface erosion - which occurs when wind, rain or frost detach soil particles from the surface, allowing them to be washed or blown off the paddock. Surface erosion can occur on any land which is exposed to wind and rain but occurs largely outside the hill country. Stock grazing influences surface erosion. Winter-forage crop paddocks represent a "low hanging fruit" when considering options for reducing sediment loads in catchments.

Sediment erosion - activities involved in earthworks, plantation forests, cropland and pasture management may all result in significant sediment loads being mobilised and often entering watercourses.

Resource: Regional maps of modelled soil loss from surface erosion for use in GIS

Examples of typical erosion types adjacent to waterways (Source: DairyNZ)

Land Management

Land management in Aotearoa New Zealand constitutes a multifaceted approach aimed at harmonising the diverse interests of economic wellbeing, good environmental practices, conservation, and cultural heritage. It involves the responsible stewardship of land resources through community engagement, environmental programmes and the research and promotion of sustainable practices. As a nation celebrated for its clean green image, diverse landscapes and special ecosystems, effective land management holds immense value in preserving these natural and cultural treasures, striving to support sustainable growth, while honouring the cultural connections to the land.

Land Management within Regional Councils

Regional Councils throughout Aotearoa New Zealand employ Catchment / Freshwater/ Agricultural and Land Management Officers (often called 'advisors'). These teams are integral to New Zealand's local councils, serving as environmental advisors, and community connectors. In some cases, they also help support regulatory implementation.

These advisors play a vital role in connecting the community and safeguarding the environment by preventing erosion, pollution, and impacts to terrestrial and freshwater ecosystems. Balancing economic growth and environmental preservation is a key focus for them.

Community engagement is central to their work. Land management advisors interact with community and catchment groups, property owners and stakeholders to educate, address concerns, implement environmental change and encourage compliance with regulations. It is typical for these teams to collaborate with Maori to help bring a Te Ao Māori perspective into any project and to respect cultural values and historical ties to the land. Often land management advisors are the first point of call for landowners to connect to environmental grants, catchment programmes and advice.

Resources

Regional Council websites

Land Use Capability (LUC)

Aotearoa New Zealand's use of land use capability (LUC) is a comprehensive approach that shapes urban and rural planning, guides agricultural practices, and influences conservation efforts. By categorising land based on its inherent potential and limitations, this system informs decisions about infrastructure development, soil conservation, agricultural decision making, forestry management, and climate change adaptation. This approach plays a pivotal role in sustainable long-term planning, balancing economic growth with ecological preservation, and safeguarding the nation's resources for future generations.

The LUC system has two major components. The Land Resource Inventory (LRI) is an assessment of physical factors considered to be crucial long term sustainable land use and management. The inventory is used for LUC classification, whereby land is categorised into classes according to its long term capability to sustain one or more productive uses.

Land use capability in Aotearoa New Zealand involves assessing and categorizing land based on its agricultural potential and limitations. The classification system, ranging from Class 1 to Class 8, informs decisions about land use, development, and conservation.

The Land Use Capability Survey Handbook has become the go to resource for practitioners to implement LUV in Aotearoa New Zealand.

Land Use Classes:

Subclasses are also used to describe a land units' limitations. Four kinds of limitation are recognised: erodibility (e), soil limitations within the rooting zone (s), wetness (w) and climate (c). The initial letter of each limitation is used to identify the subclass (e.g. 2e, 2w, 2s, 2c). Only the dominant limitation is identified in the land use capability code.

Land Use Suitability

A broader planning perspective called ‘land use suitability’ is in development. This requires more detailed understanding of the land’s natural attenuation processes, which reduce levels of contaminants like nitrogen and phosphorus, and the resilience of water bodies. Land-use suitability research is linking these natural processes with human interventions, mitigations and land-management choices, to make the consequences of our choices clearer and more predictable.

Physiographic environments

Recent research has brought together data for climate, topography, geology, soils, and hydrological controls with analytical chemistry at a national scale. This work has created a new Physiographic Environment Classification, which groups areas that have similar landscape features. Areas classified as the same Physiographic Environment will respond to land use pressure in a similar and predictable manner. This can be explored via the LandscapeDNA website.

LandscapeDNA
How to use LandscapeDNA

Resources

Common Erosion Forms and Mitigations

For a comprehensive field guide for manging erosion. Please refer to the Soil Conservation Technical Handbook. The sub sections below are by no means a comprehensive guide to this subject and we recommend referring to the resources below.

Sheet and Rill Erosion

Sheet and rill erosion are common forms of soil degradation caused by surface water movement. Sheet erosion involves the uniform removal of thin soil layers, while rill erosion forms small channels on the soil surface. These erosional processes can lead to soil fertility depletion and sedimentation in water bodies.

Common remedies encompass implementing vegetative cover, such as cover crops and grass strips, to reduce the erosive impact of raindrops. Contour farming and terracing mitigate erosion by slowing water movement, enhancing infiltration, and preserving soil structure. Conservation tillage practices diminish soil disturbance, while Critical Source Area (CSA) management, grass swales, sediment basins and sediment traps intercept eroded sediment, preventing downstream transport.

Wind Erosion

Wind erosion involves the detachment and transport of soil particles by wind forces. It can pose a significant threat to agricultural lands. The most useful strategy to reduce wind erosion is to ensure soil is not left bare. Common remedial measures encompass planting windbreaks to reduce wind velocity, enhancing soil cover through cover crops or residue management, and employing conservation tillage practices to minimize soil disturbance. Implementing mulch covers on susceptible areas serve to alleviate the erosive potential. The adoption of soil stabilisers and organic amendments aids in fortifying soil structure, mitigating the risk of wind-induced soil loss.

Shallow Mass Movement - Slip Erosion

Shallow mass movement, or slip erosion, refers to the downslope displacement of soil and rock material, posing a substantial risk to land stability. Water quality and agricultural productivity. Effective mitigation strategies are essential to ensuring sustainable farming practices.

Revegetation plays a pivotal role in addressing slip erosion, encompassing both exotic forestry and indigenous forest planting. Introducing space planted poplar trees acts as a useful mitigation measure. Poplars with their robust root systems enhance slope stability, reducing the susceptibility to slip erosion.

Other strategies could include incorporating proper drainage systems to redirect excess water, preventing the soil from becoming overly saturated, a condition conducive to slip initiation. Additionally, utilising retaining structures and erosion control blankets further fortifies slope stability.

A comprehensive approach that integrates poplar plantings, contour ploughing, drainage systems, and slope stabilisation measures is instrumental in curbing the impact of shallow mass movement.

Deep-seated Mass Movement (Slides, Slumps, and Flows)

Deep-seated mass movement, comprising slides, slumps, and flows, entails the abrupt downslope movement of large volumes of soil and rock material. This phenomenon poses substantial risks in agricultural settings, potentially impacting land fertility and infrastructure.

Common mitigation approaches include slope stabilisation through engineering measures such as retaining walls and geosynthetic reinforcement. Implementing proper drainage systems assists in reducing saturation-induced instability. Afforestation and establishing vegetation cover bolster slope cohesion, curbing erosion susceptibility. Employing contour ploughing reduces slope gradients, minimising erosion triggers.

Some monitoring techniques provide timely alerts for potential mass movements. A comprehensive strategy that matches the solution to each specific issue is needed. A combination of afforestation, engineering measures, drainage improvements, vegetation initiatives, and vigilant monitoring can contribute to the management of deep-seated mass movement in Aotearoa New Zealand.

Gully Erosion

Gully erosion is distinct from rill erosion and erosional as it has differing characteristics and scale. Rill erosion refers to the formation of small, shallow channels on the soil surface due to concentrated water flow. These channels, known as rills, are typically less than 30 centimetres in depth and are commonly observed after heavy rainfall or irrigation on cultivated fields. Rill erosion affects relatively small areas and primarily involves the detachment and transport of soil particles.

In contrast to rill erosion, gully erosion is a more pronounced process that leads to the development of larger and deeper channels. Gullies are deeper and wider than rills, with depths often exceeding 30 centimetres and widths reaching several meters. Gully erosion occurs when concentrated runoff gains sufficient energy to erode and transport substantial volumes of soil. Gullies often form in areas with higher slopes or where the landscape is predisposed to concentrated water flow.

While rill erosion operates on a smaller scale, primarily affecting the soil surface, gully erosion involves the progressive enlargement of channels, resulting in significant land degradation.

Gully erosion, characterised by the formation of deep channels, poses a considerable threat to agricultural landscapes. It results from concentrated water flow and sediment transport, often exacerbated by human activities. Common strategies to counter gully erosion encompass implementing vegetative measures, such as afforestation, space planted poplars and grass buffer strips, to dissipate water energy and trap sediment. Installing check dams and retaining structures intercept runoff, slowing its velocity, and minimising erosive forces. Employing contour ploughing reduces slope gradients and diverts water flow, mitigating gully formation. Introducing sustainable land management practices, such as reduced tillage and crop rotation, stabilizes soil structure and curtails erosion susceptibility. The integrated application of these approaches plays a pivotal role in effectively managing gully erosion in agricultural environments, preserving soil fertility, and promoting landscape integrity.

Resources

Overland flow and Critical Source Areas (CSAs)

Critical source areas (CSAs) are small, low-lying parts of farms such as gullies and swales where runoff accumulates in high concentration.

Runoff from CSAs carries sediment and nutrients to waterways. CSA are a priority area to management to reduce sediment and nutrient loss to waterways.

Research from Our Land and Water has shown that targeting mitigation actions to CSAs is six to seven times more cost-effective than an untargeted approach, and that on-farm mitigations are working to decrease concentrations of phosphorus in streams and rivers.

Actions for managing critical source areas

Identify where CSAs are.
Fence off CSAs to create a grass buffer zone to filter contaminants and prevent stock access. The faster the water is flowing across a buffer zone, the wider (distance upstream from a waterway) the buffer zone should be to provide time for effective filtering.
When constructing new tile drains, direct them into areas where runoff can be filtered, such as wetlands or grass buffers, before entering waterways.

Benefits of managing critical source areas

Loss of valuable topsoil is reduced.
Nutrient and sediment loss to waterways is reduced.
Keeping animals out of CSAs can improve hoof health and reduce the incidence of mastitis.

Highly Productive Land

Highly productive and versatile soils are a precious natural resource. Highly versatile soils cover less than 10% of Aotearoa New Zealand’s area. Due to the general growth of urban areas and the increase in land-use changes in urban fringes, many sections of productive land have been lost to housing developments. Protecting these areas is vital to ensure these areas remain available for food production and primary sector activities.

Research in 2023 investigated innovative ways of making peri-urban land (semi-rural land surrounding a town or city) more productive while also benefiting locals. The research found a strong desire to better connect food producers and consumers by growing food for locals on the land surrounding our cities, including mara kai and mahinga kai. Nearly all residents were keen to have food grown close to their homes. Likewise, most people who farm said they valued being closer to their customers. The most favoured scenario included a publicly accessible mixed-use greenbelt with food gardens, fruit trees, farmers markets and sports fields.

The Ministry of Primary Industries: Valuing highly productive land report provides further details about the proposed national policy statement for high productive land and why it needs future protection.

Resources

Winter Grazing Management

Winter grazing management is about ensuring the pasture is set up for spring to meet the feeding requirements of stock. During winter months, grass growth rates slow, or even cease, creating a feed deficit for livestock. Farmers manage this by lengthening the rotation (length of time between paddock grazing events) across autumn and winter and allowing time for feed to grow adequately. In addition, to supplement pasture feeding, stock may feed on forage crops such as kale, swede, turnip or fodder beet.

Intensive grazing at this time may result in pugging, where the soil structure is damaged and acts as a seal, limiting the rainfall from dispersing through the soil. There are environmental and animal welfare considerations to manage as well as stock feed and soil conservation, so this can be a complex and stressful situation for farmers, especially if it is an especially cold or wet winter.

Recent research has shown that grazed forage crop paddocks contribute an average rate of soil loss that is 7 to 120 times more than expected for the land area they occupy. Increasing post-grazing residual ground cover could greatly enhance soil retention.