Understanding Soils

The state of New Zealand soils

We are losing 192 million tonnes of soil a year. That’s the equivalent of four-hundred-thousand dump trucks of soil unloaded every week.

'I know that New Zealanders care about our land, and I am confident that together we can tackle the issues.'

Penny Nelson (MfE), while introducing the 'Our Land 2018' report


Declining soil quality

The idea that ‘what we cannot see cannot hurt us’ could not be more wrong in the context of New Zealand soils.

Soils provide several key services, most importantly growing our food, water drainage, storage and filtering, carbon and nutrient storage and recycling, and hosting a large part of our biodiversity. In New Zealand we rely heavily on soils for our economy, as soil provides the resources for primary production such as dairy, horticultural crops and wood. New Zealand landscapes that attract millions of tourists every year are also underpinned by soil.

our land 2018


Click on the image to watch a video summary of the Ministry for the Environment report Our Land 2018. Source: MfE

It is therefore hugely important that we manage our soils well to sustain services and production from the soil. Soils develop over long time frames – millennia or longer – but can be lost very quickly through mismanagement and natural events.

The Food and Agriculture Organization of the United Nations (FAO) has compiled a list of threats to soils worldwide. These are processes that cause loss of soil or of soil functions, and therefore a loss of the services and production we can expect from these affected soils.

The main threats to soils are:

  • Soil erosion
  • Soil organic carbon loss
  • Nutrient imbalance
  • Soil acidification
  • Soil contamination
  • Waterlogging
  • Soil compaction
  • Soil sealing
  • Salinisation
  • Loss of soil biodiversity

The most significant drivers of these threats are:

  • Deforestation
  • Population growth
  • Pollution and waste disposal
  • Climate change
  • Unsustainable soil management practices

Pressures on NZ soils, and their drivers

All FAO soil threats and drivers occur in New Zealand. However, New Zealand soils have some specific challenges, due to both the natural environment and historical management. The table below, adapted from the Our Land 2018 report by the Ministry for the Environment (MfE) and Statistics NZ, shows the most important pressures on our soils, the impacts, and where these pressures are an issue:

Soil pressure

Natural or human-influenced

Description

Potential impacts on soil

Where it is an issue

Earthquakes and landslides

Natural, although the rate of landslides is highly influenced by human land use, topography and geology.

Landslides triggered by rain and earthquakes are a leading contributor to erosion in New Zealand.

Increased erosion

Loss of topsoil and reduction in productivity

Widespread – steep terrain is commonly prone to erosion in the form of landslides.

Intense rainfall events

Natural, but human-induced climate change is increasing the frequency of intense rainfall events.

Intense rain and wind contribute to erosion. While this is part of natural processes, rates of erosion are increased in extreme weather events, particularly in the absence of a complete vegetative cover.

Increased erosion

Loss of topsoil and reduction in productivity

Widespread, but primarily in the Southern Alps and in North Island hill country formed on weak rock types. This is of particular concern in areas such as Northland and the east coast of the North Island.

Reduction in vegetation cover

Largely human-influenced, but some natural

Accelerated erosion can be caused by clearing native vegetation for farming or forestry, forestry harvesting, changing from forestry to pastoral farming, cultivation and harvesting of crops, and the effect of pests (e.g. rabbits) and plant diseases.

Increased erosion

Loss of topsoil and reduction in productivity

Loss of soil biodiversity

Loss of soil nutrients (changed nutrient cycling)

Widespread, but hill country formed on weak rock types is particularly susceptible.

Agricultural intensification

Human-influenced

Increased production per area of land, often achieved through increased inputs (e.g. fertiliser, pesticides, water, and supplementary feed), and higher stocking rates

Increased erosion

Degradation of soil and water quality

Loss of soil biodiversity

Widespread, but especially true for intensive farming areas on flat to rolling land.

Urban expansion

Human-influenced

A switch from undeveloped or rural land to urban land use leads to the loss of soil for future use. This is of particular concern for high-class soil.

Decreased availability of high-class soil

Loss of soil biodiversity

Auckland, Waikato, Tasman, Canterbury, Tauranga

Pollution and waste disposal

Human-influenced

A variety of industrial, commercial, domestic and agricultural activities can result in chemical and physical contamination of soil. Historical activities can leave legacy issues of soil contamination.

Degradation of soil quality and reduction in productivity

Loss of soil biodiversity

Widespread but usually in hotspots (e.g. landfills, mining areas, and former sheep-dip facilities).

 

Based on the soil quality monitoring results of 11 regions in New Zealand, the state of our productive soils is within the target for most measured indicators. However, specific challenges for New Zealand soils are erosion, phosphorus levels (nutrient imbalances) and macroporosity (soil compaction).

Let us look at these challenges in more detail:

Erosion

New Zealand soils are naturally highly susceptible to erosion. Two areas with the highest erosion rates are the Gisborne region in the North Island and the West Coast in the South Island. The Gisborne region has slopes with geology and related soils that are naturally highly erodible, and this natural erosion is made worse through the historical clearance of forest. On the West Coast, steep mountainous terrain in combination with high rainfall is the main cause of high erosion rates.

However, an estimated 44% of soil loss in New Zealand due to erosion - approx. 84 million tonnes per year - comes from pasture (exotic grassland), indicating that some intensive farming practices may cause considerable soil loss. Given that many New Zealand regions are naturally susceptible to erosion, intensive agriculture has to be managed cautiously to minimise the losses of valuable soil.

Phosphorus

Phosphorus is an important nutrient for plant growth, and it is often provided as fertiliser to crops and pastures. Many New Zealand soils have low natural levels of phosphorus, or have become depleted in phosphorus levels due to growing and harvesting of crops. Low levels of phosphorus may cause lower crop yield and productivity. However, a third of soils in New Zealand have too-high phosphorus levels due to (over)supply of fertilisers, and this can cause problems: erosion and run-off of phosphorus into waterways increases the risk of poor water quality and toxic algae.

Macroporosity

Macroporosity is a measure of how many large pore spaces (macropores) exist in a soil. Macropores contain either water or air. Low macroporosity indicates that soil is compacted; for example, due to heavy machine traffic or high animal stocking rates. Soil with low macroporosity is less productive because it reduces plant growth, restricts the soil drainage of excess water, and reduces soil biodiversity.

Low macroporosity also increases erosion and phosphorus run-off, because water cannot infiltrate into the soil, and so it contributes to poor water quality. Nearly half of New Zealand soils tested for macroporosity (mostly productive land) have levels that are too low, which means they are less productive than they could be and are likely to contribute to erosion and nutrient run-off.

Want more information?

Continue reading our section on Soil quality and its monitoring

Read the Ministry of Environment & Stats NZ report Our Land 2018:
http://www.mfe.govt.nz/publications/environmental-reporting/our-land-2018

Read our report on Future requirements for soil management on New Zealand:
https://www.landcareresearch.co.nz/__data/assets/pdf_file/0003/109326/Future_Requirements_Soil_Management_NZ.pdf

Read the FAO report Status of the World’s Soil Resources 2015:
http://www.fao.org/documents/card/en/c/c6814873-efc3-41db-b7d3-2081a10ede50/

Check out the infographic Our Soils under Threat (FAO):
http://www.fao.org/3/a-mn997e.pdf

Global Assessment of Human-induced Soil Degradation (GLASOD, 1988-1990):
https://www.isric.org/projects/global-assessment-human-induced-soil-degradation-glasod