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.
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.
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.
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.
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.