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:
- the country’s short history of human settlement – organic matter has not been reduced by prolonged, intensive crop-growing
- farms consisting mostly of pasture rather than crops, which conserves organic matter
- the legacy of acid-forming forests, which built up organic matter in stable forms that break down slowly.