If you dig a hole in your school playing field, will the soil there look the same as the soil in your home garden? Is soil the same all over New Zealand?
All soils have some things in common. They are all made of mineral particles, organic matter, air and water – but soils are also different due to how and where they were formed.
Five factors influence soil formation: parent material, climate, living organisms, topography and time.
Living things inherit characteristics from their parents – and soils do the same. The rock from which a soil is formed is called parent material. Sometimes the parent material stays in the same place, while other parent materials are transported by wind, water or by other means. Parent materials also have different properties, for example, the amount of nutrients. That means New Zealand soils that began as pumice from volcanic eruptions will be different to those that began as sediments deposited by rivers.
Climate refers to temperature and rain. Heating and cooling can speed up the weathering or breaking down of rocks into smaller pieces. Warm temperatures and rain encourage plants and animals to grow, adding organic matter to the soil. Rain also washes (transports) rocks and soil off slopes and can dissolve minerals, adding them to the soil.
Plant roots grow down into the soil. Roots get into cracks and release chemicals that help make nutrients available. Earthworms and other animals tunnel through soil and mix it up. When plants and animals die, they add organic matter to soil. Humans also make changes to soils. Removing vegetation from the top of soil exposes it to erosion – the soil can get blown or washed away. Humans add fertiliser to make soil more productive or lime to make it less acidic.
Topography refers to the land. If land is sloped, gravity moves soil particles downwards, deepening the soil in a valley. Topography also influences the climate. New Zealand weather patterns often come in from the west. The air cools as it rises up over the mountains, and the moisture in the air falls as rain. The air is drier by the time it reaches land in the east, so less rain falls.
It takes a long time for soil to develop – from 500 to thousands of years for every 1–2 cm. The age of soils differs around the country. Parts of Northland have quite old soils, whereas the valleys in the Gisborne area have soils that are young – some were formed on material moved by Cyclone Bola in 1988.
Nature of Science
There are many everyday words that have a totally different meaning in science. When speaking about humans, the word ‘old’ may mean several decades. When speaking about soil, the word ‘old’ may mean thousands of years.
Visit the NZ Soils website to learn more about soil formation.
The NZ Soils website has an image collection showing over 70 different soils.