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How do soils form?

Themes: Soil-forming factors | Landscape, Vegetation and Organisms | Time and Man | Quiz

Digging into the soil Soils form an almost complete skin over the earth, broken only by oceans and other water bodies and by mountains that have yet to develop a soil cover. When you walk through your garden or through fields and woods there is always this amazing carpet, called soil, beneath your feet. But how do soils form? Well, there are six main contributors that interact to produce soils: parent rocks and sediments, climate, landscape, vegetation and living creatures, time, and the effect of man. Around the world the influence of these soil forming factors will be different from one country to another and this is why there are many different soils in the world. Let's look at how soils form in a little more detail.

Parent rock in Malta showing weathering Parent material is one of the most important influences on what type of soil develops. Just as we humans all have parents, so soils have parent material from which and on which they develop. In some cases soils develop directly from the rock which lies beneath them. Gradually the rock will break down into smaller pieces under the effects of rainfall, snow, freezing and thawing, and these smaller pieces break down even further to produce soil. This fragmented rock forms the skeletal material of soils. If the rock is very hard, it can take hundreds of years just to form one centimetre of soil. If soils form on loose materials such as desert sand dunes or on more or less loose deposits left by the Ice Age, deeper soils can form quite quickly. The parent rock or sediment is the main factor responsible for the texture of the soil (i.e. whether the soil is sandy, loamy or clayey) and is also important in determining whether the soil is acid or basic and how rich it is in nutrients.

This ice on the grass reminds of the importance of the hydrological cycle for plants. Climate is the other most important soil forming influence, alongside parent material. It determines the rate of breakdown of parent rock and thus how quickly the soil will get deeper. The two main climatic influences are temperature and rainfall. Higher temperatures increase the rate at which breakdown of the underlying rock takes place and thus also the release of nutrients into the soil. Rainfall and snow melt are also important in breaking down rock to form soil and in the distribution of nutrients in the soil. In hot, wet climates such as the tropics, soils tend to be deep, whereas in the cold Arctic areas, soils tend to be thin and poorly developed.

Themes: Soil-forming factors | Landscape, Vegetation and Organisms | Time and Man | Quiz