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The beginning of soil

Themes: The earliest soil development | More recent soils | The ages of soils worldwide | Quiz

The Earth has had a very long history of geological development, in fact the first rocks and sediments are known to be over 2,000 million years old. If you consider that our own life span is usually less than 100 years, then the first rocks are zillions of years old by comparison. Since the first rocks developed this long time ago there have been periods of relatively quiet evolution separated by periods of huge turbulence. Let us take a brief look at some of these geological periods and at how soils would have formed in them.
It is thought that soils first began to form in Pre-Cambrian times (the earliest of the geological periods) dating back over 2,000 million years. Soils then would have been quite different to those now because in Pre-Cambrian times there would have been no living creatures and no green vegetation. These earliest soils were formed in an atmosphere with little or no oxygen and consisted of green clays. There would have been no organic matter in the soils and so the soils can be considered to be sterile. Ammonite from the Devonian. Can you begin to imagine how long ago 400 million years really is? Gradually, but still some 400 million years ago, in the Devonian period, soils began to develop.

These soils were reddish and brownish in colour, indicating the presence of more oxygen in the atmosphere due to the evolution of plants capable of photosynthesis. The first soil organisms also appeared and from this period onwards living soils as we know them truly began to form.