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a distinct layer of soil, approximately parallel with the land surface, whose properties develop from the combined actions of living organisms and percolating water. Because these actions can vary in their effects with increasing depth, it is often the case that more than one horizon exists beneath the surface of any soil area, at depths ranging from only a few centimetres to several metres. One or more horizons make up what is known as the soil profile, the vertical sequence of distinct layers that is unique to each soil type.