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The soils has many layers or horizon for example. Horizon O, A, B,C,E and R. Top layer or A horizon of the mineral soil horizons is also called topsoil. At the top of topsoil, a dark decomposed organic matter called humus or Horizon O. Whenever decomposing organic matter occurs it adds another layer of humus. Therefore, Humus or Horizon O is the layer of the soil which is most newly formed.