A horizon


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Related to A horizon: C horizon, B horizon

A horizon

the uppermost zone of soil, containing leaf-litter at the top, raw humus in the middle, and a dark-coloured humus layer at the bottom. See also B HORIZON.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005