A horizon

The top layer of a soil profile just under the surface O-horizon, usually dark-colored and containing orgagnic humus and from which soluble mineral salts may have been leached.

Citation: 
A horizon. (n.d.) Collins English Dictionary – Complete and Unabridged, 12th Edition 2014. Retrieved August 14 2016 from http://www.thefreedictionary.com/A+horizon
English

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