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A horizon

noun, Geology
1.
the topsoil in a soil profile.
Also called zone of leaching.
Compare B horizon, C horizon.
Origin
1935-1940
1935-40
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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British Dictionary definitions for A horizon

A horizon

noun
1.
the top layer of a soil profile, usually dark-coloured and containing humus and from which soluble salts may have been leached See B horizon, C horizon
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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A horizon in Science
A horizon  
In ABC soil, the uppermost, darkest zone that is rich in organic matter. The upper section of the A horizon usually contains humus along with plant and animal matter in varying stages of decay. The middle section usually contains a high concentration of quartz or other minerals that remain following the leaching away of clay, iron, and aluminum. The lower section is typically transitional in nature between the A horizon and the B horizon. Also called zone of leaching.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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