A-horizon

A horizon

noun Geology.
the topsoil in a soil profile.
Also called zone of leaching.
Compare B horizon, C horizon.


Origin:
1935–40

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Collins
World English Dictionary
A horizon
 
n
B horizon See C horizon the top layer of a soil profile, usually dark-coloured and containing humus and from which soluble salts may have been leached

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
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
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