Biology / Bio 2579 · Atom · ~20 seconds
Soil Horizons: O, A, B, C
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A mature soil profile layers into four horizons: O's fresh humus, A's mixed topsoil, B's accumulated fine material, and C's parent material.
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Topsoil, the A horizon, is where most microbial activity and root uptake occurs, despite often being only inches deep.
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An immature soil may show only O, A, and C — the full profile takes considerable time to develop.