Biology / Unit 11 Bio 2579 short lesson, about 20 seconds
Soil Horizons: O, A, B, C
Captions available
The idea
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.
In practice
Topsoil, the A horizon, is where most microbial activity and root uptake occurs, despite often being only inches deep.
Where people slip
An immature soil may show only O, A, and C — the full profile takes considerable time to develop.
Your place on the trail
Every lesson rests on earlier ones and opens later ones. Tap any to jump.
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