Soil Horizons: O, A, B, C
This video presents the same text shown beside it, spoken and on screen. It adds nothing the text does not say.
State
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.
Show
Topsoil, the A horizon, is where most microbial activity and root uptake occurs, despite often being only inches deep.
Watch for
An immature soil may show only O, A, and C — the full profile takes considerable time to develop.