In a soil profile, the C horizon represents weathered (but not fully pedogenically altered) parent material, where primary chemical weathering of fresh mineral surfaces continues to actively release dissolved ions (solutes) into percolating water — this is the active weathering front feeding solutes upward and outward into the rest of the soil-water system.
Of the horizons shown — O (organic surface litter), A (topsoil, mixed organic-mineral, subject to leaching/eluviation), B (subsoil, zone of illuviation where leached material accumulates),...
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