To which horizons does the term 'massive' relate regarding horizon classification?

The term 'massive' in soil horizon classification specifically relates to densic horizons. Densic horizons are characterized by a high degree of soil compaction and consolidation, resulting in a lack of significant soil structure. This condition causes the horizon to behave as a single unit rather than consisting of discrete soil aggregates, hence the designation 'massive.' These horizons often develop in soils with high clay content or where physical processes have led to substantial compaction, which can restrict root penetration and water movement.

In contrast, the other horizon types mentioned in the options do not share this characteristic of being massive. Cambic horizons may have some structure and are those that show alteration due to weathering but don't fit the massive designation. Spodic horizons are typically characterized by illuvial accumulation of organic matter and aluminum and iron oxides, displaying a layer with distinct features rather than a uniform, massive aspect. Glossic horizons are influenced by processes that create distinct features, often associated with variations in color or texture, rather than being predominantly massive in nature. Thus, densic horizons correctly align with the term 'massive' in horizon classification.