What characterizes a massive structure in soil layers?

The defining characteristic of a massive structure in soil layers is that the soil appears as a coherent solid mass, without being separated into smaller aggregates or peds. This means that when examining a massive soil structure, there will be little to no visible differentiation in the soil's composition or texture, giving it a uniform appearance.

In contrast, other options describe different soil structures. The layering of soil into distinct parts indicates a stratified structure, where different soil horizons can be identified, which is not characteristic of a massive structure. The presence of various soil textures mixed together suggests a heterogeneous mixture, rather than the uniformity found in a massive structure. Lastly, the compact state of soil in agricultural zones refers more to soil density and compaction rather than the structural form it takes; compact soil can have various structures, not just massive. Therefore, the focus on the coherence and lack of separateness into peds is what distinctly characterizes a massive soil structure.