Downward movement of water is drastically impeded when fine-textured soil overlays coarse-textured soil.

• A. Coarse
• B. Fine, coarse
• C. Medium over fine
• D. Fine over medium

Answer: (B)

Water moves through soil mainly through its pore spaces, and how easily it does that depends on texture. Fine-textured soils, like clay and silt, have very small pores and high capillary pull, so water moves slowly through them. When a fine-textured layer sits on top of a coarser layer, the top layer acts as a barrier to drainage. Water infiltrates easily at the surface but then encounters the fine layer, whose small pores and strong capillary forces resist downward flow. This creates a perched moisture condition at the interface and dramatically slows drainage into the underlying coarse material. That’s why the arrangement fine over coarse best explains the impeded downward movement. If the top layer were coarse or medium, the barrier to downward flow wouldn’t be as strong, since the upper layer would drain more readily into the layer below.