5.1.17 (HL)—A horizon (topsoil)

Topsoil combines humus, roots, microbes, oxygen, water and nutrients; losing its structure cannot be replaced by fertilizer alone.

Fertilizer adds selected nutrients but not depth, pore habitat, organic carbon, water storage or the biological networks that regulate release and decomposition.

Adding nitrogen to eroded subsoil may raise growth briefly, but poor roots and low water retention still limit the crop.

Cover, organic inputs and erosion control rebuild structure and biology over time.

Fertilizer can supplement soil; it cannot recreate lost topsoil volume and function quickly.