topsoil

Topsoil refers to the uppermost mineral layer of the soil profile, usually designated as the A horizon. It is the part of the soil richest in organic matter derived from decomposed plant and animal residues and contains most living roots, microorganisms, and soil fauna. Topsoil forms through the weathering of parent material and the accumulation of organic materials, with development influenced by climate, organisms, surface processes, drainage, and time. This layer supports most plant growth and biological activity.

The depth and composition of topsoil vary widely. In cultivated soils it commonly ranges from about 5 to 20 centimeters (2 to 8 inches) deep, though natural soils can be shallower or deeper. It is a mixture of mineral particles—sand, silt, and clay—with varying amounts of organic matter that confer structure, nutrient-holding capacity, and water-holding ability. Its color is typically darker than lower horizons due to organic matter.

Topsoil contains most of the soil’s nutrients and biology, and its chemical and physical properties depend on organic matter content and mineral composition. It may grade into an E horizon if leaching occurs or into a B horizon where clays and oxides accumulate. It is distinct from subsoil and from parent material.

Because topsoil is vulnerable to erosion and compaction, management practices aim to preserve and restore it. Conservation tillage, mulching, cover crops, crop rotation, contour farming, terracing, windbreaks, and maintaining vegetative cover reduce erosion and organic matter loss. When degraded, restoration may require adding organic amendments and careful reestablishment of vegetation.