Soil Profile and Horizons

A soil profile is the vertical sequence of horizons exposed in a soil pit, recording the cumulative effects of soil formation as a set of layers that differ in colour, texture, structure, and composition.

Definition

A soil profile is a vertical section through a soil from the surface to the underlying parent material, composed of genetically related horizons, layers roughly parallel to the surface that differ in physical, chemical, and biological properties.

Scope

This topic covers the master horizons (O, A, E, B, C, and R) and their subdivisions, the morphological properties used to describe them, and how the profile as a whole records the soil's developmental history. Reading a profile is the practical skill at the heart of soil description, mapping, and classification.

Core questions

• What are the master horizons and what distinguishes each one?
• How do colour, texture, structure, and consistence describe a horizon?
• How does the profile record additions, losses, and translocations of material?
• How is a profile described and used in soil survey?

Key concepts

• Master horizons (O, A, E, B, C, R)
• Horizon suffixes and transitional horizons
• Soil colour and Munsell notation
• Soil structure and consistence
• Solum and regolith
• Profile description

Key theories

Master horizon framework

Soils are described using master horizons, the organic O, mineral surface A, eluvial E, illuvial or weathered B, weathered parent C, and bedrock R, with suffixes recording specific features, providing a universal language for soil morphology.

Profile as a record of pedogenesis

The arrangement and properties of horizons reflect the additions, losses, translocations, and transformations that formed the soil, so the profile can be read as a history of the soil-forming environment.

Mechanisms

Organic matter accumulates at the surface to form O and dark A horizons; downward-moving water leaches and removes clay, iron, and organic matter from the E horizon and deposits them in the B horizon, while the C horizon remains little-altered parent material. Differences in colour, texture, and structure between horizons arise from these vertical transfers and from in-place weathering and biological mixing.

Clinical relevance

Reading a soil profile reveals rooting depth, drainage, fertility, and the presence of restrictive layers, information essential for crop selection, irrigation design, foundation engineering, and the classification and mapping that underpin land management.

History

The concept of the soil profile as a sequence of natural horizons emerged from Dokuchaev's school and was systematized through the 20th century into standard horizon nomenclature and field description methods used in soil survey worldwide.

Key figures

• Vasily Dokuchaev
• Nyle C. Brady
• Ray R. Weil

Seminal works

• brady2016
• soilsurveystaff2014

Frequently asked questions

What do the letters O, A, B, and C mean in a soil profile?

They are master horizons: O is surface organic material, A is the dark mineral topsoil rich in organic matter, E is a leached, light-coloured layer where present, B is the subsoil where leached material accumulates, and C is weathered parent material beneath; R denotes hard bedrock.

Why is topsoil darker than subsoil?

The surface A horizon is darker because it accumulates decomposed organic matter (humus) from plant and animal residues, whereas the deeper B and C horizons contain far less organic matter and take their colour mainly from minerals such as iron oxides.