When observed in vertical section, soil profiles nearly always have layers.
A soil horizon is a layer developed by soil forming processes that looks or feels different to from the layers below or above it. Over time, this usually results in a soil with four main horizons –
- surface soil,
- sub-surface soil,
- sub-soil and
- partially weathered or original parent material.
Soil scientists label these horizons the A1, A2, B and C horizons; together they form the soil profile.
Not all soils can be described by these four soil horizon descriptions, these are also designated by a capital letter sometimes followed by numerals. Most soils in the South East have at least three distinct layers.
The topsoil, as the ‘A’ horizons are commonly referred, are the uppermost part of a soil (often the cultivated layer) and contains the majority of organic matter, plant available nutrients and soil organisms. The depth of this layer can vary and is influenced by soil type, management and climate. The A2 horizon is usually paler in colour from the A1 horizon. It can have less organic matter, sesquioxides, and silicate clay.
The sub-soil or ‘B’ horizon often differs from the A horizon in colour, texture and structure. In many soils, the B horizon contains higher amounts of clay than the A horizon, and the depth and water holding capacity of this horizon greatly affect the value of the soil for plant growth.
The C horizon is parent material relating to the geological formation of the landscape within which the soils have formed. Parent materials are mainly sedimentary derived materials in the South East. Not all horizons are present in every soil. For example, in ‘newer’ soil or under limited rainfall, the B horizon may not yet have developed, restricting the storage of plant nutrients and moisture (Figure 3a). This compares to a soil with adequate depth and structure in the A and B horizons capable of storing sufficient water and nutrients for plant growth (Figure 3b).
Figure 3 a) shallow soil without a B horizon has low water storage and can restrict root growth and b) deeper soil with all three horizons with no physical constraints to root growth.
Most of the information for this page was kindly borrowed from the Soil Health Knowledge Bank website.