Soil | Definition, Importance, Types, Erosion, Composition, & Facts

Soil horizons

Soils differ widely in their properties because of geologic and climatic variation over distance and time. Even a simple property, such as the soil thickness, can range from a few centimetres to many metres, depending on the intensity and duration of weathering, episodes of soil deposition and erosion, and the patterns of landscape evolution. Nevertheless, in spite of this variability, soils have a unique structural characteristic that distinguishes them from mere earth materials and serves as a basis for their classification: a vertical sequence of layers produced by the combined actions of percolating waters and living organisms.

These layers are called horizons, and the full vertical sequence of horizons constitutes the soil profile (see the figure). Soil horizons are defined by features that reflect soil-forming processes. For instance, the uppermost soil layer (not including surface litter) is termed the A horizon. This is a weathered layer that contains an accumulation of humus (decomposed, dark-coloured, carbon-rich matter) and microbial biomass that is mixed with small-grained minerals to form aggregate structures.

Below A lies the B horizon. In mature soils this layer is characterized by an accumulation of clay (small particles less than 0.002 mm [0.00008 inch] in diameter) that has either been deposited out of percolating waters or precipitated by chemical processes involving dissolved products of weathering. Clay endows B horizons with an array of diverse structural features (blocks, columns, and prisms) formed from small clay particles that can be linked together in various configurations as the horizon evolves.

Below the A and B horizons is the C horizon, a zone of little or no humus accumulation or soil structure development. The C horizon often is composed of unconsolidated parent material from which the A and B horizons have formed. It lacks the characteristic features of the A and B horizons and may be either relatively unweathered or deeply weathered. At some depth below the A, B, and C horizons lies consolidated rock, which makes up the R horizon.

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These simple letter designations are supplemented in two ways (see the table of soil horizon letter designations). First, two additional horizons are defined. Litter and decomposed organic matter (for example, plant and animal remains) that typically lie exposed on the land surface above the A horizon are given the designation O horizon, whereas the layer immediately below an A horizon that has been extensively leached (that is, slowly washed of certain contents by the action of percolating water) is given the separate designation E horizon, or zone of eluviation (from Latin ex, “out,” and lavere, “to wash”). The development of E horizons is favoured by high rainfall and sandy parent material, two factors that help to ensure extensive water percolation. The solid particles lost through leaching are deposited in the B horizon, which then can be regarded as a zone of illuviation (from Latin il, “in,” and lavere).

Soil horizon letter designations
Base symbols for surface horizons
O	organic horizoncontaining litter and decomposed organic matter
A	mineral horizon darkenedby humus accumulation
Base symbols for subsurface horizons
E	mineral horizon lighterin colour than an A or O horizon and depleted in clay minerals
AB or EB	transitional horizonmore like A or E than B
BA or BE	transitional horizonmore like B than A or E
B	accumulatedclay and humus below the A or E horizon
BC or CB	transitional horizonfrom B to C
C	unconsolidated earthmaterial below the A or B horizon
R	consolidated rock
Suffixes added for special features of horizons
a	highly decomposedorganic matter
b	buried horizon
c	concretions or hardnodules (iron, aluminum, manganese, or titanium)
e	organic matter ofintermediate decomposition
f	frozen soil
g	gray colour with strongmottling and poor drainage
h	accumulation of organicmatter
i	slightly decomposedorganic matter
k	accumulation ofcarbonate
m	cementation orinduration
n	accumulation of sodium
o	accumulation of oxidesof iron and aluminum
p	plowing or otheranthropogenic disturbance
q	accumulation of silica
r	weathered or softbedrock
s	accumulation of metaloxides and organic matter
t	accumulation of clay
v	plinthite (hardiron-enriched subsoil material)
w	development of colour orstructure
x	fragipan character(high-density, brittle)
y	accumulation of gypsum
z	accumulation of salts

The combined A, E, B horizon sequence is called the solum (Latin: “floor”). The solum is the true seat of soil-forming processes and is the principal habitat for soil organisms. (Transitional layers, having intermediate properties, are designated with the two letters of the adjacent horizons.)

The second enhancement to soil horizon nomenclature (also shown in the table) is the use of lowercase suffixes to designate special features that are important to soil development. The most common of these suffixes are applied to B horizons: g to denote mottling caused by waterlogging, h to denote the illuvial accumulation of humus, k to denote carbonate mineral precipitates, o to denote residual metal oxides, s to denote the illuvial accumulation of metal oxides and humus, and t to denote the accumulation of clay.