Soil classification according to the FAO–UNESCO Soil Map of the World - Fundamentals of the Physical Environment - page 868

In addition to defining soil classes, soil classifications also define diagnostic horizons
which enable the orders and units to be identified. The FAO-UNESCO scheme for
naming horizons is as follows:
H

Peat (Histic)

O Surface organic matter other than peat

A organo-mineral topsoil

Ah mollic A humic with base saturation over 50 per cent

Ae umbric A base saturation below 50 per cent

A ochric A light coloured

Ap disturbed by ploughing

E Eluvial (leached and bleached)

B Subsoil horizon formed by weathering and/ or illuviation (in-washing)

Bw cambic B weathered

Bt argillic B clay in-washed

Bn natric B sodium-dominated

Bs podzolic B sesquioxides of Fe and Al

Bh humic B humus in-washed

C Unconsolidated parent material

R Hard rock

Other horizon designations which can be used with A, B or C horizons are:
k calcic horizon secondary lime (e.g. Bk)

g gleyic horizon waterlogging (e.g. Ag, Bg, Cg)

m cemented horizon iron pan (Bms), petrocalcic (Cmk)

y gypsic horizon secondary gypsum (e.g. Cy)

z salic horizon soluble salts (e.g. Bz)

x fragipan compact and brittle (e.g. Cx)

Other terms commonly used to describe horizons, but for which there are no horizon
symbols, are:
gelic permanently frozen (permafrost)

plinthite soft iron-rich clay which hardens irreversibly on drying (laterite)

vertic expanding clay with signs of swelling and shrinking (cracks, hummocks)