Soils in their environment - Fundamentals of the Physical Environment - page 453

Table 18.3 Soil units of the FAO classification (1988 revision)

Symbol Soil group Main features

AC Acrisols Argillic Bt of low clay activity

AN Andosols Soils on volcanic ash

AL Alisols Argillic Bt with high activity clays and aluminium

AT Anthrosols Soils affected by human cultivation and/or disturbance

AR Arenosols Coarse-textured soils without horizons

CM Cambisols Brown earth with a weathered Bw horizon

CL Calcisols Soils with a calcic or petrocalcic horizon

CH Chernozem Soils with a black humic Ah horizon

FR Ferralsols Soils have a B horizon of sesquioxides

FL Fluvisols Raw soils on recent alluvium

GL Gleysols Wet soils with gleyed horizons

GR Greyzems Soils with humic A and argillic Bt horizons

GY Gypsisols Soils with a gypsic or petrogypsic horizon

HS Histosols Soils dominated by organic material (peaty)

KS Kastanozems Soils with humic A and calcic or gypsic B horizons

LP Leptosols Shallow soils over hard rock

LX Lixisols Soils with argillic Bt horizon of low clay activity

LV Luvisols Soils with argillic Bt of medium/high clay activity

NT Nitisols Other soils with a deep argillic Bt horizon

PH Phaeozems Humic A over varied horizons

PL Planosols Heavy texture in the B horizon

PT Plinthosols Soils with plinthite causing surface wetness

PZ Podzols Soils with podzolic Bs horizon

PD Podzoluvisols Argillic Bt with irregular top

RG Regosols Soils of unconsolidated materials

SC Solonchaks Salt-enriched soils

SN Solonetz Soils with alkali B horizon

VR Vertisols Black deeply cracking clays

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 shown in Table 18.4 .
Other horizon designations which can be used with A,
B or C horizons are:

Table 18.4 Diagnostic soil horizons in the FAO-UNESCO
soil classification

H peat (Histic)

O surface organic matter other than peat

A organo-mineral topsoil

Ah mollic A: humic with base saturation over 50%

Ae umbric A: base saturation below 50%

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)

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

Other terms commonly used to describe horizons, but for
which there are no horizon symbols, are:

Bn natric B: sodium-dominated

Bs podzolic B: sesquioxides of Fe and Al

Bh humic B: humus in-washed

gelic permanently frozen (permafrost)
plinthite soft iron-rich clay which hardens irreversibly
on drying (laterite)

C unconsolidated parent material

R hard rock