Agriculture Reference
In-Depth Information
1.2
Predominantly mineral horizons
The A horizon
The A horizon normally occurs at the surface or beneath an O or H horizon, except
in the less common situations where such profiles are buried, for example, beneath
volcanic ash or alluvial deposits. A horizons are characterised by an intimate admixture
of humified organic matter and the predominantly mineral soil. They usually appear
darker than those below due to the organic matter present, some of which may form
coatings on the mineral panicles. The organic matter of the A horizon is usually derived
directly from plant and animal remains and is incorporated into it by biological activity,
rather than through physical translocation processes.
The E horizon
The E horizon usually underlies an H, O or A horizon and shows a concentration of sand
and silt-sized fractions high in resistant materials due to the loss of such relatively
mobile components as organic matter, silicate clays, iron and aluminium minerals.
The absence of organic coatings on the resistant grains remaining frequently gives this
horizon a lighter and sometimes an ashy colour.
The B horizon
This horizon is characterised by the complete or almost complete loss of original
sedimentary or rock structure. B horizons possess one or more of the following:
(i) Accumulation of illuvial materials, such as clays or organic matter translocated
downwards from overlying parts of the profile;
(ii) The presence of residual accumulations of materials such as sesquioxides
(the oxides and oxy-hydroxides of iron and aluminium); or
(iii) Evidence of alteration of the soil material from its original state.
