Agriculture Reference
In-Depth Information
II.1
SOIL HORIZONS AND THEIR NOMENCLATURE
Because of the influences of soil processes acting over time, the profile usually becomes
vertically differentiated into a variable number of master horizons or layers that reflect
the dominance of particular processes (Figure II.1). The genetic horizons and layers may
differ in morphology, texture, the types of clay and other minerals, organic and nutrient
reserves. The term 'genetic' implies that the horizon concerned has developed under
the influence of the soil-forming processes considered later in this chapter. One horizon
(the C) and one layer (the R) may occur beneath the more superficial horizons. The C
horizon has been less influenced by pedogenetic processes and the R layer only slightly
or not at all. In some soils, morphological and textural differentiation is slight with only
gradual and sometimes continuous changes occurring with increasing depth.
Various schemes exist for naming the master horizons. However, those used in
the two principal international soil classification schemes, Soil Taxonomy (Soil Survey
Staff, 1999) the FAO-UNESCO scheme (FAO-UNESCO, 1990) and its potential
successor (The World Reference Base For Soil Resources / ISSS Working Group RB,
1998) are, with the exception of the H horizon, consistent at the master horizon level and
will be used here. Full definitions of the master horizons recognised in the above
systems are available in FAO-UNESCO (1990) and Guthrie and Witty (1982).
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