Agriculture Reference
In-Depth Information
a speciÝc diagnostic horizon sequence (MacVicar, 1978). The two-tier form-series approach was
also used for the classiÝcation of soils in the Tugela basin survey (Van der Eyk et al.
1969).
ÑThe Tugela Basin classiÝcation met with considerable success. It gave planners, soil scien-
tists, agronomists and farmers a good understanding of the soils of the area, non-soil scientists
found soil identiÝcation easy, and its terminology was rapidly and widely taken into useÒ
(MacVicar, 1978). This success led to the establishment of a soil classiÝcation working group
in the early 1970s under the chairmanship of Ýrst De Villiers, then MacVicar, with the purpose
of developing a form-series classiÝcation for the whole country. This culminated in the publi-
cation of
,
Soil ClassiÝcation: A Binomial System for South Africa
(MacVicar et al., 1977).
SOIL CLASSIFICATION: A BINOMIAL SYSTEM FOR SOUTH AFRICA
As its name implies, the system was meant for soil classiÝcation in South Africa, from
national level right down to farm level. It therefore did not need a large number of categories
(tiers) of classiÝcation, in contrast to the case with Soil Taxonomy (Soil Survey Staff, 1975;
1999). Because it had to be used in detailed surveys at farm level, its categories had to be low
ones. The system was therefore constructed to have two categories in the lowest tiers. In line
with the approach elsewhere, the soil series is the lowest category in the system. Related series
are grouped into soil forms.
Each soil form has a characteristic diagnostic horizon sequence (MacVicar et al.
1977). In
some cases, soils with similar diagnostic horizon sequences are grouped into different forms on
the basis of the presence of other materials in the soil proÝle. Thus the Valsrivier and Swartland
forms both have Orthic A horizons over Pedocutanic B horizons, but in the case of the Valsrivier
the underlying material is Ñunconsolidated materialÒ (drift), while in the case of the Swartland
it is saprolite.
Well-deÝned diagnostic horizons form the basis of the classiÝcation system. Diagnostic
horizons are all designated in terms of classical traditional master horizon coding, i.e., each
diagnostic horizon is indicated as being an A, E, B, or G horizon, e.g., Orthic A, Melanic A,
Red apedal B, Prismacutanic B, etc. In the classiÝcation book, it is clearly stated that, in the
procedure for the identiÝcation of soils according to this system, the Ýrst step is the demarcation
of the master horizons present in the proÝle. This is in direct contrast to the big international
soil classiÝcation systems, such as Soil Taxonomy (Soil Survey Staff, 1975; 1999) and the FAO
system (FAO, 1990), which have steered away completely from giving diagnostic horizons any
master horizon labels.
The system distinguished Ýve diagnostic topsoil horizons: Organic O, Humic A, Melanic A,
Vertic A, and Orthic A. The Humic A, Melanic A, and Orthic A are similar in concept to the umbric,
mollic, and ochric epipedons/horizons, respectively, of Soil Taxonomy (Soil Survey Staff, 1999)
and the FAO/WRB systems (FAO, 1988; WRB Working Group, 1998). The latter names were not
adopted, because in the case of Humic/umbric and Melanic/mollic, the respective concepts are
similar, but the horizons are not identical and the deÝnitions differ somewhat. ÑOrthicÒ is thought
to be a better description of the ÑordinaryÒ (ÑorthodoxÒ) topsoil horizons, which do not have any
special features as do the others, because these horizons are not all ÑpaleÒ (ÑochricÒ).
The Vertic A horizon was a special feature of the system. Although the
,
7
Approximation
,
th
Soil Taxonomy, and the FAO system recognize Vertisols, they do not have ÑverticÒ diagnostic
horizons. The WRB system (WRB Working Group, 1998) has a vertic diagnostic horizon, but
deÝnes it as a clayey
horizon. I believe the latter is a mistake, and that the South
African approach is the correct one. Surely the classical concept of a Vertisol is a soil in which
the
subsurface
has strong swell-shrink properties, which affect the land use suitabilities and man-
agement requirements of these soils, setting them aside from other soils which may have clayey
topsoils. This aspect will be debated further later in this chapter.
topsoil
