Agriculture Reference
In-Depth Information
identiÝcation of Ñhard-settingÒ soils and the possible role of silt, in combination with Ýne sand,
in these subsoils (Turner, 2000).
The work of the Working Group culminated in the publication of ÑSoil ClassiÝcation: A
Taxonomic System for South AfricaÒ in 1991 to replace the 1977 binomial system as South
AfricaÔs soil classiÝcation system (Soil ClassiÝcation Working Group, 1991).
SOIL CLASSIFICATION: A TAXONOMIC SYSTEM FOR SOUTH AFRICA
Soil ClassiÝcation: A Taxonomic System for South Africa is again a system with two
categorical levels (Soil ClassiÝcation Working Group, 1991). The soil form is again the higher
category, as in the 1977 binomial system, but now the soil family is the lower category, instead
of the soil series. The system was supposed to have had the soil series as lowest (third) category,
but since no series could be deÝned, this category fell away. This is because the majority decision,
with which I totally disagree, was that series should represent natural soil bodies in terms of
their textural ranges. Series classiÝcation could not be done on this basis, because the data
required to do it were simply not available, and are still not available.
In the 1991 taxonomic system, the upper boundary of a horizon had to be within 150 cm
from the soil surface to qualify as a diagnostic horizon, and not within 120 cm, as in the 1977
binomial system. Master horizon designations were changed to Ýt in with the newer international
trend. The new designations were as follows (with the old equivalents used in the 1977 system
in parentheses): A(A1), E(E or A2), AB(A3), BA(B1), B(B2), BC(B3). As in the 1977 binomial
system, each soil form still had a characteristic diagnostic horizon (or diagnostic material)
sequence. In the 1991 taxonomic system, there are 73 soil forms, as opposed to 41 in the 1977
binomial system.
With one exception, i.e., the Gleycutanic B horizon, which was merged with the G horizon,
all the diagnostic horizons and materials of the 1977 system were retained. Slight changes to
the deÝnitions of some were made, however. The following are some of the most important
changes:
¤ Plasticity index was made one of the criteria that can (on its own) be used to distinguish between
Vertic A and Melanic A horizons.
¤ In the 1977 system, the E horizon had to meet the ÑgrayÒ color criteria in either the dry or the
moist state. In the 1991 system, it must meet these criteria in the dry state.
¤ÑYouthful, cross-bedded aeolian sands, usually in the form of dunes, with color that is predomi-
nantly red or yellow, no longer qualify as red or yellow-brown apedal B horizons; provision is
made for these regic sands in the Namib formÒ (Soil ClassiÝcation Working Group, 1991).
Meanwhile, the deep light gray sands, with or without lamellae, of the Fernwood form are now
considered to be E horizons and not regic sand, as was the case in the 1977 classiÝcation.
A number of new diagnostic horizons and materials have been added. Of these, the following
deserve mentioning:
¤
The placic pan, found in some Podzol B horizons.
¤
The Neocarbonate B horizon, including all the calcareous
that were formerly
classiÝed as calcareous Red or Yellow-brown apedal B or Neocutanic B horizons.
apedal B horizons
¤
The Soft carbonate (hypercalcic) and Hardpan carbonate (petrocalcic) horizons. These are horizons
of which the morphology is totally dominated by the lime. From a practical land-use point, in
marginal rainfall areas it is very important to distinguish between the two. It has been found that
large quantities of plant-extractable water are stored in the Soft carbonate horizons. Because of
this, much more consistent, and in the long term considerably higher average, maize yields are
obtained under these rainfall conditions on these soils than on the ÑbetterÒ deep soils in the same
