Agriculture Reference
In-Depth Information
areas. The Hardpan carbonate horizons do not have this advantage because they are impenetrable
to water and roots.
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Dorbank (Afrikaans: dor = very dry; bank = pan). The deÝnition of the dorbank is like that of the
duripan of Soil Taxonomy and the FAO/WRB systems. Its name indicates that it is associated with
extremely arid conditions, typically drier areas than where Soft carbonate or Hardpan carbonate
horizons are found.
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Man-made soil deposits.
Laker (2000) gives very short descriptions of the basic concepts of all the diagnostic horizons
and materials distinguished in the 1991 South African system. (He also discusses the distribution,
main properties, and land-use suitabilities of the main South African soils.) The key to the
identiÝcation of soil forms in the 1991 system (Table 16.3) is structured differently from that of
the 1977 system (Table 16.1), and is more logical and user-friendly than the latter.
With the exception of the texture criteria, which were eliminated, all the soil properties used
for series differentiation in the 1977 system are used for family differentiation in the 1991 system.
Twelve new criteria were added for family differentiation, i.e., the number of family criteria is
more than double the original number of series criteria. Some of the most noteworthy new criteria
are the following:
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Differentiation between bleached
in some soil forms. Some
Orthic A horizons have bleached (light gray as deÝned for E horizons) colors. These are not E
horizons that have been exposed at the surface. They are found mainly in relatively dry areas, with
gray mudstones or shales as parent materials. The bleached A horizons have very poor physical
properties, being especially prone to severe surface sealing (crusting). For this reason, it is important
to recognize them and to classify these soils separately from Orthic A horizons that are not bleached.
This has since also been recognized in the WRB system (WRB Working Group, 1998).
and nonbleached
Orthic A horizons
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Gray and
Some E horizons assume uniform high chroma yellow colors, as deÝned
for the Yellow-brown apedal B horizon, in the moist state. These horizons are classiÝed as E horizons
on the basis of their light gray colors in the dry state. At family level, a distinction is made between
those that remain gray in the moist state and those that become yellow in the moist state. After the
WRB workshop in South Africa in 1996, this phenomenon has also been recognized in the deÝnition
of the albic horizon in the WRB system (WRB Working Group, 1998).
yellow E horizons.
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Luvic B horizons.
Some apedal B horizons (i.e., some Red apedal B, Yellow-brown apedal B,
Neocutanic B, and Neocarbonate B horizons) have markedly more clay in B horizon than in the
overlying A or E horizon. The luvic criterion was introduced to distinguish these horizons. To
qualify as luvic, the following conditions must be met (quoted from Soil ClassiÝcation Working
Group, 1991):
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When any part of the A or E horizon has 15% clay or less, the B1 horizon must contain at
least 5% more clay than the A or E.
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When any part of the A or E has more than 15% clay, the ratio of clay percentage in the B1
to that in the A or E must be 1:3 or greater.
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Subangular/Ýne angular and medium/coarse angular structure
in Pedocutanic B and Red struc-
tured B horizons. By deÝnition the blocky structure in these horizons must be at least moderately
developed in the moist state. Horizons with medium to coarse angular blocky structure are more
common than those with subangular/Ýne angular blocky structure, and are much less suitable for
crop production than the latter. In fact, soils with such horizons are usually rated unsuitable for
crop production, both rainfed and under irrigation. Soils with Pedocutanic B horizons with medium
to coarse blocky structure are also highly erodable.
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Hard and not hard Lithocutanic B horizons
(and saprolite). Because they are found in such shallow
soils, Lithocutanic B horizons are important for root development and storage of plant-available
water. Its capacity for water storage and its space for root development are determined by how
much actual soil the horizon has. If more than 70% by volume of a Lithocutanic B horizon Ñis
bedrock, fresh or party weathered, with at least hard consistence in the dry, moist and wet states,Ò
the horizon is classiÝed as ÑhardÒ (Soil ClassiÝcation Working Group, 1991). Because of the
smaller rooting volume and lower water storage capacity, the ÑhardÒ Lithocutanic B horizons are
less favorable for plant growth than the Ñnot hardÒ ones.
