Agriculture Reference
In-Depth Information
entiation between the (ÑyoungÒ) Neocutanic B horizon and the (ÑmatureÒ) Pedocutanic B horizon
is correct.
A case in which the morphological approach of the South African system was (and still is)
far superior to the two big international systems is that of the B horizon of the ÑsolonetzicÒ soils.
In the South African system, the two key requirements for a ÑPrismacutanic B horizonÒ are:
¤
An abrupt transition from the overlying horizon in terms of texture, structure, and consistency (or
at least two of them)
¤
Prismatic or columnar structure
There are numerous soils with typical solonetz morphology and, more importantly, solonetz
behavior, in which the B horizons do not qualify as natric horizons because they do not have ESP
values of 15% or more at some depth in the proÝle. Many have the ÑancillaryÒ requirement of
lopsided Ca:Mg ratios, but some do not even have this. ÑLow sodium solonetzesÒ are also found
in other countries in the Ñthird major soil region,Ò such as Australia and Zimbabwe (Nyamapfene,
1991), and have also been reported by Soviet soil scientists to be ÑcommonÒ in parts of the former
Soviet Union (Laker, 1997). The subsoils of soils with typical solonetz morphology are notorious
for their dispersivity and extreme susceptibility to erosion, especially gully erosion and piping
(tunneling). It is, therefore, very important to classify these soils together, irrespective of the ESP
of the Prismacutanic B horizon. Nyamapfene (1991) pointed out that Ñrigorously deÝned morpho-
logical requirementsÒ would be needed in such a case. This is exactly what was done in the South
African classiÝcation system: Structured B horizons that occur under an abrupt transition, but do
not have prismatic or columnar structure, are not classiÝed as Prismacutanic B horizons but as
Pedocutanic B horizons. Likewise, structured B horizons that have prismatic structure, but are not
under abrupt transitions, are classiÝed as Pedocutanic B horizons, not as Prismacutanic B horizons.
Two horizons that deserve special attention, because of their uniqueness, are the
Soft plinthic
B
and
Lithocutanic B
horizons.
The
Soft plinthic B
horizon is a horizon that is characterized by the presence of abundant
vesicular high chroma
(bright red or reddish brown and/or bright yellow or yellow-brown) mottles
in a matrix that has at least some gray areas in it. The cores of the mottles are often black, and
occasionally some of the mottles may be black, indicating the presence of manganese oxides.
Some mottles may be hard in the dry state and some individual mottles might have hardened
into iron or iron-manganese concretions. The presence of gray colors in the matrix and the
localization of ferric oxides (indicated by the vesicular nature and bright red or yellow colors
of the mottles) indicate that this horizon is formed in a zone of the soil proÝle that is subject to
a Þuctuating water table (alternate wetting and drying).
Although the Soft plinthic B horizons form under the same conditions as the plinthite of Soil
Taxonomy (Soil Survey Staff, 1999) or plinthic horizon of the FAO/WRB systems (FAO, 1988;
WRB Working Group, 1998), these horizons
do not qualify as plinthite or plinthic horizons
because they do not harden irreversibly upon alternative wetting and drying. It was initially (in
1993) extremely difÝcult to convince the members of the WRB Working Group that such horizons
actually exist, although we South African soil scientists knew that we had several million hectares
of soils with Soft plinthic B horizons. It was only when they saw these horizons in the Ýeld
during the 1996 WRB workshop that the WRB members had to agree that it was different from
plinthite, and made provision for it as a ÑparaplinthicÒ horizon (WRB Working Group, 1998).
None of the South African soil scientists that I contacted have ever found plinthite, i.e., Ñsoft
plinthiteÒ that hardens irreversibly, in South Africa. These include P.A.L. le Roux, who did his
Ph.D. on plinthic soils (Le Roux, 1996). It should be pointed out that large areas with ÑHard
plinthic B horizonsÒ (petroplinthite) are found in South Africa.
From a land suitability and land use planning point of view, it is very important to identify
Soft plinthic B horizons correctly in soils, especially for rainfed cropping. For example, in the
