Agriculture Reference
In-Depth Information
Thirteen diagnostic subsurface horizons and two diagnostic materials (regic sand and stratiÝed
alluvium) were deÝned (MacVicar et al.
1977). The thirteen subsurface horizons included eleven
diagnostic B horizons, plus an E and a G horizon. For each horizon, a set of speciÝcations was
given, followed by a general discussion explaining the underlying concept of the horizon and its
linkages with other horizons. To be deemed diagnostic, the upper limit of a horizon had to be
within 120 cm (the normal length of a soil auger) from the surface. Morphological features,
which could be easily observed in the Ýeld, were almost exclusively used for the identiÝcation
of different diagnostic horizons. Most of these implied certain land qualities. This characteristic
of the system greatly contributed to its success.
Two of the most important features used for the identiÝcation of diagnostic subsurface
horizons were soil color and structure. Extensive color speciÝcations were given, especially for
the so-called Red apedal B, Yellow-brown apedal B, and Red structured B horizons, in terms of
permissible hues, values, and chromas. In addition to the color speciÝcations, the colors of these
horizons had to be Ñsubstantially uniform.Ò Horizons with signiÝcant color variegation were not
permitted. This was to ensure that only perfectly drained and aerated horizons were included
under these brightly colored subsoils, which included the best subsoils from a crop production
viewpoint in the country. A consequence of this color emphasis was that a soil form such as the
Hutton form, having an Orthic A horizon over a Red apedal B horizon, included soils as diverse
as an almost pure eutrophic sand (with less than 6% clay) at the one extreme and a highly
weathered dystrophic clay at the other extreme. That is, it spanned several orders in the U.S.
Soil Taxonomy. This has drawn heavy international criticism. It should be kept in mind that the
soils in a form were separated very neatly into several texture and weathering/leaching class
combinations at series level. The most important aspect is that it worked successfully in practice.
Soil structure was another important feature used for separating certain diagnostic subsoil
horizons. Again, as in the case with the ÑchromicÒ horizons discussed above, it has important
implications with regard to land suitability for rainfed cropping or irrigation. The B horizons
with moderate to strong blocky structure have poor qualities (unfavorable for root development;
highly erodable) and must therefore be separated from their more weakly structured counterparts,
which normally have much better qualities.
An important example with regard to differentiation on the basis of structure, not only from
a land suitability point of view but also in terms of understanding pedogenesis, is the distinction
between the Neocutanic B horizon and the Pedocutanic B horizon. The concept of the Neocutanic
B horizon is that it is a young (neo = new) horizon which has developed in transported material.
The Pedocutanic B horizon is a ÑmatureÒ horizon which has developed from clay illuviation over
adequate time. The difference between the two is that the Neocutanic B horizon has structure
that is weaker than moderate, while the Pedocutanic B horizon has at least moderate to strong
structure. It is easy to perceive that in most cases Neocutanic B horizons, over a long period of
time, will reach the stage at which it can become a Pedocutanic B horizon. This sequence is
clearly seen on some narrow river terraces in South Africa, e.g., along the Tyumie River, near
Alice in the Eastern Cape. The soil on the youngest terrace on the river bank has alluvial
stratiÝcations throughout the proÝle below the A horizon. The subsoil on the second (older)
terrace has a typical Neocutanic B horizonÏalluvial stratiÝcations have been obliterated, but no
structure has developed. The subsoil on the third (oldest) terrace is clay-enriched and has strong
course blocky, and in places even prismatic, structure.
Conceptually, the Neocutanic B horizon is similar to the cambic horizon of Soil Taxonomy and
the FAO/WRB system, while the Pedocutanic B horizon could be expected to resemble the majority
of the argillic/argic horizons of the latter systems. In these international systems, however, minimal
structure requirements are reversed: The cambic horizon must have structure Ñwhich is at least
moderately developed,Ò whereas the argic horizon has no minimal structure requirement (WRB
Working Group, 1998). For our purposes, I believe that the South African approach toward differ-
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