Agriculture Reference
In-Depth Information
drier western part of the maize quadrangle, maize yields (summer crop/summer rainfall) are,
during normal years and especially during low-rainfall years, higher (and more reliable) on soils
with Soft plinthic B horizons than on other soils. This horizon indicates a Þuctuating water table,
i.e., a much higher plant-extractable water reservoir in the deeper soil layers. In the higher-rainfall
eastern parts of the quadrangle, maize does not perform as well on the soils with Soft plinthic
B horizons as on the better drained soils, because the former tend to be too wet during the
growing season. In these areas, the soils with the Soft plinthic B horizons are the best wheat
(winter crop grown totally on stored water) soils, however, and outyield the Ñbetter soils.Ò From
a land suitability evaluation point, it is very important to distinguish correctly between Soft
plinthic B horizons and the following situations, which do not qualify as Soft plinthic B horizons:
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Horizons with abundant low chroma (dull) yellow, usually streaky and not vesicular, mottles. These
indicate unfavorable, excessively wet conditions.
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Uniformly red horizons with abundant iron or iron-manganese concretions. Absence of gray colors
indicates that these horizons are not under the inÞuence of Þuctuating water tables, and thus much
drier than Soft plinthic B horizons.
¤
Light gray horizons with abundant iron or iron-manganese concretions that have formed from the
weathering of ferricretes (hard plinthite; petroplinthite). These horizons have not formed under the
inÞuence of a Þuctuating water table. They have extremely poor physical properties (being hard-
setting when dry) and are very infertile. Hubble et al., (1983) also refer to such infertile Ñyoung
soils formed in lateritic detritus and exposed remnants of laterite proÝle following dissection.Ò
horizon Ñis one of minimal development of an illuvial B
horizon in weathering rockÒ (MacVicar et al.
The concept of the
Lithocutanic B
1977). It is typically characterized by prominent
tonguing of soil into partly weathered rock. It often shows clear signs of illuviation in the form of
clay skins, which are often deposited around a small stone. In a country where shallow soils
predominate, it is important to distinguish soils with these horizons from other shallow soils,
because these horizons offer at least some room for root development and for additional storage
of plant-extractable water. This type of material is classiÝed as a Lithocutanic B horizon only if it
occurs directly under an Orthic A, Humic A, or Melanic A or E horizon. Soils of the Glenrosa
form (Orthic A over a Lithocutanic B) are possibly the most widespread soils which are found in
South Africa. We are consequently so used to them that they are not regarded as special. We were,
therefore, very surprised when Hari Eswaran, at the time Chairman of Commission V of the then
ISSS, in 1993 insisted that we must include it when an international soil classiÝcation workshop
is held Ñbecause no provision is made for it in the international classiÝcation systems.Ò An example
of a Glenrosa soil likewise attracted intense interest and debate during the 1996 WRB workshop
in South Africa. During the workshop we noted that the WRB Working Group classiÝed a non-
calcareous series of our Mayo form, which has a Melanic A (mollic horizon) over a Lithocutanic
B horizon as a Glossic Phaeozem, the ÑGlossicÒ indicating the tonguing nature of the horizon.
Once all the diagnostic horizons in a soil proÝle have been identiÝed, the diagnostic horizon
sequence of the soil can be determined. Once the diagnostic horizon sequence has been deter-
mined, the soil form is determined by using the ÑKey to the soil formsÒ in the classiÝcation topic
(Table 16.1). Thus no reading through a descriptive key is required to determine the soil form.
Each soil form was given the name of a place, normally where the Ýrst good example of such
soil was identiÝed, e.g., Hutton, Clovelly, etc.
The key to soil forms also indicates the page on which a matrix table with the series for each
form is given, with a color photograph of a typical proÝle for that form on the opposite page. The
three types of soil properties most widely used as series differentiae in the South African binomial
system were texture, degree of leaching/weathering, and presence or absence of lime. It must be
kept in mind that these were not the only properties used and that they were not used in all soil forms.
Two textural parameters were used, namely, clay content and sand grade. The clay content
classes were 0Ï6%, 6Ï15%, 15Ï35%, 35Ï55%, and >55%. Not all the classes were used in all
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