Agriculture Reference
In-Depth Information
forms. In some, the sandy classes (0Ï6% and 6Ï15% clay) were excluded, because no soils in
the form had such low clay content. In other forms, the high clay classes were excluded, because
no soils in the form had such high clay content. Three
sand grade classesÐÝne, medium, and
coarseÐwere used. Sand grades were derived from a sand grade triangular diagram similar in
concept to the well-known texture triangles (MacVicar et al.
1977). Sand grade subclasses were
distinguished only in the relatively low clay classes. This is because in these classes, the sand
grade has major effects on soil physical conditions, such as plant-available water storage capacity
and susceptibility to soil compaction or hard-setting. Where the B21 (upper B) horizon was the
series classiÝcation control horizon, sand grades were used in the 0Ï6% and 6Ï15% clay classes
only. Where an Orthic A or E horizon was the control horizon for series classiÝcation, sand
grades were used in the 0Ï6%, 6Ï15%, and 15Ï35% clay classes.
Although the concept of Ñcontrol horizonsÒ was nowhere mentioned in the classiÝcation
system, it was in fact applied. Some examples are the following:
,
¤
soils with Red apedal B or Yellow-brown apedal B horizons, the B21 (upper B) horizon was
the horizon which had to meet certain requirements with regard to the two textural parameters and
degree of leaching/weathering (where the latter was applicable).
In
all
¤
soils with E horizons, the E horizon had to meet certain requirements with regard
to the two textural parameters and was the control horizon, although in some cases certain properties
of the underlying horizon were also used in series classiÝcation.
In the case of
all
¤
soils with Melanic A (mollic) horizons, properties of the A horizon, including
clay content classes, were used for series classiÝcation.
In the case of
all
Three leaching/weathering classes were distinguished. These were deÝned in terms of the
amount of exchangeable (i.e., not including soluble) bases (Ca, Mg, K, and Na)
,
as expressed in me/100 g clay, i.e., the system did not use percentage base saturation, as is done
in Soil Taxonomy and the FAO/WRB systems. The following three classes were distinguished:
per unit clay
¤
Dystrophic: < 5 me exchangeable bases/100 g clay
¤
Mesotrophic: 5Ï15 me exchangeable bases/100 g clay
¤
Eutrophic: > 15 me exchangeable bases/100 g clay
These criteria were applied to the upper parts (B21 horizons) of the Ñhigh qualityÒ B horizons,
namely, the Red apedal B, Yellow-brown apedal B, and Red structured B horizons.
Table 16.2 gives the series classiÝcation matrix for the Hutton form, the form generally
considered to include the highest potential soils in the country. The diagnostic horizon sequence
for this form is an Orthic A over a Red apedal B horizon.
The system quickly found wide acceptance in the country, not only among soil scientists,
but also among the users of soil information. This can be attributed to the following:
¤
Its simple, straightforward structure
¤
Its emphasis on morphological features, making it easy to identify soils
¤
The fact that only a few simple routine laboratory analyses are required
¤
The fact that most of the parameters used in the classiÝcation can be directly related to some
practical land use implication
¤
The fact that no ÑdifÝcultÒ terminology was used
An advantage of the system was its ÑopenÒ nature, i.e., if new diagnostic horizon sequences
were found in nature, these could be added as new forms without disrupting the existing
classiÝcation. The same would be true if the need arose for deÝning additional diagnostic
horizons.
