Agriculture Reference
In-Depth Information
Dystraquerts. In central Thailand and elsewhere in Southeast Asia, there are extensive areas of
hydromorphic Vertisols that have a sulfuric horizon, and are therefore extremely acid. Their clas-
siÝcation as Sulfaqueptic Dystraquerts implies, at the great group level, that the soils have a pH
value of 4.5 or less within 50 cm depth, and, at the subgroup level, jarosite concentrations and a
pH value of 4.0 or less within 100 cm depth.
The most constraining property of the soil is its extreme acidity, and this attribute is only
considered at the subgroup level. The Asian pedologists argue that the shrink-swell nature of the
soil is less important to the use and management of the soil. As the soil is classiÝed as a Vertisol,
it is also not shown on small-scale soil maps at the great group level, and hence misleads potential
interpretations. They believe these soils should be classiÝed as Vertic Sulfaquepts. This classiÝcation
would indicate, at the great group level, that these soils have a pH value of 3.5 or less within 50
cm of the mineral soil surface.
This taxonomic change could be accomplished by amending item F in the Key to Soil Orders
to deÝne Vertisols (Soil Survey Staff, 1999) as ÑOther soils that do not have a sulfuric horizon or
sulÝdic materials within 1 m of the mineral soil surface.Ò It would further require establishing and
deÝning a Vertic subgroup of Sulfaquepts.
The Ambiguity of “Pale” Great Groups
Soil Taxonomy recognizes ÑpaleÒ great groups in the AlÝsols, Aridisol, Mollisol, and Ultisol
orders. Most of the 14 great groups have an argillic horizon, although some Palecryalfs, Paleudolls,
and Palexerolls qualify on the basis of other differentiae. The common feature of the pale great
groups with argillic horizons is Ña vertical clay distribution in which the clay content does not
decrease by as much as 20 percent from the maximum clay content within a vertical distance of
150 cm below the soil surfaceÒ (Soil Survey Staff, 1999). In the Palexeralfs and Palexerolls, this
clay distribution must occur within the argillic horizon; in all other pale great groups only depth
is diagnostic. Consequently, pale great groups may have argillic horizons that do not extend to 150
cm below the soil surface. This situation is common throughout the tropics where Ultisols and
AlÝsols are developed in clayey parent materials. In these soils, the clay distribution is a lithological
rather than pedogenetic feature. In Puerto Rico, for example, all of the 13 series of Paleudalfs,
Paleudults, and Paleustalfs are developed in clayey sediments on late-Pleistocene or younger
surfaces, and have not been found on autochthonous parent materials.
According to Soil Taxonomy (Soil Survey Staff, 1999), ÑpaleÒ is derived from the Greek word
Ñpaleos,Ò meaning old, and has the connotation of Ñexcessive development.Ò The narrative descrip-
tions in Soil Taxonomy also refer to old and stable land surfaces of mid-Pleistocene or older age,
and occasionally mention thick argillic horizons.
The current deÝnitions of pale great groups result in a pedogenetically heterogeneous group of
soils. Although the argument could be made that for practical interpretations, it is irrelevant whether
the clay is pedogenic or lithological, we believe the present deÝnitions do not conform to the
original intent of the pale concept. We propose, therefore, that they be changed to specify clay
distribution in argillic or kandic horizons that extend to a depth of 150 cm below the soil surface.
Part of the problem also stems from the fact that the lower limit of the argillic horizon is not deÝned
in Soil Taxonomy, in contrast to the upper boundary. More studies are needed to arrive at an
appropriate resolution to this issue.
The Dilemma of Paddy Soils
Paddy soils, or soil used for wetland rice cultivation, have yet to be appropriately classiÝed.
The problem is shared by all classiÝcation systems. The puddling action of management that is a
prerequisite for rice cultivation destroys all surface attributes in the upper 25 to 40 cm of the soil
surface. In addition, in these seasonally Þooded soils, the anthropic change induces changes in
