Agriculture Reference
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of bleached sand. The soils have a very poor vegetative cover, further indicating that there is an
inadequate supply of organic matter to enrich the Ñspodic horizonÒ and develop the giant podzols
of the Amazon described by Jenny (1941). On the raised coastal plains around Miri, Sarawak,
further accumulation of organic matter is terminated because of the uplift. Since the uplift, the
material has hardened to form a rock-hard pan, for which there is no adequate term.
We believe that the present deÝnition of the spodic horizon can result in Spodosols that fail to
conform to the genetic concept of Spodosols. This is a situation in which similar morphology
results from distinct and different processes, and this must be alluded to in the system. The process
described earlier is not exclusive to the tropics, and could occur in other similar landscape condi-
tions. There are many ways in which the deÝnition and concept could be modiÝed to reduce the
ambiguity. Regarding large areas of soils that now, perhaps illogically, key out as Spodosols, we
propose that the deÝnition of spodic materials be revised to require the presence of active amorphous
materials. This is the unifying feature of all Spodosols. In view of the tropical evidence, it should
also be examined if the spodic horizon should always be illuvial. Subsequent to geomorphic and
laboratory studies of such soils, it would be useful to differentiate them from the classical Spodosols
of the temperate regions.
The Case of Aridic Mollisols
Soils that have a mollic epipedon, an isomesic or warmer iso soil temperature regime, and a
drier than ustic soil moisture regime are currently placed in aridic, torrertic, torripsammentic,
torriorthentic, or torroxic subgroups of Ustolls. The soil moisture regime of these subgroups requires
them to be moist in some or all parts for fewer than 90 days, and dry in some or all parts for six
tenths of a year (i.e., 220 days) or more of cumulative days. This regime is similar but not identical
to the aridic soil moisture regime, as the control section in soils with an aridic regime must be dry
in all parts for more than 180 cumulative days. The mentioned subgroups, therefore, may be more
or less dry than soils with an aridic regime. Also, there may not be too many such subgroups, as
item 8 of the deÝnition of the mollic epipedon requires that Ñsome part of the epipedon is moist
for 90 days or more (cumulative) in normal yearsÒ (Soil Survey Staff, 1999).
We are not familiar with the rationales that support these deÝnitions, but we Ýnd them somewhat
confusing and difÝcult to establish. In the absence of extensive Ýeld measurements, which would
allow a positive placement in one of the subgroups, there is the possibility that many soils are
misclassiÝed, leading to erroneous interpretations. Further, the soils are indicated as Ustolls or
Xerolls on small-scale maps, although the actual moisture regime is aridic.
In order to reduce the margin of error in taxonomy and interpretation, we propose that item 8
be deleted from the deÝnition of the mollic epipedon, that the mentioned subgroups be eliminated,
and that a suborder of Torrolls for Mollisols is established with an aridic soil moisture regime. The
proposed change would have a precedent in the Torrand, Torrert, and Torrox suborders and in the
Torriarent, TorriÞuvent, Torrifolist, Torriorthent, and Torripsamment great groups. The change
would make the system logical and would maintain the integrity of the system.
The Problem of Extremely Acid Vertisols
The suborder of Aquert was recently introduced based on extensive occurrence of such soils.
However, though the need was established and rationale was defensible, the consequences of the
change were not anticipated. There are soils with shrink-swell potential that are continuously wet
and as they did not crack, they were classiÝed as Inceptisols in the previous versions of Soil
Taxonomy. Those with a sulfuric horizon were classiÝed as Sulfaquepts, or the actual acid sulfate
soils. Some with only sulÝdic materials (the potential acid sulfate soils) are the Sulfaquents.
With the change in deÝnition of Vertisols and the introduction of Aquerts, some of these soils
are currently classiÝed as Haplaquerts or Dystraquerts, and at the subgroup level, as Sulfaqueptic
