Agriculture Reference
In-Depth Information
hydrologic properties of the soil, altering the innate pedological processes. Seasonally Þooded soils
are water saturated for 4 to 6 months of the year, and are subject to the climate-induced moisture
stress for the remaining part of the year. The use of soil moisture regimes, as deÝned in Soil
Taxonomy, is inappropriate. During some parts of the year, the soil has an aquic soil water condition,
while during the remaining parts, the soil moisture regime is undeÝned (as it would not meet the
deÝnition of ustic, xeric, aridic, or udic). This periodicity in soil water conditions, either natural
or human-induced, affects land use and directs the mode of genesis of the soil.
Soils converted to paddy soils from former well-drained upland soils have the previously stated
difÝculties in classiÝcation, hence application of the system for use and management. An example
is the sequence of soils studied by Hseu and Chen (1996). The unmodiÝed upland soil developed
on basic materials is a red Paleudult. After about 30 years of rice cultivation, iron is removed from
the upper 30 cm of the soil and the reduction and removal of the iron proceeds in the subsoil as
vertical streaks. The present-day subsoil has the morphology of plinthite, but would not qualify as
plinthite as deÝned by Daniels et al. (1978). This is a clear example of anthropogenesis, except
that the process is not implied by the classiÝcation.
The solution rests in the concept and deÝnition of anthropic processes and speciÝcally as it
applies to paddy soils. It also requires the identiÝcation and deÝnition of contrasting soil moisture
conditions, as in the case of paddy soils.
The Predicament of the Kandic Horizon in Oxisols
The kandic horizon (Beinroth et al., 1986; Moorman, 1985), which was introduced on the
recommendation of the International Committee on the ClassiÝcation of Soils with Low Activity
Clays (ICOMLAC), is a hybrid between an argillic horizon and an oxic horizon. Its introduction
constitutes the revival of an old concept, and it may be appropriate to cite what Guy Smith (1965)
had to say about this issue: ÑIn the 7
Approximation we tested the possibility that a single horizon
might be both an oxic and an argillic horizon. This was not well received, so we have abandoned
the idea.Ò Apparently, this notion has now, in the form of the kandic horizon, become acceptable
again.
As a consequence, soils that have 40% or more clay in the surface horizon and a kandic horizon
that has the weatherable-mineral properties of an oxic horizon, with an upper boundary within 100
cm of the mineral soil surface, key out as Oxisols. Unlike the argillic horizon and most other
horizons, which have a morphogenetic connotation, the origin of the kandic horizon is attributed
to several processes.
As far as we know, no systematic testing of the implications of the kandic horizon has been
conducted, and we therefore have no factual basis for assessing the faults and virtues of the new
horizon. One consequence was that it resulted in the reclassiÝcation of many Ultisols and some
AlÝsols as Oxisols, and thus in a signiÝcant increase in the area occupied by Oxisols. In Puerto
Rico, for example, the Oxisol area increased from 2.5 to 6.2%. (The reclassiÝcation may not stand
close scrutiny, however, as the amount of weatherable minerals was not determined).
There is also an inconsistency in the deÝnition of the kandic horizon, which states that Ñthe
percentage of clay is either measured by the pipette method or estimated to be 2.5 times [percentage
of water retained at 1500 kPa tension minus percent organic carbon], whichever is higher, but not
more than 100Ò (Soil Survey Staff, 1999). However, the default coefÝcient for the oxic horizon is
3. (We have not been able to establish the reason for this discrepancy.) Further, to qualify for
Oxisols, a kandic horizon must have more than 40% clay in the surface horizon. Yet there are soils,
for example the Rengam series in Malaysia (Paramanathan, 1977), which have less than 40% clay
in the surface horizon and a kandic horizon with distinctly oxic properties, but nevertheless classiÝed
as Ultisols. The Rengam Series of Malaysia lacks cutans (as conÝrmed by micromorphological
analysis by Eswaran and Wong, 1978) to a depth of 18 m. Many such studies have shown that
th
