Tuesday, 11 July 2006

Humid Pampa - Argentina: Consequences of the Lack of a National Soil Classification System.

Mabel Susana Pazos and Nuria Roca. Facultad de Agronomía - UNCPBA, Republica de Italia 780, CC 47, Azul, 7300, Argentina

Argentina never had a national soil classification system; consequently Soil Taxonomy (ST), at that time the 7th Approximation, was adopted in 1970 for soil surveys in the country. Previously, the systems of Baldwin, Kellogg & Thorp (1938) and Thorp & Smith (1949) had been utilized in soil studies.

The systematic soil survey of the Pampean Region started in the x60, mostly performed by geologists and a strongly based on geomorphology. The wide area known as Humid Pampa is an extended plain of 300.000 km2 covered with loess; in a seemingly uniform landscape abundant examples of soil polygenesis are found.

Soon it appeared that the peculiarities of the area were not sufficiently recognized by the system. Among many others, two features mark the main difficulties in the utilization of ST in the Pampean Region.

On one hand, the tosca, 40.000 km2 of a subsurface layer of CaCO3 accumulation underlying lime free soils. Not foreseen in ST until 1992 when the Petrocalcic Paleudolls were included, its presence was locally solved utilizing different shortcuts: phase qualifiers for taxonomic units, coining local subgroups. Incoherence persists today in ST with respect to depth to the petrocalcic horizon required in two subgroups that can be found closely associated in the landscape. Petrocalcic Paleudolls require a petrocalcic horizon within 150 cm from the mineral soil surface while Petrocalcic Natrudolls consider the upper limit of the petrocalcic horizon within 100 cm from the mineral soil surface.

On the other hand, the strong geomorphological influence in the soil survey led to attribute an excessive importance to lithological discontinuities, actual or apparent, associated with abrupt textural changes above argillic or natric horizons. Without the support of studies on soil forming processes which are very intense in the region, once more, local names were coined for naming the soils. In apparent consonance with the rationale on buried soils in ST, the coined subgroups Thapto Argic Hapludoll and Thapto Natric Hapludoll were profusely utilized in the soil maps.

Afterwards, following research on soil forming processes and proposals submitted in accordance with the philosophy of ST, amendments were accepted in the system and Abruptic Argiudolls were incorporated in ST 1996 and Natrudolls in ST 1998.

Other hot spots are those related with the recognition of aquic soil moisture regime, and the need of some other subgroups in the Mollisols to cover the variability of the Humid Pampa and clearly display soil lateral transitions.

A mutually beneficial reciprocating exchange still has to be done between soil knowledge in the Humid Pampa as a source for proposals for amendments in ST.

As a result, at present there are numerous misunderstandings when teaching soil classification, exchanging scientific information, and dealing with soil information both with technical advisers and farmers.

It is expected that these misunderstandings will be solved upon the updating of soil maps of the area, a very expensive and time consuming activity. The success of this enterprise is fully dependent on the methodology to be applied. If the updating is based on a simple process of taxo-transference, the result will be just the multiplication of errors. Pedon by pedon should be reclassified, obtaining new original information in place when the available soil profile descriptions do not fit the present standards.

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