Tuesday, 11 July 2006

Characterization of Typic Natrudolls and Petrocalcic Soils in the Center-Southwest of Buenos Aires province, Argentina.

Julio M. Sánchez, Gervasio Carboni, Gustavo A. Cruzate, and Ricardo C. Díaz. INTA-CIRN-Instituto de Suelos, Las Cabañas y De Los Reseros CC 25, Castelar, Argentina

Until 1998, Molisolls were identified in center-southwest of Buenos Aires province as udic humidity regime and natric horizon. The 1992 USDA Soil Taxonomy (S.T.) did not include Udolls with natric horizon. These wide plains, have high percentage of sodium interchangeable, Bt horizon, and calcium carbonate accumulation in depth. Later, the soils with natric horizon were classified as Natraquolls, Hapludolls and Natralbolls. Godagnone (1991), and Pazos, Stoops (1987) suggested a review of the USDA S.T. Classification to include Natrudolls. The Great Group Natrudolls, and eight Subgroups were incorporated in 1999. The area is a wide plain with non consolidated sediments of the quaternary, deposited by wind, and redistributed by water. Two mountain systems (Tandilia, Ventania), occupy the center and southwest presenting rocks from the Precambric and Paleozoic. Tandilia and Ventania, recent sediments, are over Paleozoic rocks covering crystalline basament. The mountain area has a cover of calcium carbonate cemented silt "tosca", also silt sediment from wind action or "pampean loess" that represent the soil parent material for the province. These were divided into seven geomorphological units according to the characteristics and evolution of the soil parent materials, actual topography, drainage network, and incidence of those factors in the distribution of soil types. Annual average temperature and precipitations are around 14°C, and 800-900 mm (1971-2000), respectively. They are located in the geomorphological subregions: sector occidental of depressed Pampa, intermountainous Pampa, Tandilia mountains, and valleys. The objective of this study was to characterize and confirm their correct taxonomy. New criteria were used to classify twenty soil profiles corresponding to four identified soil series comprising around 1,600,000 ha. The methodology included: 1) Topographic Argentine maps (IGM), INTA photo mosaic 3760-13, 19, 25, 31, 3763-18, 24, 30, 36, and 3960-1,7 Scale 1:50.000; 2) Geomorphological units of the province maps; 3) Soil identification criteria; 4) Productivity index and parametric methods of soil evaluation; 5) Land capability classification; 6) Keys to soil taxonomy (S.T., 2003); and 7) Soil survey laboratory methods manuals (1992). According to new S.T. (2003) criteria. Twenty soil profiles allowed to differentiate Udolls and Aquolls Suborders. The 44% had the required criteria to be included as Typic Natrudolls (IHAH) with humidic udic regimen. These soils are in high and wide plains, low hills with good to poor drainage. La Tigra Series included the horizons A-E-Bt-BCn-Cn-Cck-2Ckm. General Lamadrid Series was classified as Typic Natrudolls (Ap-Btn-BCc-C1-C2-2Ckm) without transition horizon in extended low hills, and 37% were Petrocalcic Natrudolls (IHAA) with udic humidity regimen, mollic natric epipedon horizon, calcium carbonate, and loess as original material or petrocalcic horizons. These soils are in extended plains up to 2,000 meters of the intermountain Pampa and bottom of the Tandilia range of Las Hermanas Series (An-Btn-BCn-Cn-2Ckm) with 79 cm depth. Low plains with undulations in Cruzada Series (Ap-A1-Bt-2Ckm) up to 46 cm depth where the calcium carbonate appear. They have moderately good to poor drainage. The natric horizon has 70% of exchange sodium from the surface and 15% of sulphates at 15 cm depth (VIws-VIIws) with 5-10 productivity index indicating livestock use. The 19% of the soils were included as Typic Natraquolls (IBCC) because humid aquic regimen. They were in low parts with a fluctuating water table. Total surface reached 1,600,000 ha with 327,500 ha of Natrudolls. The Typic Natrudolls cartographic units (La Tigra, General Lamadrid Series) included 277,484 ha (84.7%) and were associated to a less representative soils (Typic Natraquolls). The remaining, 50,047 ha, were the dominant Cruzada and Las Hermanas Series classified as Petrocalcic Natrudolls (15.3%). An adequate taxonomical location of the soils and land use of the center-southwest of the province was possible after the incorporation of the Natrudolls in the Key to S.T. (1999). Therefore, Typic and Petrocalcic Natrudolls were included to the province soil type list. Previously, Natraquolls and Natralbolls were used to designate low areas while Typic Argiudolls and Petrocalcic Paleudolls were used for higher sectors. Typic Natrudolls have agriculture-livestock land capability (IVws), but Petrocalcic Natrudolls are only for livestock use (VIws). Keywords: Typic Natrudolls, Petrocalcic Natrudolls, Soil Taxonomy, pampean region soils.

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