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Bimodal Zone of the Soil Textural Triangle: Common in Tropical and Subtropical Regions

^a Lab. for Transfers in Hydrology and Environment (LTHE), UMR 5564 CNRS-INPG-IRD-UJF, BP 53, 38041 Grenoble cedex 9, France
^b Indo-French Center for Groundwater Research (IFCGR), National Geophysical Research Institute (NGRI), Uppal Road, Hyderabad 500007, India
^c Institut de Recherche pour le Développement, 08BP 841, Cotonou, Bénin
^d Bureau de Recherches, Gèologiques et Minières (BRGM), Water Division, 1039 rue de Pinville, 34000 Montpellier, France

View larger version (34K): [in this window] [in a new window]   Fig. 1. The USDA soil textural triangle. Open square: a sandy clay with 50% sand, 5% silt, and 45% clay; filled circle: a silty clay with 5% sand, 50% silt, and 45% clay. The shaded area corresponds to bimodal soils satisfying Eq. [2].

View larger version (10K): [in this window] [in a new window]   Fig. 2. Bimodal cumulative particle-size (D) distribution of a sandy clay soil with 53% sand, 12% silt, and 35% clay. The soil sample was taken from the B horizon of an Alfisol at the Maheshwaram watershed (South India). The vertical dotted line indicates an inflection point in the sand zone.

View larger version (15K): [in this window] [in a new window]   Fig. 3. Textural classification of Alfisols collected at various depths in the Maheshwaram watershed (South India) plotted on the USDA soil textural triangle: (A) E horizon, (B) B horizon, and (C) C horizon.

View larger version (15K): [in this window] [in a new window]   Fig. 4. Textural classification of Entisols collected at various depths in the Maheshwaram watershed (South India) plotted on the USDA soil textural triangle: (A) E horizon, (B) B horizon, and (C) C horizon.

View larger version (15K): [in this window] [in a new window]   Fig. 5. Textural classification of Alfisols collected at various depths in the Ouémé watershed (Benin, western Africa) plotted on the USDA soil textural triangle: (A) A horizon, (B) B horizon, and (D) C horizon.

View larger version (35K): [in this window] [in a new window]   Fig. 6. Sample population of 666 soils of the UNSODA soil database (Leij et al., 1996) plotted on the USDA soil textural triangle.

View larger version (35K): [in this window] [in a new window]   Fig. 7. Sample population of 660 soils of the GRIZZLY soil database (Haverkamp et al., 1998) plotted on the USDA soil textural triangle.

View larger version (31K): [in this window] [in a new window]   Fig. 8. Sample population of 9607 soils of the Soil Information System of the Netherlands, adapted from Nemes et al. (1999). Adapted and reprinted from Nemes et al. (1999) with permission from the publisher.

View larger version (31K): [in this window] [in a new window]   Fig. 9. Sample population of 771 tropical and subtropical soils extracted from the IGBP-DIS database plotted on the USDA soil textural triangle, adapted from Hodnett and Tomasella (2002). Calibration (492 soils) and Validation (279 soils) points indicate the data sets that Hodnett and Tomasella (2002) used to derive and validate, respectively, their pedotransfer function. Adapted and reprinted from Hodnett and Tomasella (2002) with permission from the publisher.