Friday, 14 July 2006

Rendzinas: a Soil Group with Primitive Original Attributes?.

Jorge E. Gama Castro, Instituto de Geología, Universidad Nacional Autónoma de México, Cda. Universitaria, México, D.F., Circuito exterior, área de Institutos, México, D.F., 04510, Mexico

Over the last decade, WRB(FAO-ISRIC-SICS,1994; 1998), has developed an internationally accepted system to describe soils. However, we consider that there are still some weaknesses in this system such as the artificial inclusion of Rendzinas in the Leptosol group, for example. This design affects the WRB's main objective that purports to give a solid scientific basis to the 1988 revised FAO Legend. At present time, the concept that defines rendzic Leptosols, excludes the variety of Rendzinas located in different areas of Mexico, forming soil units which are cartographically recognized even at a scale of 1:50 000. In addition, our work recently made at the field in the Yucatán Peninsula, Mexico, strongly suggests that it is possible to consider the existence of a variety of Rendzinas that are out of the concept of rendzic Leptosol. The study showed that weathering, soil formation and constant evolution take place at depths of a few centimeters. The objective of this paper is to emphasize the properties and soil processes characteristic of Rendzinas and their intergrades in the Mexican humid tropics. Thus, the proposal that Rendzinas must constitute a different group from that of Leptosols, it is supported by the fact that their genesis and evolution are influenced by several pedological processes, as humification and in some cases hydromorphism, observed in their profiles, and by the development of some properties as : (1) very low content or absence of primary and secondary carbonates, which is congruent with a neutral or slightly acid reaction; (2) high level of weathering; (3) high content of clay, and (4) a very well developed structure; these properties are evidences of a higher degree of evolution in comparison to Leptosols. Finally, we consider that the decision adopted by the WRB (1994-1998), with regard to the possibility of including the soils classified as Rendzinas and Rendzinas-like soils within the odd and very artificial group of Leptosols, requires a new review. Authors would like to thank Vallejo-Gómez, E. and Kumiko Shimada for technical assistance.

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