Morphology, genesis and classification of three soils over limestone, Jamaica

Abstract

This paper describes three Jamaican soils over limestone that are widely different in their genesis, morphology and classification. Following descriptions of the locations of the profiles, the environmental conditions governing their formation are discussed. Two of the profiles are derived from limestone; the third is developed in bauxite. In all three profiles intense weathering preceded actual soil formation. In profiles 1 and 2, weathering of the limestone rock through dissolution of CaCO 3 resulted in the residual accumulation of clay minerals such as kaolinite and chlorite. The bauxite deposit in which profile 3 developed, represents a very advanced stage of weathering and desilication. The origin of the Jamaican bauxites is not clear, however. Soil formation in the two soils over limestone (an Ultisol and an Alfisol) is characterized by downward movement of clay, iron and organic matter and by the formation of an argillic horizon. Biological mixing, oxidation-reduction and formation of nodules are additional processes. One of these profiles shows a lithological discontinuity. In this soil, transformation of rock fragments (chert) into pseudo iron-manganese nodules has contributed to soil formation. We postulate that the dominant processes in the profile developed in bauxite (an Oxisol) are desilication and residual accumulation of oxides of aluminium and iron into an oxic horizon. Other soil-forming processes in this profile comprise biological activity, some iron segregation and nodule formation. The three profiles are classified according to the USDA Soil Taxonomy, the legend for the FAO-Unesco Soil Map of the World and the French soil classification system (CPCS). Rationales for placement in these systems are discussed. Detailed profile descriptions and laboratory data are presented in an Appendix.