Journal of Arid Environments (2001) 47: 145–152 doi:10.1006/jare.2000.0712, available online at http://www.idealibrary.com on Calcic horizon distribution and soil classification in selected soils of north-western Jordan Sa’eb A. Khresat Department of Natural Resources and the Environment, Jordan University of Science and Technology, P.O. Box. 3030, Irbid-22110-Jordan (Received 17 March 2000, accepted 17 August 2000) Calcic horizons are formed in most of the soils in northern Jordan where a dry, warm climate prevails. Soil samples were collected from four soil sites repres- enting different parent material types (colluvium, alluvium). The purpose of this study was to discuss the possible genesis of calcic soils in north-western Jordan and to classify these soils according to the USDA soil taxonomy. Calcium carbonate accumulations in the studied soils varied from slight car- bonate filaments to prominent stage II and stage III carbonate nodules and plugged horizons. Carbonate and clay eluviation-illuviation were the dominant pedogenic processes in the study area. These soils developed in a humid climate that shifted gradually towards aridic.  2001 Academic Press Keywords: pedology; calcium carbonate; aridisols; inceptisols; arid and semi- arid environment Introduction Calcic soils contain a significant amount of secondary carbonates in the form of calcic horizons (Machette, 1985). The accumulation may be in any horizon in the profile and generally has identifiable characteristics related to both form and content. Calcic hor- izons range from layers of carbonate-coated pebbles or carbonate filaments and nodules to massive indurated carbonate enriched layers (Machette, 1985). Calcium carbonate accumulation is relatively easy to identify in the field and carbon- ate levels can be easily measured in the laboratory. The depth of accumulation varies according to changes in climate and landscape stability. The formation of these hor- izons, as suggested by many soil scientists, is attributed to translocation and net accumulation of pedogenic carbonates from several sources (Birkeland, 1984; Raben- horst et al., 1991; Flach et al., 1969; Gile et al., 1966). The calcic horizon is an illuvial horizon in which secondary carbonates, usually calcium or magnesium, have accumulated to a significant extent. The requirements for calcic horizon classification are explained in the Soil Taxonomy (Soil Survey Staff, 1999). Gile et al. (1966) introduced the concept that carbonate morphology in soil changes with time and can be described by a sequence of morphologic stages related to soil age (Gile et al., 1981). Stage I carbonate horizons occur in Holocene soils, while stage II, III, and IV carbonate horizons occur in Pleistocene soils in New Mexico (Gile, 1995). The thickness of matrix carbonate increases substantially with increasing soil age, suggesting 0140-1963/01/020145#08 $35.00/0  2001 Academic Press