Application of the WRB (FAO) and US Taxonomy Systems to Gypsiferous Soils in Northwest Isfahan, Iran

Gypsiferous soils occur in the arid and semi-arid regions of Iran and are found in different geomorphic units. Few studies are available on the genesis and classification of these soils. In the present study, fifteen pedons from an area extending from Jaafarabad Mountain to the central piedmont plain are studied. The objective is to classify the gypsiferous soils studied according to the USDA Soil Taxonomy (1990, 1994, 1996, 1998, and 1999) and WRB (FAO, 1974, 1988- FAO, ISSS, ISRIC 1994, 1998) systems. For this purpose,the designation, amount of gypsum, depth and the thickness of gypsic horizons will be taken into account. The study area includes different geomorphic units such as gravelly fan, quaternary gravelly alluviums and piedmont plain. The 1994 version of the USDA Soil Taxonomy allows for more characteristics of the soils under study to be defined as compared with the previous versions, while the more recent versions (1996, 1998, and 1999) have remained unchanged in this regard. FAO (1974, 1988) and its successor WRB (1994, 1998) show continual and immense progress allowing for greater differentiation of various soils. It will be shown that the WRB 1998 version offers greater possibilities for more detailed characteristics to be included in the classification system; hence, its higher efficiency in comparison to the USDA system.