N. Asgari Hafshejani - Department of Soil Science, Ramin University of Agriculture and Natural Resources, Ahvaz University,Ahvaz, I.R. Iran
S. Jafari - Department of Soil Science, Ramin University of Agriculture and Natural Resources, Ahvaz University,Ahvaz, I.R. Iran

ABSTRACT-This experiment was carried out to study particle size distribution (PSD) of carbonates in soils and its effect on some calcareous soil properties in Khuzestan Province, Iran. Soil samples (n=۷۲) were collected from different regions. To evaluate the effect of carbonates on water contents at pressure head of -۱۵۰۰kpa and PSD of soil samples, these properties were determined before and after removing their calcium carbonate. Based on the results, removal of CaCO۳ led to a change in PSD in all the samples and ۷۰% of those changed textural class. Carbonates were most accumulated in clay and sand size in Abadan region while they were accumulated in silt and sand fractions in other areas. Also, evaluation of moisture retention at permanent wilting point (PWP) showed that this feature has been reduced in all samples after removal of carbonates. Following these changes, verification of key to soil taxonomy equation [Clay%= ۲.۵(% water retained at -۱۵۰۰ kPa tension-%organic carbon)] in calcareous soils of Khuzestan Province showed that this equation can be used with high reliability in calcareous soils of Abadan, Karoon, Haft-Tape agro-industries. However, this equation is not valid in Amir-Kabir and Debal-Khozaiea gro-industries. Our fitted equation [Clay % = (۴.۳۵*% water retained at -۱۵۰۰ kPa tension +۳۵.۳* % organic carbon)] has high reliability in predicting clay content in non-carbonate samples. This result suggests that the presence of carbonates and its distribution in soil particle size is an important factor in the variability of soil properties at arid and semiarid areas.

Keywords:, Free carbonate clay, Soil taxonomy, Water Retention