Abstract
The study was conducted on nine clay samples collected from three different soils of northern Iraq (Mukhmour, Shikhan and Zakho) which varied in climatic conditions (precipitations and temperatures).Permanent surface charge on clay surfaces was measured through adsorption of lithium chloride experience before and after removing iron oxides. Results showed that the highest values of permanent negative charge on clay surfaces before removing iron oxides was found in Mukhmour soil with average 34.5 C.mol. kg-1. clay, while the lowest values appeared in clay fraction of Zakho soil (18.1 C.mol. kg-1. clay), but in Shikhan soil,the negative charge values was at average of (20.0 C.mol. kg-1 clay). The highest stability of negative surface charge was found at pH between values (3-5). The distribution of negative charges within soil profiles showed an increase in values with depth and in all studied soils. Results also reveals very low positive surface charge compared with negative surface charge. However, the results of the lithium chloride adsorption experiment have indicated that the removal of iron oxides from clay fraction had a clear effect on increasing in the values of negative surface charge in all study soils.The rates of increase associated mainly with the iron oxides content of the clay fractions but the impact of removing iron oxides on the values of positive surface charge were minor and had no clear behavior. From the above study it can be concluded that the variations in the values of surface of clay surface charge clearly reflects the levels of weathering and the degrees of soil development.
