Effect of Iron Oxides on Positive a N D Negative Charges I N Clays and Soils

The contribution of free iron oxides to positive and negative charge distribution has been studied on soils and synthetic kaolinite-iron oxide complexes over a wide range ofpH values. The charge on the iron oxides is pH-dependent, being positive at low and negative at high pH values, respectively, and accounts for a considerable proportion of the pHdependent negative charge in soils. Iron oxides increase the buffer capacities of all soils and kaolinite-iron oxide complexes. A considerable proportion of the surface of the clay fraction is covered by iron oxides, resulting in a decreased negative charge on the clay. The isoelectric points of iron oxides in soils are considerably higher than those reported for pure iron oxides. A number of the soils studied were isoelectric at their field pH values, so that under field conditions they would be expected to be extremely infertile. Evidence is presented to show that kaolinite has a permanent negative charge, confirming the view that isomorphous substitutions occur.