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Sorption of added Zinc to irrigated soils in arid region is an important process that may control the availability of Zinc to growing plants. Two soil surface samples varying in clay organic matter and calcium carbonate contents were selected from central and southwestern regions of Saudi Arabia and prepared in order to give different initial moisture contents ranging from air dry to 100% of field capacity. Sorption experiment was conducted using Zn concentrations ranges from 5 to 25 mg L-1, prepared from ZnSO4 either in distilled water or solutions containing 75 mg L-1 Dissolved Organic Carbon (DOC).

Results indicated that adsorption of Zn follows the Freundlich isotherm and two linear portions were found in most cases. The amount of Zn sorbed in the presence of DOC was relatively high compared with the absence of DOG. Also, equilibrium Zn concentrations were quite low, while Zn retentions were high in all treatments. In the absence of DOG, retention of added Zn was controlled by the available exchange sites and/or the precipitation of Zn as sparingly soluble forms. Zn ions in the presence of DOC were able to form a soluble Zn-organic complex which adsorbs on the soil surfaces. The extent of such behaviour was related to the variations in clay, organic matter and calcium carbonate contents as well as the initial moisture of the soil. Results also indicated that addition of DOC reduces the amount of extractable Zn from either soil-Zn or the sorbed Zn by AB-DTPA. More than 80% of the sorbed Zn was extracted by AB-DTPA and the percentage of extracted/ sorbed Zn decreased with the increase in sorbed Zn. The obtained results give evidence that initial moisture content and addition of DOC reduces the extractability of applied inorganic Zn by AB-DTPA extract in arid soils.