17. Zinc

Zinc (Zn) is naturally present in all soils in typical background concentrations 10–100 mg Zn kg^–1. Human activities have enriched topsoils with Zn through atmospheric depositions, fertilization and sewage sludge application. Zinc contaminated soils with negative impact on the soil ecosystem are found around Zn smelters, near Zn mining sites and under galvanized structures. The solubility of Zn in soils is almost invariably controlled by sorption reactions. Pure Zn minerals (carbonates, silicates, hydroxides) have been detected at high total soil Zn concentrations (>1,000 mg Zn kg⁻¹) but are rarely controlling Zn solubility. Zinc is specifically sorbed as Zn²⁺ on pH-dependent binding sites of oxyhydroxides and organic matter and, at high concentrations, by ion exchange reactions on clay minerals. In general, soil solution Zn concentrations increase fivefold per unit pH decrease. Zinc deficiency for agricultural crops is found in about 1/3 of worldwide soils due to low total Zn concentrations and/or high pH. Soils containing less than 0.5 mg Zn kg⁻¹ diethylenetriaminepentaacetic acid (DTPA) extractable Zn are potentially Zn deficient. Dietary Zn deficiency in humans is often associated with Zn deficient soils and crop Zn biofortification is now a global initiative through selection for Zn-efficient crops or judicious fertilisation. Zinc toxic soils are less widespread than deficient ones. Risk of Zn toxicity is manifested by effects on soil dwelling organisms, i.e. plants, invertebrates and soil microorganisms. Toxic effects are identified at total Zn concentrations 100 to >1,000 mg kg⁻¹ and toxicity decreases with increasing soil CEC. Risk assessments of Zn have proposed maximal additions as low as 26 mg added Zn kg⁻¹ in the EU to maintain soil ecosystem structure and function.