Micronutrient additions to soil result mainly from return of crop residues and application of fertilizers, farmyard manures and municipal wastes. Crop removal and erosion of surface soil constitute the main micronutrient losses. Only a small fraction of total micronutrients in surface soils is cycled via cropping. Plant availability of all micronutrients except Mo increases with increasing soil acidity. Toxicities due to Mn and Al in extremely acid soils may result in decreased crop yields. Changes in soil pH due to acid depositions are minimal in most agricultural soils because of relatively high buffering capacities of these soils. Modern farming practices such as liming and return of crop residues also may override depositional effects. Therefore, micronutrient cycling in most agro-ecosystems should not be significantly affected by acid depositions. The long-term effects of acid depositions are not known; research is needed to study effects of acidification on the dissociation characteristics of functional groups in soil organic matter which may affect metal complexation. Micronutrients are those elements which are essential to the completion of the life cycle of biological systems, but which are required in relatively small amounts. Plant micronutrients generally included in this group are B, Cu, Fe, Mn, Mo, and Zn. Although Cl and Co are also micronutrients, their supply in most agro-ecosystems is usually sufficient, and they will not be discussed further. Micronutrient cycling in agro-ecosystems is a complex series of processes involving micronutrient additions to and removal from soils and changes in the chemical forms, solubilities and consequent availabilities of micronutrients to plants induced by changes in the ecosystem. Micronutrient additions to soils result from alluvial or aerial depositions and applications of fertilizers, farmyard manure, or municipal wastes. Crop removal and erosion of surface soils result in losses of micronutrients from agricultural soils. Return of crop residues to soils mitigates such losses by returning some of the plant-contained micronutrients and by reducing the potential for erosion. Chemical changes occurring in soils also greatly influence the solubility and mobility of micronutrients and hence, the amounts taken up by crops and removed during harvest. One of the soil chemical properties influencing micronutrient cycling is soil acidity or basicity, as reflected by soil pH. Effects of soil pH on nutrient availability and mobility have been studied in considerable depth; however, the effects of acid deposition from atmospheric sources per se on micronutrients have received little attention. For this reason, much of the following discussion will deal with amounts of micronutrients in soils, sources of their additions and losses and general effects of soil pH on micronutrient chemistry and plant uptake. A brief discussion concludes of the probable effects of acid deposition on soil pH and micronutrient cycling in agro-ecosystems.