Nickel occurs naturally in soils as a result of the weathering of the parent rock (McGrath, 1995). The highest concentrations are found in ultramific igneous rocks with much lower levels found in sedimentary rocks including shales, clays, limestones, and sandstones. The underlying geology and soil-forming processes strongly influence the amount of nickel in soils with higher average concentrations reported in clays, silts, and fine grained loams relative to coarser grained loams, sandy and peaty soils. Soil nickel concentrations vary widely, ranging from 0.7 to 259 mg Ni/kg soil on a global basis, with an arithmetic mean of 23.9 mg Ni/kg. Nickel has been established as a micronutrient for plants, and some soils have been categorized as being deficient in nickel content. Excess nickel concentrations do, however, lead to adverse impacts to soil organisms. Further information on soil ecotoxicity of nickel can be found here.
Anthropogenic activity has resulted in the atmospheric deposition of nickel from the burning of oil and coal. Localised nickel contamination with high concentrations of nickel may also occur near to a nickel smelter, plating works or steel mill. Agricultural fertilisers, especially phosphates, are also a significant source of nickel in soil but it is unlikely to build-up in soil in the long term from their use. More important is the application of wastes to land including sewage sludge.