Ecotoxicity of Ni in soil is often studied by adding soluble Ni salts to soil and assessing the health or function of soil organisms or plants. However, after addition of soluble Ni to soil, several soil physico-chemical reactions are active which not only reduce the concentration of Ni in the soil pore water, they also reduce the fraction of Ni in soil which is in equilibrium with the soil pore water (the labile pool). We studied these reactions in 16 European soils covering the major soil types found in Europe. These soils were spiked with Ni at two levels, incubated outdoors and sampled several times during a period of one year. Reductions in the potential ecotoxicity of Ni were assessed by determining pore water and extractable concentrations of Ni, and by determining the labile Ni pool through isotopic dilution using ⁶³Ni. Reductions in pore water Ni concentrations were observed in all soils, and several factors were identified as the causal agents. Initially, pore water Ni was reduced through increased Ni adsorption to soil surfaces through reductions in pore water solution ionic strength by leaching of soluble anions. The reduction in labile Ni (pore water+sorbed) concentrations in soil over time, so called “aging” reactions, were rapid initially and were highly pH dependent, being of a greater magnitude in alkaline soils. A semi-mechanistic model of aging in soil was developed that indicated the principal factors controlling aging of Ni are soil pH and incubation time. The results indicate that ecotoxicity of Ni in soil will be highly dependent on the time of organism exposure following spiking of soluble Ni salts.