Abstract: To improve the prediction and management of problems associated with toxicity in nickel (Ni)-contaminated soils, a better understanding of their sorptive and desorptive behavior is required. In this research, the sorptive and desorptive behavior of Ni in a clay loam soil (Fluventic Xerochrept), typical of central Italy, was measured using a stirred-flow reactor where the desorption products are continuously removed from solution. In addition, the effects of the presence of carbonates (calcite) and organic matter on Ni adsorption and desorption by the soil were investigated. For this purpose, samples obtained depriving the whole soil (S) of carbonates (S-C) and, successively, organic matter (S-C-OM) or only organic matter (S-OM) were prepared. In the samples containing carbonates, Ni adsorption was characterized by a fast initial reaction in which all the Ni added to the stirred-flow chamber was sorbed; following this, a slower and incomplete sorption occurred. This Ni adsorption process was totally nonreversible. This behavior enables hypothesizing the formation of a complex Ni calcite that can be considered to be mainly responsible for Ni sorption in calcareous soils. When the pH of the whole soil was lowered, depriving the soil of carbonates, its Ni sorptive capacity was significantly reduced and the Ni sorption process became partially reversible. The removal of organic matter decreased both the adsorbed and the desorbed Ni percentage only in the soil samples previously deprived of carbonates.