Phytotoxic effects caused by soil contaminants depend both on their total concentrations and also on their forms, in particular those that can be easily released into soil pore water. The contaminants introduced into soil in water soluble forms undergo various transformations referred to as "ageing" which causes reduction of their solubility and toxicity. In this study, we examined the dynamics of such changes under controlled conditions in an incubation experiment. Two relatively light soils, differing in texture (loamy sand and sandy loam), and brought to three various pH ranges, were spiked with water solution of sodium arsenate to obtain different soil concentrations of As, up to 1000 mg kg^-1. The soils were incubated at constant moisture (80% of water holding capacity) for three months. The changes in water extractability of As over time were examined. The phytotoxicity of As was assessed based on the reduction of germination, using the seeds of two different plant species: white mustard and red fescue. We found that the process of As immobilization in light mineral soils, poor in organic matter, proceeded quickly, and the concentrations of water-soluble As in these soils dropped significantly within one week. The dynamics of those changes depended on soil properties and pH, and As immobilization was most efficient under acidic conditions. There were no significant differences between the toxicity of As to both plant species examined, as measured in the germination test. The effective concentration of soluble As in soil, resulting in a 50% reduction in the number of germinated seeds, was assessed at about 100 mg kg^-1. The results provide a reference base for further experiments with spiked soils, and will be used in examination of As binding mechanisms in soils.