Antimony trioxide (Sb2O3) is a widely used Sb(III) chemical that can be emitted to soil. Here we report the gradual dissolution of Sb2O3 in soil by measuring concentration and redox speciation of Sb in soil solution with reference to that the more soluble reference form SbCl3. A silt-loam soil (pH 7.0, background 0.6 mg Sb kg-1) was amended with Sb2O3 or SbCl3. More than 70% of Sb in soil solution was present as Sb(V) within 2 days as revealed by IC-ICP-MS analysis. Soil solution Sb concentrations in Sb2O3 amended soils gradually increased between 2 and 35 days. Soil solution Sb concentrations in SbCl3-amended samples were stable beyond 2 days after spiking and were equivalent to those in Sb2O3-amended soils equilibrated for 5 years at equivalent total soil Sb. Our data indicate that the Sb solubility in this soil was controlled slow dissolution of Sb2O3 with a half-life ranging between 50 and 250 days, sorption on the soil surface and Sb precipitation as Ca-antimonate at the higher doses. Plant uptake of Sb from soils freshly spiked with SbCl3 or Sb2O3 or from soils with 5 year aged Sb2O3 was correlated with soil solution Sb across all soils. However, toxicity to plant or soil nitrification was far larger in SbCl3 amended soils than that Sb2O3 and was unrelated to soil solution Sb. This is because effects of SbCl3 were confounded by a decrease in pH and an increase in salinity as proven with CaCl2 reference treatments. The 10% effect concentration of Sb after complete Sb2O3 transformation is found at 500 mg Sb/kg and above suggesting that Sb is not toxic to soil organisms in the environmental relevant concentration range.