Abstract: Improved methods and compositions for decreasing the bioavailability of metals in soil are provided. Broadly, the methods comprise mixing a source of phosphorus and an oxide of manganese with the contaminated soil so as to reduce the metal bioavailability in the soil. The phosphorus source and oxide of manganese can be individually mixed with the soil, or can be provided as a premix powder or granule to be mixed with the contaminated soil. Preferably, the pH of the soil is then adjusted to, and maintained at, a level of at least about 7.0. Preferred phosphorus sources include phosphate rock, alkali and alkaline earth metal phosphates, ammonium phosphates, ammonium orthophosphates, orthophosphoric acid, and superphosphates. Preferred oxides of manganese include MnO2, Mn3O4, birnessite, cryptomelane, and psilomelanes.