Comparison of Phosphate Materials for Immobilizing Cadmium in Soil

A study was conducted to compare the effects of phosphate (P) materials in reducing cadmium extractability. Seven P materials (commercial P fertilizers—fused phosphate (FP), ‘fused and superphosphate' [FSP], and rock phosphate [RP]; P chemicals—Ca[H₂PO₄]₂·H₂O, [NH₄]₂HPO₄, KH₂PO₄, and K₂HPO₄) were selected for the test. The selected P source was mixed with Cd-contaminated soil at the rate of 0, 200, 400, 800, and 1,600 mg P kg⁻¹ under controlled moisture conditions at 70% of water holding capacity, then incubated for 8 weeks. FP, Ca(H₂PO₄)₂ ∙ H₂O, KH₂PO₄, and K₂HPO₄ significantly decreased NH₄OAc-extractable Cd (plant-available form) concentrations with increasing application rates. Compared to other phosphate materials used, K₂HPO₄ was found to be the most effective in reducing the plant-available Cd concentration in soil, mainly due to the negative charge increase caused by soil pH and phosphate adsorption. Contrary to the general information, FSP and (NH₄)₂HPO₄ increased Cd extractability at low levels of P application (