Not All Phosphate Fertilizers Immobilize Lead in Soils

The effects of six phosphate (P) fertilizers in mobilizing and immobilizing water-soluble lead (Pb) were determined in a contaminated soil (Alfisol from Shaoxing) from China and four Australian soils (an Oxisol from Twonsville Queensland and three South Australian soils from Cooke Plains (Typic Palexeralf)), Inman Valley (Vertisol), and Two Wells (Natric Palexeralf). The fertilizers tested were single superphosphate (SSP), triple superphosphate (TSP), monoammonium phosphate (MAP), diammonium phosphate (DAP), monocalcium phosphate (MCP), and dicalcium phosphate (DCP) to produce an initial P concentration of 1,000 mg/L. The Chinese soil contained 16,397 mg/kg total Pb, but the Australian soils were uncontaminated. The four Australian soils were each spiked with 1,000 mg Pb/kg soil (as Pb(NO3)(2)) and incubated for a month. Single superphosphate treatments decreased total soluble Pb in soil solution to 2-14 % of those of the nil-P (0P) treatment in the four Pb-spiked soils and to 48 % in the Chinese Pb-contaminated soil. The DAP treatment followed by the MAP treatment greatly increased the total soluble Pb in soil solution up to 135-500 % of the 0P treatment, except in the Two Wells soil. MCP could decrease the total soluble Pb in Cooke Plains, Inman Valley, Shaoxing, and Two Wells soils while increase it in the Queensland soil; DCP decreased the total soluble Pb in Cooke Plains and Queensland soils while increased it in the Shaoxing and Inman Valley soils. There were close relationships between the total soluble Pb, total soluble Al, and total soluble Fe in the water extracts of each. Soluble Al and Fe ions in soil solution increased soluble Pb concentrations. We conclude that not all phosphate fertilizers immobilize Pb in soils equally well. SSP and TSP are excellent Pb-immobilizing fertilizers, while MAP and DAP are strong Pb-mobilizing fertilizers. MCP and DCP are either Pb-immobilizing fertilizers or Pb-mobilizing fertilizers depending on their reactions with individual soils.