Interactions of N, P and K fertilizers in soil-plant systems are widely recognized. This study focused on the transformations of monocalcium phosphate (Ca(H2PO4)2) (MCP) with co-application of ammonium and potassium fertilizers in three different soils. The results showed that after 1 d incubation a large portion of the MCP applied in the paddy, calcareous and red soils became the water-insoluble form and the recoveries of P applied as Olsen P varied greatly in these three soils. Application of ammonium sulfate ((NH4)2SO4) (AS) or potassium chloride (KCl) reduced WSP significantly in the soils with AS more effective than KC1 in the calcareous soil, while the reverse occurred in the red soil. Meanwhile, in the paddy soil, co-application of the two fertilizers reduced WSP more than when the fertilizers were applied individually. The co-application of AS with MCP in the paddy and calcareous soils significantly reduced Olsen P, but the opposite occurred in the red soil. The experiment on the effect of diffe Interactions of N, P and K fertilizers in soil-plant systems are widely recognized. This study focused on the transformations of monocalcium phosphate (Ca (H2PO4) 2) (MCP) with co-application of ammonium and potassium fertilizers in three different soils. The results showed that after 1 d incubation a large portion of the MCP applied in the paddy, calcareous and red soils became the water-insoluble form and the recoveries of P applied as Olsen P varied greatly in these three soils. Application of ammonium sulfate ( (NH4) 2SO4) (AS) or potassium chloride (KCl) reduced WSP significantly in the soils with AS more effective than KC1 in the calcareous soil, while the reverse occurred in the red soil. Meanwhile, in the paddy soil, co-application of the two fertilizers reduced WSP more than when the fertilizers were applied individually. The co-application of AS with MCP in the paddy and calcareous soils significantly reduced Olsen P, but the opposite occurred in the red soil. The experiment on t he effect of diffe