The study looked at how different types of phosphate fertilizers affect soil phosphorus levels over time. They analyzed soil samples and conducted greenhouse experiments to compare the residual effects of superphosphate and rock phosphate. They found that superphosphate increased total and soluble phosphorus in the top soil layer, while rock phosphate had less soluble phosphorus but more total phosphorus deeper in the soil. After three years, both fertilizers had similar residual effects on crop yields. The sand fraction of the soil contained most of the phosphate applied as rock phosphate.