Weigh a clean, dry 250 mL beaker and add to it about 20 g of Epsom salts (magnesium sulfate). Add approximately 200 mL of water and stir to dissolve.
You should now have a plant food solution and an Epsom salt solution in two separate beakers. In order to make sure you precipitate all the phosphorus from the plant food, you need to calculate how much of the Epsom salt solution you should use based on the amount of P2O5 on the label of the plant food, calculate the grams of P2O5 in your sample. You will need 50 mL of Epsom salt solution for every 1.0 g of P2O5 in the sample. Calculate the total volume needed and add this volume to your plant food solution. Stir to mix.