To avoid possible precipitation of Pb caused by the presence of phosphorus (P) in the nutrient solution, I used a KH2PO4 concentration of 0.04 mM, but my solution is still precipitated. What to do to avoid precipitation in the nutrient solution?

Salt of Pb is lead nitrate, I am using deionized water to make the Hoagland solution and the pH of the nutrient solution is 5.5.