This document specifies a method for the extraction and determination of phosphonates (P-PO3) in plant biostimulants using ion chromatography and conductivity detection (IC-CD). This document is applicable to the fertilizing product blends belonging to PFC 7 where the EU fertilising product plant biostimulant contained in the blend represents the highest % by mass in the blend. In case of equal shares, the user may apply either this or the standard(s) applicable to the other component product(s).