This document specifies a method for the determination by ion pair chromatography of the iron chelated by each individual ortho(hydroxy)-ortho(hydroxy) isomer of the chelating agents [o,o] EDDHA, [o,o] EDDHMA and by HBED in organo-mineral fertilizers, having an organic matrix based on vegetal residues (cocoa shells, grape residue, soybean residue, …), algae extract, and animal meal (feather, bones, blood, ...) and containing one or more of these substances, except for [o,o] EDDHMA and HBED mixes.