Characterization of the metal-complexing ability of polyfunctional macromolecules through the determination of stability constants of their metalic complexes is discussed along with the validity of ascribing a relationship between the metal-ion complexing properties of organic matter extracted from the soil and the role of naturally occurring organic matter in soil in the processes of plant nutrition and soil genesis.The type of complex obtained when mixing metal-ion and complexant solutions will be a function of the molar ratio metal-ion/complexant. If Schubert's original experimental conditions (e.g., complexant concentration ≫ metal-ion one) are used, the MCh- or MChb-type complexes will always be formed regardless of the polyfunctional character of the organic macromolecule (Ch). It is suggested that the overall stability constant of the saturated complex MaCh would describe in a comprehensive way the metal-complexing properties of such complexants.No reliable information about the role of soil organic matter in plant nutrition or soil genesis processes can be obtained by studying the metal-complexing ability of extracted soil organic matter. This is due to the fact that the metal-complexing properties of these materials—or the number of active anionic group on the extracted molecules—will depend on the extent of depolymerization suffered by the naturally occurring soil organic matter during extraction.The methodology used up to the present to study the involvement of soil organic matter in plant nutrition or soil genesis processes through its metal-complexing ability urgently needs reevaluation.