We applied geochemical speciation calculations to study the effects of ionic strength on the stability of [Zn(PDMA)] -and [Zn(DMA)] -complexes. The inclusion of Zn-DMA complexes aimed to address the questions of whether the behaviour of PDMA is comparable to the natural ligands and whether application of PDMA as fertilizer could interfere with metal complexation by DMA. The range of ionic strengths were chosen based on the those found in agricultural soil solution considering the modifications of the conditions after soil amendments and fertilizer application 31 . Ionic strengths of 0.02 M are typical for soil solutions of weathered soils, while higher ionic strength values are typical for soils following amendments and fertilizer application . In solutions of I = 0.02 M the formation of Zn II -PDMA starts approximately at pH 5 and increases to 50 % and 100 % at pH 5.8 and 7 respectively.