To simulate the effect of metal ions addition in soil as consequence of fertilization treatment on the formation of [Zn(PDMA)] -complex in paddy soil, different Zn II , Cu II , and Fe III concentration ratio were modeled (Table and). The pH chosen for the models were selected to cover all the pH range of paddy soils and based on the selectivity diagram (Figure Chemical model and protonation constants of PDMA determined potentiometrically at I = 0.15 M in NaCl(aq) and T = 298.1 K. Table . Stability constant (logK) values of Zn II , Fe II , Fe III , Cu II , Co II , Ni II , Mg II , and Mn II -PDMA species determined potentiometrically at I = 0.15 M in NaCl(aq) and T = 298.1 K. Table . (a) Effect of Zn II fertilization treatment on the formation of [Zn(PDMA)] -in presence of Cu II and Fe III in saline solutions (I= 0.1 M NaCl) with different pH and redox conditions. PDMA is present as free ligand and its concentration is 10 -6 M. Initial metal ratio used in the model are based on the average ratio of Zn II , Cu II and Fe III present in soil and Zn II concentration is increased according to the ratio indicated in the table. Fe II is not included because complex formation with PDMA is insignificant in waterlogged condition. (b) Effect of increased concentration of Cu II and Fe III , following soil fertilization on [Zn(PDMA)] -complex formation at 0.1 M NaCl and different pH and redox conditions. In the model PDMA is present as free ligand and its concentration is 10 -6 M. Initial metal concentrations used in the model are 10 -6 M. Cu II and Fe III concentrations increase according to the ratio indicated in the table. M for Cu II and Fe II /Fe III and 10-5 M for PDMA. Different type of lines represents different ionic strength. Mn II , is also redox sensitive, but it does not form strong complexes with PDMA, due to its low logK value, and for that reason it is not included in the figure.