In the pH range commonly found in soils (i.e. between 4 and 9), PDMA forms 1:1 M:L complexes with Mg II , the weakest complex (logKML = 3.69±0.05), and Fe is probably due to the effect of the -OH group on the azetidine ring on the pKa of the nitrogen in the ring. Azetidine has a pKa of 11.3 while 3-hydroxy azetidine, has a predicted pKa of 14.5 . Furthermore, the -OH groups impose some geometrical constraints on the fourmembered ring. Together, these factors could change HMA's ability to coordinate metal ions compared to the other ligands.