Seems to come from water / soil contamination from ore smelting, oil refining, soluble in water but complexes with alkaline soil. (nutritionfacts.org)
Acidic soils have reduced calcium and magnesium content, while highly alkaline soils affect iron, manganese, copper and zinc availability [29]. (dralisonchen.com)
The sodium salt of the soluble iron complex, [Fe (III)- EDMA]-, is used in large quantities as an iron micronutrient in citrus agriculture, for plants that can not extract iron from alkaline soil in which the iron is insoluble. (scienceinschool.org)