Is the Soil in Allotment Gardens Healthy Enough?—Relation Between Organic Matter Content and Selected Heavy Metals

This study was conducted in nine allotment garden complexes in Wrocław, West Poland (Central Europe). Soil samples were collected from each garden and analyzed for their total concentrations of Zn, Cu, Pb and Cd, alongside the percentage of organic carbon C. Contaminant levels varied widely between sites: Zn ranged from 101.1 to 3464.5 mg/kg, Cu from 24.93 to 322.45 mg/kg, Cd from 0.51 to 6.31 mg/kg, and Pb from 19.92 to 401.85 mg/kg. The highest metal contamination was found for the garden complex placed on the former impact of the Hutmen. The organic carbon content ranged from 2.12% to 7.64%, indicating substantial variability in organic matter levels across the studied sites. This variability may significantly influence the soils’ capacity to retain heavy metals. A significant positive correlation was observed between soil organic carbon and the total concentrations of Pb, Cu and Zn, suggesting that soils richer in organic matter may retain higher levels of heavy metals. These findings underscore the dual role of organic matter as both a beneficial soil component and a potential contributor to heavy metal retention in urban garden soils. Protecting and enhancing SOM in polluted soils is a beneficial strategy, remediating environmental damage while aligning with global sustainability goals.