Kim et al. 2024 — Metals in kale from urban versus rural farms, Maryland
This pilot study from Johns Hopkins University (co-funded by USDA) measured eight non-essential metals (As, Ba, Cd, Cr, Pb, Ni, U, V) alongside essential nutrients in Darkibor kale grown at three urban and four rural farms in and around Baltimore City, Maryland. The study found that mean concentrations of nine non-essential metals were higher in urban kale samples compared to rural field samples, with Pb and Cd concentrations for all samples remaining below public health guidelines. Compared to rural samples, urban field samples had higher concentrations of eight nutritional elements (by as much as 413% for iron), suggesting that urban soils, while more contaminated with non-essential metals, also have higher nutrient availability. Within-urban and within-rural variance exceeded between-group variance for six elements, indicating that farm-level site history and soil conditions may matter more than urban vs. rural location per se.
Key numbers
Non-essential metals measured: As, Ba, Cd, Cr, Pb, Ni, U, V. Specific concentration means and ranges: the full numerical table is in the paper’s Supporting Information; the abstract states all Pb and Cd concentrations were below public health guidelines (CDC blood lead reference 3.5 µg/dL corresponds roughly to soil/food guideline context; no explicit food concentration limits cited for Pb in leafy vegetables in this study). Urban field mean concentrations were higher than rural field means for 9 of the 8 non-essential metals measured. Rural carotenoid and vitamin concentrations were 22–38% higher than urban means. Iron concentrations were 413% higher in urban field samples vs. rural.
Sample collection: kale seedlings (Darkibor variety) planted simultaneously across all farms; harvested at same maturity stage. Metals measured by ICP-MS at USDA Beltsville. All values reported as wet weight basis.
Note: “As” is total arsenic (tAs) measured by ICP-MS; inorganic vs. organic speciation not performed.
Methods (brief)
Three urban farms in Baltimore City; four rural farms in Baltimore County/surrounding region. Kale planted by research team, cultivated by farmers using their usual practices. Samples collected from fields and from points of distribution (post-harvest). Analysis: ICP-MS for non-essential metals; HPLC and colorimetric methods for nutrients. Statistical analysis: mixed models; variance components analysis to partition within- vs. between-farm variance.
Limitation: pilot study with small n per site (3–4 samples); insufficient power to detect statistically significant differences for some metals despite potentially meaningful effect sizes. Darkibor variety only; other kale varieties may differ. Sample size for distribution-point samples was particularly small.
Implications
Certification: Kale grown at urban farms shows elevated non-essential metals relative to rural farms, but all values below public health guidelines in this US pilot. Relevant to any certification program with kale or leafy greens as direct-use ingredients, particularly if supply chains use urban-agriculture sourcing.
Courses: Useful for modules on urban agriculture food safety; demonstrates that soil contamination (urban legacy Pb, industrial deposition) affects leafy vegetable composition and that the nutrient-contamination tradeoff is real and geography-dependent.
App: Supports geography-level flagging for kale and leafy greens: urban-grown leafy greens carry higher non-essential metal burden even when absolute concentrations remain below current guidelines. Ba, V, and U data (not routinely in HMT&C analyte vocabulary) provide breadth for the wiki’s broader metals coverage.