Chromium in Soil and Plants
Chromium is a naturally occurring element in rocks and soil. Natural soil typically contains between 10 and 50 mg of chromium for every kg of soil. Chromium can exist in its elemental form (chromium 0) as well as chromium III and chromium VI ‑ the Roman numerals denote the valency. Chromium is toxic in high amounts to both plants and humans but the toxicity depends on the valency; chromium VI being substantially more toxic. Chromium affects seed germination, plant growth, photosynthesis and the uptake of a variety of nutrients.
Chromium in Humans
It is important to note that chromium is beneficial to humans in very small amounts (in fact it has been deemed an essential element) as it is involved in the action of insulin. As it is naturally present in a wide range of foods, a deficiency is unlikely.
Inhalation of chromium VI causes a variety of effects but this is unlikely to be a problem in a garden situation except if large amounts of very dry soil are inhaled. Chromium can cause skin sensitisation and cancer, however research has demonstrated that there is very little risk of this when exposure is via contact with contaminated soil. Ingestion of chromium can cause kidney and liver damage, stomach ulcers, vomiting, haemolysis, heart problems and possibly reproductive problems.
How Much Chromium is Too Much?
If less than 10% of the fruit and vegetables you eat comes from your garden and the concentration of chromium in your soil is greater than 100 mg/kg, it would be a good idea to have your home grown produce analysed and then seek expert advice on whether any chromium detected constitutes a health risk. If more than 10% of the fruit and vegetables you eat comes from your garden, you may wish to test some of your produce even if your soil contains less than that amount. The Canadian Soil Quality Guidelines for the Protection of Environmental and Human Health (2007) recommends that soil in residential areas and parklands should contain less than 64 mg/kg of total chromium and 0.4 mg/kg of Chromium VI. These guidelines specify the same values for agricultural land so if you are a farmer growing produce for commercial purposes and your soil contains chromium in excess of either of these values, I would recommend you seek expert advice for your situation.
Treating Chromium Toxicity
In the event that high chromium levels are detected in soil, there are some options for remediation. Nutrient deficiencies resulting from chromium toxicity can be ameliorated by mycorrhizas (beneficial fungal associations). As a result, digging should be kept to a minimum on soils that contain high levels of chromium. It may be possible to remediate soils high in chromium using Albizia amara. As with other forms of contamination, vegetables and fruits can be grown in raised beds containing clean soil.
- Arun K. Shanker, Carlos Cervantes, Herminia Loza-Tavera, S. Avudainayagam, 'Chromium toxicity in plants', Environment International, Volume 31, Issue 5, July 2005, Pages 739-753, ISSN 0160-4120, http://dx.doi.org/10.1016/j.envint.2005.02.003. http://www.sciencedirect.com/science/article/pii/S0160412005000231
- http://www.foodstandards.gov.au/publications/documents/FSANZ%2023rd%20ATDS_v8_.pdf [PDF]
- Paustenbach DJ, Rinehart WE, Sheehan PJ. 'The health hazards posed by chromium-contaminated soils in residential and industrial areas: conclusions of an expert panel', Regul Toxicol Pharmacol. 1991 Apr;13(2):195-222. PubMed PMID:1852930. http://www.ncbi.nlm.nih.gov/pubmed/1852930
- [Australian] National Environment Protection (Assessment of Site Contamination) Measure 1999 Schedule B1 (F2013C00288, measures as amended, taking into account amendments up to National Environment Protection (Assessment of Site Contamination) Amendment Measure 2013 (No. 1)) http://www.comlaw.gov.au/Details/F2013C00288/Html/Volume_2
- Canadian Soil Quality Guidelines for the Protection of Environmental and Human Health (2007) [PDF]