When trying to find affordable planting containers in the developing world, organizations and workers all over have promoted the use of a readily-available waste resource: tires. Over the years, many have asked whether or not tires contain harmful chemicals that could potentially be taken up by your crops. This article is written to communicate what we found from our search of literature on this topic.
Much of the literature on the subject pertains to tires that have been recycled into small particles. In comparison to the side wall of a tire container, the tire surface area in contact with growing media is much greater with small chunks of rubber. Much of the information available also pertains to toxins in the ash of burnt tires, or those leached from tire material subjected to strongly acidic solutions. Tire garden containers, of course, are not being converted to ash.
Furthermore, the media used to grow plants in tires is not nearly as acidic as the solutions often used to study contaminants in tire leachate.
Nevertheless, tires do contain trace amounts of four metals that are known to be toxic to humans. Most of the discussion below relates to metallic elements, but there is also brief discussion of organic contaminants. The article concludes with suggested practices to make tire gardening as safe as possible. This write-up is not meant to be exhaustive or definitive. Depending on feedback and what we learn going forward, we are open to follow-up articles.
Cadmium can be highly toxic to humans. Fortunately, in tires, it is only found in trace amounts if at all. In a study in the UK, fragments of tires made by ten different companies were exposed to an acid (pH of 2.5) solution to see how much of each of the metals would leach out (Horner 1996). In that study, the concentration of cadmium in the leachate was considered negligible, ranging from 0 to 3 ppm. In a study done in Nepal, the ash of burnt tires from a company in China (Yin Zhu) contained considerably more (27 ppm) cadmium (Shakya et al. 2006); it was the only one of tires from four Asian companies with ash concentrations of cadmium above 0.1 ppm. Note that even soils with no industrial pollutants contain some cadmium. Concentration of cadmium in agricultural soils of the United States---where there is no metal contamination from pollutants---varies from about 0.1 to 1.0 ppm (Holmgren et al. 1993). Soils near industrial sites often contain 24 or more ppm cadmium.
In a study done at Redeemer University College, a tire sample was found to contain 0.9 ppm cadmium. However, tire fragments soaked in solutions with pH ranging from 3 to 8 did not leach measurable amounts of cadmium over a period of six weeks (Berkelaar 2016). Also, lettuce plants (known for their ability to readily accumulate cadmium) growing in hydroponic growth solutions containing tire fragments did not contain more cadmium than plants growing in solutions with no tires. Basically, while tires contain some cadmium, there was no evidence that it could easily leach out of tires or be taken up by plants—at least for the time frame of the study.
Chromium also can have negative human health impacts. Its toxicity depends on the form (valence state) in which it exists. Weathering of minerals results in naturally-occurring Cr3+ (Ahmad et al. 2013). Of greatest concern is chromium as Cr6+, which results from industrial activity (dyes, paints and the tanning of leather) and is mobile in soil. This metal was not even mentioned in the UK study (Horner 1996). In the Nepal study, chromium in tire ash was considered low (0.14 to 1.18 ppm). In non-contaminated soil, chromium occurs at concentrations from 10 to 50 ppm (Risikesh Thankur et al. 2016). Soil organic matter is able to bind (adsorb) and/or convert (reduce) Cr6+ to the less toxic Cr3+ (Bartlett and Kimble 1976; Lee et al. 1999).
Lead is more concentrated (8.1 to 22.33 ppm according to Horner, 1996) in tires than cadmium or chromium. Its concentration in soils ranges from 10 to 50 ppm, with much higher amounts possible (150 to as high as 10,000 ppm) in urban areas (Stehouwer and Macneal 2016) due to historical burning of leaded fuels. Only a small fraction of lead in soils is available for plants (Porrut et al. 2011). It has no essential function in plants but can be absorbed from the soil solution through the roots. In the previously referenced UK study, the amount of lead found in the leachates was considered negligible; however, they measured 1160 ppm lead in the soil of a tire dump site. This fact is something to consider because it is likely the eventual breakdown and severe degradation of the tires are the source of this contamination. Plants are able to take up some forms of lead and will sometimes start to show toxicity symptoms themselves, so that is something to look out for. It also remains to be seen how available the lead is to the plants. To complicate matters, plants can contain concerning amounts of trace elements and not show toxicity.
The final trace metal is found in the greatest concentrations and has been found to be a significant pollutant and leachate from tires: zinc. Zinc is found in considerable quantities in tires, ranging from 2524-6012 ppm in the UK study. The Nepali study also found very large amounts of zinc. Zinc is actually an essential micronutrient for plants and for human beings; in humans, it has been shown to help fight the common cold and to have other health benefits. Zinc can be toxic to humans, but in my research I found that those amounts have to be quite high (e.g., more than 50 mg/day according to Brown et al. 2001), and symptoms will retreat by simply discontinuing consumption of excessive amounts, due to the fact that people are able to metabolize zinc. In a study looking at leachates from ground-up rubber tires, zinc was said to be present in amounts that may be harmful to plants but the study authors concluded that toxicity to humans was unlikely (EHHI 2007).
How much of an element in soil is considered unsafe?
It was difficult to glean exact values from the literature as to what constitutes safe or unsafe soil concentrations of each the trace metals discussed above. In large part, this is because the uptake of metals by plants varies with a number of soil-related factors including pH, organic matter, temperature and concentration of other soil minerals such as phosphorus or calcium. Grubinger and Ross (2007), however, provided values from the New York Department of Environmental Conservation (December 2006) that can serve as a starting point in assessing the upper acceptable limit of each element for agricultural land use. Those values, in mg/kg (ppm), are: 0.43 (cadmium); 11 (chromium); 200 (lead) and 1100 (zinc). Recognize that 1) some metals may be naturally present in the soil placed in tires; 2) some contaminants, especially near roadsides or in urban areas, may be from other sources besides the tires; and 3) guidelines for other regions and countries may differ.
These include carbon disulfide, toluene, phenol and benzene. In a separate review of research on tire contaminants, Sullivan (2006) cited several World Health Organization documents in stating that tires do contain potentially harmful organic substances, but toxicity to humans would be unlikely in amounts leached from rubber. Sullivan stated that organic contaminants are most likely to leach at high soil pH.
Best practices to reduce risk
Taking all these things into consideration, here are some recommendations regarding the use of tires as planting containers:
- First of all, do not use tires that are heavily degraded. It is probably just fine to use tires that have been disposed of because of balding of the tread, but avoid those that are crumbling or tearing into pieces.
- Avoid soil contact with cut surfaces, since leaching will more likely occur along those surfaces than others. If avoiding cut surfaces is difficult, the tire could be lined with some sort of material to help minimize contact.
- Consider your crop selection. Trace metals are most likely to concentrate in the roots, with less in the leaves and stems, and even less in the fruits and flowers. Other than the fact that root vegetables are already difficult crops to grow in the limited space of the tire, this may be another reason to grow fruiting vegetables instead. Also, if concerned about toxicity, avoid brassicas (e.g. cabbage, broccoli, cauliflower), which readily accumulate trace metals.
- Use non-acidic media with plenty of organic matter. At a soil pH near neutral (7.0), most trace metals are less available to plants, and organic contaminants are also less likely to leach. Organic materials such as composted leaves and manure have an abundance of negatively-charged exchange sites that bind positively-charged metal ions, preventing them from being taken up by plants.
Ahmad, A., I. Khan, and H. Diwan. 2013. “Chromium Toxicity and Tolerance in Crop Plants.” In Crop Improvement Under Adverse Conditions, edited by Narendra Tuteja and Sarvajeet Singh Gill, 309–32. Springer New York. http://link.springer.com/chapter/10.1007/978-1-4614-4633-0_14.
Bartlett, R. J., and J. M. Kimble. 1976. “Behavior of Chromium in Soils: II. Hexavalent Forms1.” Journal of Environment Quality 5 (4): 383. doi: 10.2134/jeq1976.00472425000500040010x.
Berkelaar, E. 2016. Personal communication.
Environment and Human Health Inc. 2007. Artificial Turf: Exposures to Ground Up Rubber Tires-Athletic Fields, Playgrounds, Garden Mulch. http://www.ehhi.org/reports/turf/health_effects.shtml.
Grubinger, V. and D. Ross, 2011. Interpreting the Results of Soil Tests for Heavy Metals. University of Vermont Extension.
Horner, J. M. 1996. “Environmental Health Implications of Heavy Metal Pollution from Car Tires.” Reviews on Environmental Health 11, no. 4: 175–78.
Lee, Suen-Zone, Lizone Chang, and Robert S. Ehrlich. 1999. “The Relationship between Adsorption of Cr(VI) and Soil Properties.” Journal of Environmental Science and Health, Part A 34 (4): 809–33. doi: 10.1080/10934529909376867.
Shakya, P.R., P. Shrestha, C.S.Tamrakar, and P.K. Bhattarai. 2006. Studies and Determination of Heavy Metals in Waste Tyres and their Impacts on the Environment. Pak. J. Anal. & Envir. Chem. Vol. 7, No. 2.
Stehouwer, R. and K. Macneal. “Lead in Residential Soils: Sources, Testing, and Reducing Exposure (Crops and Soils).” 2016. Crops and Soils (Penn State Extension). Accessed January 13. http://extension.psu.edu/plants/crops/esi/lead-in-soil.
Sullivan, J.P. 2006. An Assessment of Environmental Toxicity and Potential Contamination from Artificial Turf using Shredded or Crumb Rubber. Submitted to Turfgrass Producers International. www.ardeaconsulting.com/pdf/Assessment_Environmental_Toxicity_Report.pdf.
Thakur, Risikesh, G. D. Sharma, and B. S. Dwivedi and S. K. Khatik. 2016. “CHROMIUM : AS A POLLUANT.” I Control Pollution. Accessed January 12. http://www.icontrolpollution.com/articles/chromium--as-a-polluant-.php?aid=45697.
Fisher, B. and ECHO staff 2016. Tire Contaminants from a Container Gardening Perspective. ECHO Development Notes no. 130