Soil quality and lead (Pb) levels were determined in 10 Chicago gardens, in both raised bed and non-raised bed food growing areas, and in nearby paths or playgrounds. Soil Pb was measured through acid digestion with the Environmental Protection Agency (EPA) 3050B method and a Mehlich-III (M-III) extraction. Soil in raised beds contained less Pb and thus reduced potential Pb ingestion from plant uptake. Higher soil Pb levels in nearby areas suggest possible contamination to raised beds. Results suggest that the M-III test could be developed as a less expensive Pb assay. Participating gardeners learned about soil quality, contamination, and testing.
Urban gardening is a popular activity that offers many benefits to participants. Urban gardeners, however, face unique soil quality challenges, like compaction, low organic matter, and pollution. This project focuses on the issue of soil lead (Pb) contamination by evaluating the soil Pb levels in different types of areas within 10 urban garden projects in Chicago.
The nonprofit GreenNet lists over 600 community gardens currently in Chicago (GreenNet, no date). Forty community gardens exist within the Chicago Park District alone (Chicago Park District, 2008). Gardening has increased importance in low-income neighborhoods, where access to fresh food may be limited.
In addition to food production, gardening projects offer numerous other benefits to urban participants: increasing neighborhood stability, helping people meet self-esteem and social needs, providing a place for interracial interaction, and encouraging healthier eating habits (Tranel and Handlin, 2006; Shinew et al., 2004; Waliczek et al., 2005; Koch et al., 2006). Environmental benefits of urban gardening include increasing biodiversity and wildlife habitat while reducing soil erosion, air pollution, and waste (Brown and Jameton, 2000; Doron, 2005; Garnett, 1997).
Unfortunately for urban gardeners, urban settings may contain contaminants that can pose risks to them, children who play in the gardens, and consumers of garden produce (Clark et al., 2006; Finster et al., 2004; Hough et al., 2004; Sipter et al. 2008; Tokalioglu and Kartal, 2003).
Currently, the Environmental Protection Agency (EPA) method for measuring soil Pb is a total digest of soil, a process that attempts to measure all forms of Pb within the soil (USEPA, 1996). Much research is devoted to finding a measurement that predicts only bioavailable forms of Pb. Despite this, Menzies et al. (2007) reviewed literature covering extractants and metal phytoavailability and concluded many common extractants poorly estimated plant availability (Menzies et al., 2007). Simple extracts like Mehlich-I (M-I) and Mehlich-III (M-III) are attractive options since these are routinely used by commercial soil testing labs.
Numerous agencies and studies advise urban gardeners to avoid exposure by importing soil materials and growing produce in raised beds (Chicago Park District, 2008; Peryea, 1999; Finster et al., 2004; Stilwell, et al., 2008). Little research has been done to verify the effectiveness of this solution.
Original objectives include:
• Informing users of many Chicago community and school gardens about soil quality and how to approach the challenge that lead poses to human health and plant productivity
• Increasing the knowledge about local food production and the scientific process among students of participating schools
• Helping gardeners whose soil is found to contain high levels of lead take steps to remediate the problem.
• Helping gardeners from around the city may communicate more frequently with each other.
• Making soil testing a habit among urban gardeners and that by including underserved students, we will encourage them to feel excited about science and consider higher education.
For garden selection, I contacted representatives (garden managers or volunteers) of over 30 community gardens, school gardens, and other urban agriculture projects in the city of Chicago that were found through nonprofits, websites, and word of mouth. Sixteen gardens expressed interest in the study. Complying with the University of Illinois at Urbana-Champaign Institutional Review Board’s (IRB) procedures for human subjects, a questionnaire was developed and distributed online to those 16 representatives. The 10 self-selected gardens that completed the questionnaire were invited to and agreed to continue participation. Soil was sampled from the gardens in late May and early June of 2008. A total of 86 soil samples (0 to 30 cm deep with a 5 cm diameter volume corer) were taken from the 10 sites. At most sites, four soil cores were taken from food-growing areas and three soil cores from nearby areas of soil not used for growing food (exposed soil in pathways, ornamental beds, places where children play).
Physical and chemical characteristics of the samples were determined. For standard nutrient and Pb analysis, samples were sent to Brookside Laboratories in New Knoxville, Ohio which uses Gavlak et al. (2003) to determine percent organic matter (OM), pH, and potassium (K), phosphorus (P), copper (Cu), aluminum (Al), and zinc (Zn) with a M-3 extraction. Pb was determined with both a M-3 extraction and through USEPA Method 3050B using ICP (USEPA, 1996).
In general, studies suggest lettuce and other leafy crops accumulate Pb more than other vegetables. Lettuce seedlings were grown in a sub-sample from vegetable gardening areas to directly evaluate plant uptake. Sub-samples of soil from each garden area were treated as replicates and were divided among four plastic flats. After 30 days, lettuce was harvested. Each plant was gently rinsed with water in a sieve under the tap to wash away soil particles, then rinsed in soapy water, then washed again with tap water and finally washed with deionized water. Roots were separated from the leaves and stems and plants were then oven dried and ground. Plants were analyzed for total Pb content at Brookside Laboratories using ICP after acid digest with EPA method 3050B (USEPA, 1996). To meet weight requirements for analysis, the roots or shoots (leaves plus stems) were pooled for some gardens.
The MIXED procedure in SAS (PROC MIXED, SAS v9.1.3, SAS Institute, Cary, NC, USA) was used to compare different garden areas based on least-squares means for the variables OM, estimated nitrogen release (ENR), P, K, pH, EPA Pb, M-3 Pb, Al, Cu, and Zn. The three types of garden areas (raised beds, non-raised beds, other areas) were treated as fixed effects and garden site was a random effect. All variables (OM, ENR, P, K, EPA Pb, M-3 Pb, Cu, Zn) except for Al and pH were not normal and were transformed before analysis.
Simple regression was used to evaluate the relationship between M-3 and EPA Pb and between Pb concentration in lettuce leaves and soil Pb fractions. Sample sizes of roots were too low to perform meaningful analyses between root Pb and soil Pb levels. Stepwise multiple regression analysis (PROC REG, SAS v9.1.3, SAS Institute, Cary, NC, USA) was used to find which variables (pH, OM, M-3 Pb, EPA Pb, and lettuce biomass) were most important in determining total leaf Pb. To enter the model, the significance level needed was 0.5 and to stay in the model was 0.05. Non-normal variables, M-3 Pb and EPA Pb, were transformed. Simple regression was then performed between total leaf Pb and biomass.
The 10 gardens have unique site histories, with former uses ranging from sanitariums to parking lots. Questionnaire respondents were asked to estimate the number of participants or volunteers that use the garden and the total number of grocery bags full of food produced per season. They were given a wide range of fixed values to choose from to help make this estimation. In total, between 294 and 600 people participate in gardening at the sites and between 193 and 410 grocery bags full of food are produced.
Only one of the respondents was aware of previous soil testing for metals at their site, and that volunteer did not have the testing results. Another respondent reported soil fertility had been tested in 2006. This underscores the need to educate the public about soil testing.
Treatment-based differences (raised beds, non-raised beds, other areas) between all fertility variables (OM, N, P, K, pH), were significant at an alpha level of 0.10 or less. Raised bed garden areas contained higher amounts of OM and N than soils in non-raised bed garden areas or other areas, while non-raised bed gardens and other areas contained similar amounts of OM and N. Raised bed garden areas also contained more P and K than non-raised bed garden areas and other areas. These differences were significant for each type of area regarding P, but only for raised bed gardens and other areas for K. Raised bed gardens contained excessive amounts of P (a mean of 266 ppm P) and all three types of areas averaged excessive amounts of K. Raised bed gardens had the lowest pH, at 7.3. This pH was significantly different from non-raised bed garden (pH 7.8) and other areas (pH 7.7), but non-raised bed gardens and other areas were similar. Because P was determined through M-3, the fact that the soil was alkaline is less likely to have caused an underestimation of P. The Cooperative Extension System recommends a pH of 6.0 to 7.0 for vegetable gardens (2008), but a pH higher than 7 may preferable in urban areas. This is because at a higher pH, Pb is less soluble and thus less available to plants for uptake (Martínez and Motto, 2000).
Average soil Pb levels in this study were lower than in other studies sampling in Chicago. The overall mean total Pb level for the study was 142.3 ppm. Shinn et al. (2000) reported one of their two study areas in a residential Chicago neighborhood averaging over 2,000 ppm soil Pb. Another study sampling partially in that same study area, but specifically within gardens, found an average of 639 ppm Pb amongst 87 samples (Finster et al., 2004). A third study collected 57 samples from properties owned by the city of Chicago and found an average of 395 ppm Pb (Kay et al., 2008).
Treatment effects for metals were significant. Least-squares means showed that raised bed garden areas had significantly less Pb as reported by EPA than non-raised bed garden areas, though their mean could not be separated from other areas. With the M-3-based estimation of Pb, raised bed and non-raised bed garden areas were significantly different, though other areas and non-raised bed garden areas could not be separated. The lower Pb levels in raised beds is possibly due to the fact that raised beds hold more clean, imported materials than gardens without raised beds, verifying their importance in urban gardens. Despite the wide variety of raised beds in this study – some were only a few inches high and had no barrier between original soil and imported soil – the technique worked to reduce Pb and metal levels.
Total Pb levels and M-3 Pb levels were highly correlated. The M-3 method may offer a less-expensive alternative to using the total soil EPA digest for the types of soil in this study, mostly garden soil high in OM. Because M-3 is a more affordable procedure, its use for soil Pb testing could encourage more urban gardeners to test. The high correlation to the EPA method means an easy calculation would allow them to convert their M-3 number to a number based on the methodology used for the EPA (and other) soil Pb recommendations.
The mean shoot Pb concentration was 7 ppm and for roots was 11.8 ppm. Higher concentrations in roots is consistent with other studies (Finster et al., 2004; Liao et al., 2007). No correlation existed between total or M-3 soil Pb and shoot Pb concentrations in the lettuce.
The Stepwise multiple regression analysis found lettuce biomass to be the only variable among pH, OM, EPA Pb and M-3 Pb to be related to total leaf Pb. Simple regression showed an R2 of 0.75 between these two variables. In general, the plants absorbed a small amount of Pb, and this suggests the plants would have absorbed more Pb if they had been allowed to grow larger.
Educational & Outreach Activities
This project resulted in a masters thesis, a publication (status pending), a newsletter about urban soil quality, and an interactive website with videos about soil testing.
Outreach efforts also included participation in two field days, two workshops, two seminars, and an online training session.
Impacts of this project include:
• Ten urban garden sites were accessed for soil fertility and lead, aiding production in those gardens.
• Urban gardeners learned strategies to avoid elevated soil lead levels.
• Urban gardeners learned about testing soil for metals and fertility.
• A website about urban soil was created:
• The possibility of using a less-expensive Pb assay for urban gardeners (M-3) was explored
Because the ten gardens tested in this project have between 294 and 600 total participants, this project likely affected hundreds of people, all benefiting from the soil testing in their gardens. In addition, 47 people took part in learning about urban soil quality at the two festivals where we displayed information and gave demonstrations. And a total of 64 people took part in our live and online trainings about urban soil. The urban soil website which was created reaches even more people.
The economic impact of this project has great potential – if a M-3 soil Pb test were adopted for use in urban gardens to replace the EPA test, gardeners could a significant amount of money when testing.
We believe the likelihood of farmer adoption is high. Sixteen of 21 participants at two workshops we helped conduct indicated that as a result of the workshop they were likely or would definitely have their soil tested in the next four years. Nineteen participants said they definitely increased their knowledge about soil contaminated and 15 said they defiantly increased their knowledge about soil quality.
Practical recommendations for urban gardeners to avoid Pb:
• Regularly testing urban soil for lead
• Using raised beds for growing food, filled with uncontaminated compost and other materials
• Replacing compost in raised beds regularly
• Gardening with gloves
• Washing urban-grown produce with soap and water
• Selecting urban garden sites away from roads and frames of houses
• Having a blood test for lead levels
Areas needing additional study
More resources, in particular online resources, are needed to explain urban soil quality and risks to the public. We also recommend further studies to develop using M-3 as a comparable soil Pb assay to the EPA test.
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