Urban Gardening: 12x More Cadmium in Your Tomatoes Than in the Conventional Produce? Plus: Domestic vs. Foreign & Conventional vs. Organic - What's Healthier?

Image 1: Allotment gardens are the new trend among young families in Germany, 45% of the 1,000,000 allotment gardens that were previously the territory of stuffy men and their garden goblins are already in their hands - that equals a total "acreage" of 21,000hectar - let's hope none of these gardens is right next to a street.
Those of you who have listened to the "Urban Gardening" episodes Alisa Profumo did on Super Human Radio back in 2011 (click here to download Part I and Part II) may remember that it is not impossible to grow your own vegetables, herbs and more - and that not just in a large garden. In fact, "urban gardening", which has here in Germany long been regarded as antiquated and "for grandmas and grandpas only" has become fashionable again, as more and more people enjoy the comfort of "knowing what they eat". Unfortunately, a recently published German study shows that believing does not mean knowing; or do you believe that people who knew that their homegrown tomatoes have 5.7x more nickel, basil more than 5.4x more anorganic chromium and their carrots 4.2x more cadmium than their conventionally grown counterparts from the supermarket around the corner would still not unsettle their faith in the general and unquestionable superiority of home-grown produce?

From the exhaust pipe into your garden and from your garden into your mouth :-(

Just to make sure that we understand each other: This is no anti-gardening post, it is just a brief reminder of the differences between hearsay and nostalgia, on the one hand and facts and reason, on the other. In that, it is very reasonable to assume that the constant pesticide assault is a problem, it is however not reasonable to assume that veggies and herbs you grow in the toxic environment of a major city like Berlin or its rural outskirts are per se "pollutant free".

Figure 1: Main importers of vegetables, root and tubers in 2011 (ITC calculations based on UN COMTRADE statistics)
Did you know that Katz et al. have shown in 2008, that...
  • the pesticide residues in domestic (US) produce (fruits and vegetable) were significantly higher for 11 pesticides while imported exposures were higher for the remaining four. 
  • all five pesticides that were found in potentially hazardous (yet on a per serving base still licit level) were the higher in US than in imported produce
  • the mean daily exposure estimate for one pesticide, methamidophos, was above the reference dose for domestic fruits and vegetables while slightly below the reference dose for imported fruits and vegetables
... probably not, right? If you want to avoid pesticides, all-together it is advisable to chose organically grown produce, anyway - also for it's slightly higher nutrient content (see next infobox)
This is all the more true in view of previous exploratory studies, which have already shown that horticultural crops in urban or peri-urban areas are generally exposed to a higher level of pollutants of both inorganic and organic origin than the conventional produce they are supposed to replace (Shinn. 2000; Alloway. 2004). And as Ina Säumel and her colleagues point out,
"the contamination of urban horticultural products can exceed the precautionary values, and a dietary exposure to trace metals can result in significant human health risks." (Säumel. 2012)
That said it was only logical that the researchers from the Technische Universität Berlin and their colleague from the Botanical Garden of the Khmelnitskij National University in  Khmelnitsky, Ukraine, set out to compare the positive view the public has on urban- and peri-urban gardning (according to a survey among gardeners, producing "fresh and healthy fruits and vegetables" is one if not the main motivations for gardening; cf. ) to the actual level of potentially hazardous substances in their produce.

The samples were collected from 28 randomly chosen sites within inner city neighborhoods in Berlin, Germany. As the authors point out,
"[t]hese sites represent a mixture of different local settings of horticultural plantings within Berlin’s centre  and were characterized according to the following parameters: (a) overall traffic burden (otb) within a radius of 1 km around planting sites [...] (b) traffic burden of the nearest road per day [...], (c) distance to nearest road (d; in meters); and (d) presence or absence of barriers between planting sites and next street which might reduce airborne pollution." (Säumel. 2012)
In combination with the data on the planting styles of the individual produce this allowed for a very detailed analysis of the confounding factors that influenced the the zinc, copper, lead, cadmium, chromium and nickel concentrations of the 12 different horticultural crop species (tomatoes, green beans, carrots, potatoes, kohlrabi =Brassica oleracea var. gongylodes, white cabbage, nasturtium, parsley, chard, basil, mint, thyme).
Figure 2: Trace metal content of 12 different horticultural crop species from 28 randomly chosen sites within inner city neighborhoods in Berlin, Germany; data expressed relative to conventionally produced produce from the local supermarket, the dotted black line indicates the respective 100% content of the conventional produce - everything thats above this "demarcation line" has more, everything below less of the metal (calculated based on Säumel. 2012)
If you take a look at the results you may initially be shocked - 12x more cadmium in "home-grown" tomatoes than regular produce? The currently allowed maximal level of this toxic trace metal depends on the respective legislation (I guess I don't have to rant about what those differences tell you about the arbitrariness of "allowances" & co) and is in the range of 0.1mg/kg for vegetables with 0.12mg/kg the mean content is thus already right at the red line of the official "danger zone" - the maximal detected value of 0.79mg/kg is 7.9x higher and certainly no healthy alternative to the conventional supermarket tomatoes with 0.01mg/kg (i.e. 10x below the tolerable limit) of cadmium in them.
Figure 3: Soil, overall traffic burden and the distance to the nearest road are all confounding factors that will influence how much unwanted trace metals your pesticide-free produce will contain.
As you may already guess, both the overall trace metal load as well as the high discrepancies between individual vegetables and herbs are largely dependent on their individual traffic exposure with zinc, lead, nickel and chromium showing statistical significant correlations with the overall traffic burden and lead, chromium and cadmium with the distance to the nearest road.

Plant away from roads and/or behind fences or walls + other things to protect your crops

Did you know that one of the latest meta reviews confirmed the superiority of organic vs. conventionally grown produce as far as its nutrient content is concerned? According to Hunter at al. the nutrient content of organically grown vegetables was 5.9% higher, that of legumes 5.7% (for fruits the 6.5% difference did not reach statistical significance; cf. Hunter. 2012). There is however a significant difference between screened and non-screened trials, with the former having a clear-cut bias towards an even higher difference than the latter (mean difference 8%). It is however at least questionable how valuable a classification like this is, when the "superior" mineral content of organically grown veggies, for example, is mediated among others by a statistically significant +6% higher phosphor content, the level of calcium and magnesium, however is only non-significantly higher (0.6% and 3%, respectively).
Aside from planting further away from, or next to low traffic streets, placing your plants behind fences, walls or other protective barriers can be a valid method to reduce the trace metal content of your crop. Moreover, Säumel et al. point out that regardless of the traffic burden,
"[...]the vegetables planted in urban soil beds were less likely to have lead values above the critical values than vegetables planted in pots or beds that had been supplemented with commercial garden soil. This might result from the use of compost, which increases metal solubility, but more research is needed to reveal underlying causes as we could not analyse soil trace metal contents." (Säumel. 2012)
This does not necessarily mean that planting in the local soil must be better, nor is it certain that the commercial garden soil was already contaminated, it should yet raise your awareness that even when you are planting outside of the 10m danger-zone of the next street in which 67% of the analyzed crop has lead values that exceeded the standards of the European Union, the lead, cadmium, nickel and what not could already be in the soil of your garden or be accidentally introduced by the commercial garden soil you bought and used to increase your produce (for tips on safer ways than the next best fertilizer from the garden center, listen to the initially mentioned gardening episodes of Super Human Radio).

Picking the right crop, such as kohlrabi, green beans or basil and installing extra fences using the back instead of the front yard are other means that would allow for "safer gardening" even in the inner city. When you are living right next to the highway, all that will yet probably be useless. I don't know if you have something like that wherever you are from, but the good old German "Schrebergarten" (=allotment garden), is becoming increasingly popular, over here - and, as I initially mentioned, not just among old stuffy men and their garden goblins; for me personally that would not be an option, but who knows, maybe it is for you?
Bottom line: Done right, i.e. not directly next to a high traffic street and without the use of standard commercial soils and fertilizers, yet fully aware that neither the "remarkable taste" of your tomatoes, carrots & co, nor being able to say "I produced it in your own garden" let alone "on my own balcony" will  protect you from the toxins that may be present in what your 100% pesticide free produce, gardening does still offer a safe and above all self-empowering alternative to buying all your goods at the farmers market, super market or wherever else you prefer to shop.
  • Alloway, BJ. Contamination of soils in domestic gardens and allotments: a brief review. Land Contamination and Reclamation. 2004; 12:179-187
  • Burda GmbH, 1993. Wohnwelten und Gärten in Ostdeutschland: Alltagsästhetik, Wohnmotive, Wohnstile, Gartenwerte und Gartenstile in den neuen Bundesländern. ein Forschungsbericht der Burda-GmbH, Offenburg, und Sinus, Heidelberg, 108 p. (German).
  • Hunter D, Foster M, McArthur JO, Ojha R, Petocz P, Samman S. Evaluation of the micronutrient composition of plant foods produced by organic and conventional agricultural methods. Crit Rev Food Sci Nutr. 2011 Jul;51(6):571-82.
  • Katz JM, Winter CK. Comparison of pesticide exposure from consumption of domestic and imported fruits and vegetables. Food Chem Toxicol. 2009 Feb;47(2):335-8. Epub 2008 Nov 27. 
  • Säumel I, Kotsyuk I, Hölscher M, Lenkereit C, Weber F, Kowarik I. How healthy is urban horticulture in high traffic areas? Trace metal concentrations in vegetable crops from plantings within inner city neighbourhoods in Berlin, Germany. Environ Pollut. 2012 Jun;165:124-32. Epub 2012 Mar 22.
  • Shinn NJ, Bing-Canar J, Cailas M, Peneff N, Binns HJ. Determination of spatial continuity of soil lead levels in an urban residential neighborhood. Environmental Research. 2000; 81:1-7.
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