The Loam Ranger – Community orchards and soil contamination

SESL Australia

Loam Ranger logoI am generally looking at the feasibility (cost) of getting public land tested before planting fruit trees on them to avoid potential health risks from contaminated soil. Do you know whether fruit absorbs any heavy metals or contaminants? I was under the impression it is absorbed into the trunk/bark and leaves. Do you know if this is true and a good publication that simply states this?

There is a general rule that heavy metals (HMs) do not get into fruiting structures easily, and hence grains and fruit are nearly always protected.

The second rule is that phytotoxicity prevents ecotoxicity. This means that if a plant were to take up enough HMs to cause harm to grazing animals or any animal that ingests the foliage, the plant would be dead and hence the grazer would be protected anyway. This is not quite true with chronic high-level intake (eating the same contaminated food every day for months or years).

These two mechanisms explain why HM toxicity in humans from eating food grown on contaminated soils is uncommon. The only exception is where leaf vegetables like spinach or silver beet (and root crops like potato) grown on lead-contaminated soils can contribute to the overall dietary input of a young child. This is secondary, however, as pica (the direct eating of soil) is the main dietary input in infants.

In more detail, plants fall into one of three groups in relation to HM uptake: the excluders, which have mechanisms in the roots to avoid taking up HMs in the first place; the tolerators, which take up HMs and then park them out of the way; and the intolerant species, which take up HMs and die (and therefore will never be eaten).

Bioavailability

But any plant’s reaction to HMs in soil depends first and foremost on the bioavailability of the HMs. Bioavailability is defined simply as the availability of a chemical to an organism. An extreme example of a totally bio-unavailable chemical is titanium dioxide (TiO2). This brilliant white chemical is the main pigment in white paint and is included in thousands of food products where whiteness is required, as well as in personal care products such as toothpaste. Titanium is, by some definitions, a heavy metal (see the companion article on HMs in this current issue of Fertile Minds), yet TiO2 is utterly insoluble. You can eat as much as you want (within reason), but your body will be unable to absorb any.

In the context of plants in soils, many HMs occur as both natural components and accidental contaminants in many different forms. The bioavailability of HMs depends principally on the chemical form of the HM (for example, soluble or insoluble) and on the soil pH (a low, or acidic, pH makes many HMs more soluble and thus more bioavailable). For example, zinc (Zn) is commonly present in urban soils as zinc oxide (ZnO), from corroding galvanised steel; ZnO is insoluble, so plants will not take up Zn from ZnO. But Zn is present in the soil in other forms as well, some of which plants will take up.

It also depends on many (some poorly understood) factors that operate singly and together, including how much of the HMs is present, which HMs inhibit or enhance the uptake of other HMs, soil minerals present, microorganisms that might release the HMs into the soil solution, and the presence of plant roots that exude acids to make nutrients more available and coincidentally make HMs more available.

It is not possible to know all of these factors. But it is possible to use lab tests that estimate the bioavailability of HMs in the soil. SESL uses a range of standard tests that meet the requirements of the legislation concerning HMs and that give a good indication of whether any HMs in the soil will be taken up by plants.

In addition to soil tests (to show what’s there), if you are concerned, arrange some plant tissue tests as well, which will show definitively which HMs (if any) the plants have taken up.

Further reading

Chaney R. USDA. Rufus Chaney argues that HM poisoning in humans is virtually impossible.

Department of Environment and Conservation NSW. 2006. Contaminated sites. Guidelines for the NSW Site Auditor Scheme, 2nd ed. DEC 2006/121. DEC, Sydney.

Wikipedia. Bioavailability.