Heavy metals are found in soil in many places. Super plants are currently being researched that can remove these hazardous substances from the soil again.
Heavy metals can get into a garden soil in different ways. The problem is: These can accumulate in food chains, for example in vegetables that are grown on polluted soil. Regular intake in small doses can even lead to chronic heavy metal poisoning. In order to be able to use polluted soil completely and safely again, science is now resorting to the power of nature - and you as a garden owner can also benefit from this.
In various countries around the world, intensive research is currently being carried out into so-called super plants. These also grow on sites that are heavily polluted with heavy metals and are considered a promising solution when it comes to detoxifying heavily polluted soils. In the following explanations you can find out what super plants are, against which heavy metals they can be used in the garden and which plant is suitable for removing which heavy metal.
What are super plants?
In France, the ecology professor Claude Grison is researching the decontamination of a former mine site and is primarily relying on kidney vetch (Anthyllis vulneraria) and Hellerkraut (Thlaspi caerulescens). ) to remove arsenic, lead, cadmium and zinc from soil. In Canada and Albania, too, plants are used to renaturalize polluted soil. Various plant techniques are used here: while kidney vetch and Hellerkraut, for example, actively absorb heavy metals and store them in their plant body ("hyperaccumulators"), willow (Salix) is able to absorb pollutants with the help of symbiotic microorganisms in the soil harmless ("phytodegradation"). Around 500 such super plants have already been identified worldwide, which enable the improvement of polluted soils in various ways.
Heavy metals in the garden
Excessive levels of he alth-related heavy metals can also be found in house gardens and allotments. Elevated concentrations can often be found on former industrial areas or landfills, as well as in former flood zones of rivers and in the vicinity of transport routes. In addition, the increased use of mineral fertilizers and pesticides can be an entry route. And of course the distribution of contaminated soil, waste materials or domestic ashes is also a way of getting heavy metals into the garden. The good news is that heavy metal pollution in fertilizers is government controlled. However, this control is subject to constant change and is considered to be insufficient, and therefore one should not fully rely on the protective effect of the existing regulations. When buying fertiliser, therefore, always make sure that it is of high quality.
Tip: Mineral phosphorus fertilizers in particular play a role, because rock phosphates often have increased levels of cadmium and uranium. This special article deals with soil pollution from fertilizers.
The following heavy metals can be detected in increased concentrations in German gardens:
| Heavy Metal | Element Symbol | Sources | Effect on humans |
|---|---|---|---|
| Lead | Pb | Fertiliser, Industrial Dust/Rain, Lead Shot, Lead-Sheated Wire | Uptake via the respiratory tract or food leads to accumulation, lead is not excreted. Chronic poisoning is possible |
| cadmium | Cd | Various consumer goods produced before 2011, today's paints, electrical appliances, (artificial) fertilizers and pesticides | Can cause severe chronic and acute poisoning when ingested through food |
| Chrome | Cr | Chrome alloys, leather goods, industrial residues, emissions from waste incineration, fungicides | Uptake as dust leads to diseases of the respiratory tract, ingestion through food leads to chronic poisoning |
| Nickel | No | Nickel-bearing ore deposits, industrial contamination, emissions from waste incineration plants | Vital trace element for plants and humans. Has an allergenic effect on skin contact, inhaled nickel dust is carcinogenic |
| Mercury | Hg | dumps, coal burning, waste incineration,Cement Plants, Steelmaking, Fertilizers | Uptake from floor dust and vapors are highly toxic. Mercury vaporizes at room temperature. Unlikely to be ingested through food and less dangerous |
| Copper | Cu | Natural release from rocks in soil, Mining, Industrial, Pesticides containing copper, Slurry (from contaminated feed) | Vital trace element for plants and humans. Increased intake promotes arthritis and high blood pressure |
| Zinc | Zn | Dust and waste water from power plants, industry and mining, sewage sludge | Vital trace element for plants and humans. Throughout Germany there is more of a zinc deficiency. Dust from heavily polluted soils can cause poisoning |
| Arsenic | Ace | Arsenic-bearing, near-surface rocks, ore mining or enriched groundwater | Possibly vital trace element for plants and humans. Excessive intake leads to severe chronic or acute poisoning and is carcinogenic |
| Uranium | U | From soil-borne rocks, phosphate fertilizers, contaminated fertilizers for animal feed, also less in organic fertilizers (slurry, manure), contaminated sites from historic mines | Depending on the intake concentration, the toxic or radiological effect predominates. Accumulates in bones when ingested orally. Toxic effect primarily affects the kidneys |
| Thallium | Tl | From historic cement works and mines and their overburden, from near-surface ore deposits | Toxic, causes skin changes, hair loss, liver and kidney damage, mental changes |
| Antimony | Sb | Made of ammunition containing lead, metal alloys, textiles containing antimony, plastics, brake pads, paint, pesticides or fireworks | Gase antimony most dangerous. Solid forms less toxic. It has a toxic effect on numerous organs: digestive tract, liver, kidneys, heart, nervous system. Small doses irritate skin and mucous membranes |
Tip: Some heavy metals - such as nickel, copper, zinc and iron - are among the essential nutrients for plants and animals and occur naturally everywhere in our environment. However, they are only harmless to us and plants in these small concentrations.
Soil analysis: finding pollutants in the garden
Soil analyzes are very useful to check the condition and nutrient supply of your soil and to adjust fertilization or humus management accordingly. You can also have a specialist laboratory carry out a soil analysis to find out whether there are any pollutants in your garden soil. To do this, take soil samples, proceeding as follows:
- A minimum of five soil samples are required per 1000 square meters.
- Samples are taken from different, evenly distributed locations from a depth of 20 to 25 cm. Untypical spots (molehills, dead-end paths, lawn edges) should be left out. For lawns, sampling from a depth of 10 cm is sufficient.
- The individual samples are very thoroughly crushed and mixed in a bucket. The quantity required by the laboratory is taken from this mixed sample.
The pollutant analysis shows the heavy metals listed above in various investigations. The laboratory will inform you whether the detected values are too high with reference to the applicable limit values. If you have the test result for your soil, you can rest easy - or react appropriately to any increased values. A possible response on your part may be to choose the vegetables you grow carefully, as some heavy metals are only found in high concentrations in a few plant families. If you want to remove heavy metals from your soil, you can find out more about suitable super plants in the following paragraph.
Super plants for the garden
Because research into super plants is currently in full swing, we were able to find a suitable plant for your garden for (almost) every heavy metal mentioned above. You can find pictures of the super plants in the slide show.
Warning: Even if some of the plants mentioned can otherwise be used as kitchen herbs, they are no longer suitable for consumption after cultivation on a contaminated area! They accumulate the respective heavy metals in he alth-relevant concentrations in their roots, shoots and leaves.
1. Kidney vetch (Anthyllis vulneraria)
Nursery vetch effectively binds zinc (Zn) from the soil. The perennial likes sunny, dry locations and bears numerous yellow flowers in spring and summer.
2. Banded mulberry fern(Pteris vittata)
Banded ferns primarily absorb arsenic (As) from the soil. The 30 to 80 cm high, evergreen fern prefers stony soil and only thrives in partially shaded or shady locations. It is only partially hardy in Germany and should be protected against frost accordingly.
3. Wall Stonewort (Alyssum murale)
The stonecrop effectively binds nickel (Ni) from the soil. The small, bushy perennial needs full sun, grows to a maximum height of 30 cm and bears small yellow flowers in panicles. It prefers well-drained, stony subsoil, but can also do well in normal garden soil.
Avena strigosa)
Sand Oat effectively binds cadmium (Cd) from the soil. The old European cultivated plant is an annual and grows well in all soils. The cereal plant grows up to 120 cm tall and has blue-green foliage.
5. Brown Mustard (Brassica juncea)
Brown Mustard effectively scavenges Copper (Cu), Nickel (Ni), Lead (Pb) and Zinc (Zn) from soil. The annual cabbage plant forms upright shoots that are no more than 100 cm high. All soil types are tolerated, but fresh, moist and nutrient-rich soils are preferred. The yellow flower appears between July and August.
6. Broom cypress (Bassia scoparia syn.: Kochia scoparia)
The summer cypress effectively binds chromium (Cr), lead (Pb), mercury (Hg), zinc (Zn) and uranium (U). In addition, the annual ornamental leaf plant with the fine foliage is undemanding and thrives best in sun or semi-shade in permeable soil that is not too nutrient-rich.
7. Parsley (Petroselinum crispum)
Parsley effectively binds mercury (Hg). The actually biennial plant loves well-drained and humus-rich soils as well as sunny to partially shaded locations.
8. Smooth Spectacles (Biscutella laevigata)
The smooth goggle tube effectively binds thallium (Tl) from the soil. The low perennial bears decorative, yellow flowers and seed pods at a height of 25 cm. The name comes from the interesting shape of the seed pods. The spectacled shell prefers fresh and at the same time stony soil in a sunny location.
Tip: Unfortunately, no effective superplant has yet been found for the semimetal antimony (Sb).
Dispose of heavy metals
With thatthe content of pollutants in your soil decreases in the long term, on the one hand the repeated or permanent cultivation of super plants is necessary. On the other hand, the parts of the plant enriched with the heavy metals must be removed and disposed of naturally and must never remain on the area or be composted. This could even make the pollution worse: After the decomposition of contaminated plant material, the harmful element goes into the soil in a more easily soluble form and is also found in the top soil layer. Therefore, first collect the cut parts of the plant and dispose of them in the hazardous waste container at the recycling depot.
Conclusion on the super plants
In the coming years, research will certainly identify more plants that reliably draw toxins from the soil. This may also be relevant for home gardens. The good news is that you can also take precautions in your home garden to prevent heavy metal pollution - for example, avoiding unnecessary chemicals. If you want to learn more about sustainable gardening, here are 9 tips.