Lead Contamination in Soil / Improving Soil


Hi Dr Kris, What a good article on soil testing thank you. My problem is all the plants that I’ve planted whether from cuttings or seed start off fine and grow, but won’t flower or set fruit, and after a short while stop growing and become retarded. I have fertilised with cow poop and chemicals with no difference. I really am at a loss as what to do next. Any ideas please. I’m now going to try your testing suggestions. Many thanks, Andrew.

Dear, Dr. Kris, I’ve read that living near heavy traffic and main roads increases the risk of lead contamination in the soil and is it bad for vegetable gardening? What are the risks or ways to make an edible garden in this situation? Thanks in advance, Jan.

The soil is where a good garden starts, and if you have a poor soil you likely be limited in what you can grow. The main factors that affect soil quality for growing flowers, fruits and vegetables depend on whether it is sandy which will mean it tends to dry too quickly, or if it is clay which means there is less aeration and a tendency to waterlog.

Rejuvenating a poor soil is more involved then pouring fertiliser over the ground. Natural fertiisers such as cow dung and chicken manure need to be worked in along with compost over time to gradually improve the soil, otherwise try raised garden beds with a whole new ‘imported’ soil for an instant solution.

As for lead contaminated soil which is sometimes an issue in urban areas, replacing the soil or amending it is also the solution. If you live near main roads, industrial areas or old buildings you would expect an elevated level of lead in your soil.

A recent Australian study led by RMIT University, Melbourne, found that 20% of edible home gardens are contaminated with levels of lead exceeding the safety guidelines of 300 mg/kg – whereas natural background levels are usually in the range of 15-40mg/kg. The figure is worse in Sydney, where a similar study found 40 per cent of vegetable patches had levels of lead that exceeded the safety guidelines. I can imagine that a similar statistic plays out across gardens in most cities and crowded urban areas the world over.

High levels of lead remain in the soil in urban areas – the result from adding the toxic metal to paint and petrol for decades. Living close to main roads is an issue due to lead accumulation in soils from traffic pollution. Vegetable gardens should be established away from busy roads – at least 20 to 80 metres away, lead levels are usually elevated directly next to main roads.

In addition – generally the older the house, or any other building for that matter containing lead-based paint, then the more lead found in the surrounding garden beds. Most often areas most affected are under the dripline of the eaves due to the weathering and runoff of leaded paints throughout the seasons.

Lead in the soil presents a risk if it is either inhaled or ingested. Lead in the soil can also be absorbed by vegetables grown in it. Children who play in the garden are also at risk.

If living near main roads, consider growing vegetables in raised garden beds filled with clean imported soil and compost. Limit potential exposure by avoiding plants known to be lead accumulators such as low-growing leafy vegetables like lettuce, as well as root vegetables including carrots, onions, turnips and radishes. As a general rule, the worst affected vegetables are either unwashed or unpeeled root vegetables. Leafy greens can be affected although much of this could possibly be surface borne dust. The least affected plants are those that produce fruiting parts – whether or not we consider them ‘fruit’ or ‘vegetables’. There’s very little danger of lead toxicity in anything from tomatoes to papayas and pumpkins and all fruit trees in general.

Plants are fairly good at keeping lead out of themselves in most circumstances, but if the vegetables are covered in soil, or even an invisible layer of dust settles on fruit or leafy greens, you may be consuming lead from the soil in that way. All fruit and vegetables taken from the garden should be washed to remove all traces of soil before consumption.

A good diet with adequate amounts of calcium and phosphorous has been shown to reduce absorption of dietary lead. Similarly, those same two nutrients help reduce lead uptake in plants. Seaweed and spirulina when eaten have also been demonstrated to bind heavy metals and remove them from the body. It’s not clear from the studies whether you have to eat them concurrently with the potentially contaminated foods or just every now and then – so to be safe just put spirulina on everything.

Any good soil will be ‘living’ filled with organic matter. Organic matter also helps bind the lead up in the soil, rendering it less available to plants. So, get composting, and adding it to the garden beds. Besides reducing lead uptake, it is also the key solution to a poor soil, producing healthier and more productive plants. Otherwise import a clean soil for an instant fix and a fresh start.

I don’t think that there is a need to worry excessively about lead in the soil – just be cautious, be alert not alarmed!

Wash your edible plants, mulch your soil, and work in compost and organic matter which will help to bind any lead, reducing the bioavailability but also improving the overall soil health at the same time.

On a final note – Old-school paints were up to 50% lead and even with newer layers painted over the top, the levels of lead left contained on the interior and exterior walls of old buildings are still likely to be high. The safe level of lead in food is measured in parts per million so it’s easy to see how disturbed, crumbling, scraped, sanded or flaking paint can be a major source of direct contamination – whether from inhalation or ingestion. Please be careful when renovating your home. Have a professional remove flaking exterior lead-based paint to prevent future contamination.

 

Dr. Kris

Garden Doctor Contact: gardendoctor@hotmail.com

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