Are chemical contaminants killing my plants?
Healthy soil is essential for healthy vegetable and fruit plants, but often we don’t know what is lurking in the ground beneath our gardening feet.
And when growers watch their plants wither and die, or fail to launch, they naturally wonder if it’s because there’s something sinister in the soil.
This was the question asked of us at NZ Gardener by Eve Osborne, of Clevedon, who wondered why her vegetables failed to grow.
“Despite putting lots of organic vegetable compost in the bed before planting out tomatoes, they simply did not grow. Nor did the beans, capsicum, and lettuce,” Osborne says.
“I think the ground is polluted with the sheep pellets and the pesticide that was in them.
“I have tomatoes growing elsewhere in the garden and they are doing well without sheep pellets.”
It would be good to know what residues are in my garden and how to remove them, she says.
The good news is that her soil is unlikely to be contaminated with high levels of zinc, lead, arsenic, copper, clopyralid (a herbicide used to control broadleaf weeds in turf and lawns) or glyphosate weed killer — all chemicals that have been tested for in New Zealand gardens by two scientists from the the School of Environment at Auckland University in a survey.
Environmental scientist Dr Melanie Kah (who completed her PhD on the fate of pesticides in soils) and Dr Emma Sharp (her research interests
are food politics and citizen engagement in science, and the environment) are the backbone of Soilsafe Aotearoa, set up in partnership with researchers at GNS Science and Sydney’s Macquarie University.
And the research reveals that levels of arsenic, lead, copper and zinc would have to be very high to be toxic to all plants, Kah says.
As for the sheep pellets, they’re unlikely to contain herbicide residues that are sufficiently high to prevent plant growth, she says.
“And we tested a series of commercial compost last year and found no herbicide residues at all,” Kah says.
However, she does suggest Osborne check her soil for pH levels, which can be done easily with kits available at
garden centres. A pH of 6.5 is ideal and the range from pH 6 to 7 is good for vegetables. But below 5.5 and 7.5, soil treatment and modification is needed.
Further scientific testing can get expensive, Kah says. “To be honest, unless there is a clear point source, the levels of (metal) contaminants are unlikely to prevent plant growth, so probably not worth the investment.