There are many soil additives on the market, all offering gardeners the appealing promise that they will make your plants bigger or healthier. Often there is little, if any, way of knowing whether they will work - the claims made by the products may simply not have been adequately tested, and it's hard to tell what's in them.
Volcanic rock dust is touted as having many useful benefits – but is it worth buying rock dust? We asked ecologist Ken Thompson for his advice.
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What is rock dust?
Rock dust is finely ground volcanic rock.
What is rock dust supposed to do?
Apparently there’s almost nothing this finely ground volcanic rock can’t do, including producing bigger yields, healthier crops, better flavour and improved resistance to pests, disease and drought.
What do trials on rock dust show?
Fortunately, there have been two big, well-designed scientific studies of the effects of rock dust, one in Scotland and the other in Sweden, and both found exactly the same: nothing. There was no effect on yield, plant nutrient content or soil chemistry.
These studies also show why they found nothing. As the Scottish analysis puts it, ‘a high degree of rock weathering was required to release small quantities of trace elements from rock dust samples’. In short, it’s not easy to get anything out of rock dust, and even when you do it’s mostly sodium, with small amounts of calcium and even smaller amounts of other constituents. So the most rock dust might sometimes do is raise soil pH a little, although in the Scottish study it didn’t.
Should I buy rock dust?
Soil is mostly rock already, so adding a bit more was never likely to make much difference. But rock is pretty inert stuff, so if you still feel like using it, it’s very unlikely to do any harm.
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