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Columnist Ken Thompson urges caution over so-called miracle garden products; they're not always all they're cracked up to be

There is seemingly no end to miracle additives on the market, all offering gardeners the appealing promise that they will make your plants bigger or healthier - or possibly even both. Often there is little, if any, way of knowing whether or not they work the claims made by these products may simply not have been adequately tested, and sometimes it's hard to tell what's in them.

There are at least three that should make you pause for thought before parting with your hard-earned cash: rockdust, biochar and mycorrhizal fungi.

Rockdust

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. Fortunately, there have been two big, well-designed scientific studies of the effects of rockdust, one in Scotland and the other in Sweden, and both found exactly the same: nothing. 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 rockdust samples. In short, it's not easy to get anything out of rockdust, and even when you do it's mostly sodium, with small amounts of calcium and even smaller amounts of other constituents. So the most rockdust might sometimes do is raise soil pH a little, although in the Scottish study it didn't.