On the allotment site in Northern England where I grow food, there’s a fashion for rockdust. Some of my neighbours swear by it. I’m not tempted, and my plot’s the envy of the site (well it was so, until my worsening disability put a kibosh, but that’s another story).
I’m not tempted by rockdust partly because I distrust any claim about ‘soil remineralisation’ that doesn’t involve naming minerals, appropriate to particular sites. You can find a list of what’s in the rockdust online, but I’ve seen nothing about what that means in practice.
There’s plenty of calcium and magnesium in there. Calcium carbonate is also known as lime, used by many gardeners (including me) to raise soil pH. It’s often used with magnesium sulphate, also known as Epsom salt. Epsom salt can be made from magnesium oxide. Here’s what the European Commission says about the lime, magnesium oxide and cement industries. These materials are useful in many ways.
Farmers can choose to buy a field-by-field soil test, leading to specific advice. That makes sense. I’d do it if I were farming. Plants, and the livestock and people who eat plants, need the right minerals. We need the right pH in each of our tissues, and we can be short of calcium and magnesium.
But ‘minerals’ in rockdust to put on your garden? Meaning? I don’t think the same bag of ‘minerals’ would be useful on every soil. I suspect rockdust of being just overpriced lime and magnesium. With added woo, woo.
Also, I’m not tempted by rockdust because a claim about ‘eco-friendly’ is made alongside a claim about ‘420 million years old, freshly ground, untreated, volcanic rock from Scottish quarries’. Mines in one place to feed gardens in another place? That’s what liming the soil involves too, and I don’t claim that all my gardening choices are perfect (far from it) but I don’t trust claims of ‘eco-friendly’ quarrying.