Well, my Rockdust arrived. This stuff is pretty dense, because the 25kg sack was quite small:
The recommended rate of application for it is a minimum of 500g per square metre, so I weighed some to see what 500g looked like. Not a lot!
So, I then measured out for each of my raised beds 3 generous "portions" of 500g (more like 600g in many cases, I suspect). Each bed is 1 metre x 2.4 metres - let's call it 2.5 square metres.
Then I mixed the Rockdust into the soil with a hand fork.
Having done this I thought "That doesn't look like enough", and I did the same thing all over again! The manufacturer's instructions say that you can apply as much as 5kg per square metre, but then they are trying to SELL the stuff, so I reckon they probably want you to use more than is strictly necessary. 3kgs per 2.5 metres sounds like enough for me. Hopefully enough to be able to judge whether it makes any difference. We shall see...