Without a soil test, it is impossible to say precisely. It is quite possible you don't need to use any. Recommendations run anywhere from .25 to 2 pounds per 10 square feet.

What reason do you have for using the rock dust and which rock dust is it? The compost in Mel's Mix is supposed to provide all the nutrients the plants need to grow.

kimmq is kimmsr

As I understand it you would add rock dust for the trace minerals. Now Mel's Mix is relying on compost for all its nutritional content. The question really depends on the quality of compost you are adding. With that said, and as a broad generalization, most soils I've seen (New England) seem to benefit from rock dust. There are a hundred videos on this topic. I like this guys enthusiasm. https://www.youtube.com/watch?v=xYV_HK0cTFc

5 lb is not that much in 3 beds that size. I would just split it up evenly and use it up. It's not soluble like granular fertilizer, so you don't really have to worry about putting in too much. Well, if you had 100 lb I might worry, but 5 lb is not very much.

Go ahead and use the dust. Don't worry about a soil test for micro minerals...hundreds of dollars likely!!!!

Not saying you need a test but it is not that expensive to get the 'major minors' - Ca, Mg, Mn, Fe, Zn, Cu and maybe a few others were included in my last test for about $20 total. Now if you want to go REALLY deep with some of the more obscure ones, it's going to add up.

You are going to have sufficient minerals with this mix so I would not worry about it. Use the stuff and your garden will grow. But gardengal's point is a good one. If you don't have any actual evidence that you're deficient in one or more trace minerals badly enough to affect plant growth, then your effort and fundage could be better spent on something other than trace mineral supplements. And it's fairly rare that compost amended soil is deficient. Amending with compost with varied ingredients over the years will give the soil everything it needs.

It seems to me that removing minerals with produce and only adding back the residue or other residues similarly depleted will come up with some micro shortages after a while. I supplement my human diet. I don't want to be in the hospital or an 6 drugs before I wise up.

Lordy, lordy!! I think there's a bit of overthinking going on here :-)) I have never, ever added any sort of trace element/mineralization to any of my gardens in a lifetime of gardening experience and have never failed to get healthy, productive and I would assume, completely nourishing crops and harvests!! Neither have I ever taken a vitamin pill or food supplement in my life. And despite a few bad habits, I am as healthy as horse. And I am just a couple years shy of 70.

Unless you are growing on seriously depleted soils or are a farmer pulling bushels of produce annually out of your plot, the trace mineral/micronutrient levels are very likely perfectly adequate. And tox is dead-on........they most certainly would be with a Mel's Mix in a first time raised bed or square foot gardening bed.

Many years ago (1960's) we organic growers were told often that we needed to add rock dusts to our soil to "mineralize" those soils and to build up and bank those minerals in the soil. We found later that excess amounts of nutrients in soil did not stay put but would be washed out with the rains and cause pollution of the ground water, that water we drink. I found that once the soil had adequate amounts of organic matter in them the nutrients were there in balanced amounts without the addition of rock dusts.

If the soil does not have adequate amounts of organic matter, to feed the Soil
Food Web that will convert that to plant food, adding rock dusts will do little while if the soil has adequate amounts of organic matter the rock dusts will not be needed since the nutrient levels will be adequate.

Theoretically Mel's Mix has all the nutrients plants need to grow most plants , so adding some rock dust is a waste of time, energy, and money.

kimmq is kimmsr

People often lost sight of the fact that most soils have gobs of trace minerals, most of which are not being utilized (or even mobile at any one time). I blame the mineral supplement people at least partly. It all depends on what your soil is, so the discussion is at a disadvantage without that. But unless you are growing in pure silica sand (which rarely occurs anywhere, if you look closely at sand it will usually have all different colors of rocks represented), you will likely have plenty.

As for depleting it and not putting it back, I suppose if you didn't import anything to your yard, there would be a nonzero risk of that if your soil was poor. But most of us import compost, and we import food as well, and compost the scraps. We don't live in a bubble.

Just for kicks, let's look at one mineral and one vegetable. How about zucchini, which according to the following link

http://www.lenntech.com/fruit-vegetable-mineral-content.htm

has 0.7 mg Zn per 100 mg. Background Zn in my Missouri clay runs about 60 mg/kg. How many pounds of zucchini would I have to remove from to top 6" of a 20x20 ft garden to deplete the Zn by 50%?

It comes out to about 80 lb of zucchini by my reckoning. Which sounds like not very much for a 20x20 plot. Am I in danger of depleting my minerals? Note, this is assuming that: the Zn level IS background, i.e. I've added no compost, etc.; that I add NOTHING back such as compost from outside or compost I make at home from imported materials such as kitchen scraps; that everything takes place only in that 6" and roots and worms don't bring up any minerals from below; and so on. Most of which will not occur in the average garden.

I'm assuming that the Zn content of zucchini was by wet weight, but the table didn't say, which bugs me. I picked one that was fairly high - note how many are listed as 'trace amount' in that table.

Here's a much better page from USDA that lets you pick up to 3 nutrients and sort by food group...for example I got Zn, Cu and Mn content of 31 pages worth of vegetables with just a few clicks. In case you're interested.

https://ndb.nal.usda.gov/ndb/nutrients/index

It did come pretty close to the same zinc in zucchini value I used above (0.8 vs. 0.7).

First, I would have to ask is this homemade Mel's Mix or the purchased version? All too often people say they are using Mel's Mix when it really is "Their Mix" of whoknowswhat?. No clue why they do that.

Also I would have to know IF this is a new bed and where on this glorious planet it is located before I would even fire up my guessin machine.

You have a guessin' machine? Awesome. Most of us just make stuff up. :-p

There are some here that do no doubt.

I'm hopin maybe their machine is broke? Out for repair? Or just not calibrated? Spin.... lol.