This is a solution to the predicament that comes if you’re mid-cycle but want to amend a bed to fix mineral deficiencies, or make adjustments to mineral balance. Watering in a liter of 220ppm copper for instance is probably not the best way to treat a plant and the microbes in the soil. Even over multiple waterings it would be challenging to get it to a level that doesn’t cause some eyebrows to raise.
I acknowledge that you could try to evenly sprinkle the minerals over the soil surface then till them in. The crystals would eventually all melt and the soil life would work them into the soil. That’s probably a fine solution and I’ve overthought this. That said…
I take the minerals, typically sulfate salts or stuff like sodium molybdate, and dissolve them in hot water. If they’re not dissolving, heat the water up some more and stir, or add more water. Once everything is dissolved, I use the mineral water to hydrate some dry hardwood fuel pellets. I like Kirkland brand because it’s cheap, I can buy it local, and it doesn’t have fillers or adhesives. Since the water is hot it doesn’t take long to get the pellets to puff up. From here, it’s ready to be applied as mulch, but I’ve found I can do a little more to really get things grooving.
Since sawdust has such a high carbon:nitrogen ratio, it makes sense to add some nitrogen to this to produce better biological activity and accelerate its breakdown. High microbial activity will also attract worms to the mulch layer and of course as they go back into the soil and poop it does the work of moving the minerals evenly throughout the soil profile. It also makes sense to mix in a bit of good compost with the sawdust, for the same reason as above. I don’t add but a few tablespoons and mix it together, and of course I regularly water in compost extract during all grow stages. Another option at this point is to add something like calcium bentonite to the mix, particularly if the soil mix has a low clay component. A final option, but a highly recommended one, is to add something like mosquito bits. For the best results, you’ll want to keep the mulch layer moist, and the mosquito bits will help a ton with controlling gnats in that environment.
Hopefully this helps someone… Happy Growing!