Incidentally, I had the same idea last week. I used to make root tabs with just a block of red clay from the craft store and some dry ferts. Worked fine years ago, but it was way more clay than I needed, and it took up a lot of space and inevitably dried out on me. I was thinking about the seed bomb mix while I was at the nursery recently, so I picked up the same clay powder that you listed in your first link. Mixed it up tonight with some organic soil-less compost mix that I have for the yard. I figure that has a lot of nutrients, so maybe I'll forego the added dry fertilizers this time. The clay mixed up fine with just a little water, and I've got about 20 little tabs drying in the back yard now. I'm restarting an old tank, so I thought it might help to revitalize the substrate a bit, and I didn't feel like completely tearing it down and putting soil beneath the gravel. We'll see how it works out over the next few months, I suppose. The biggest drawback I've had in the past is that the clay is just a mess to clean if you're later tearing down the tank. It just turns into red muddy goo when disturbed, so you've got to stick it down in there and then leave it alone...for good.
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