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I believe the redness in clay denotes iron content to some extent. How much is the clay you are looking at? I just ran across my stash of carbondale red clay powder in the fish room today and can probably get it to you cheaper.
I got it for 3.80 shipped. Only an ounce though ,kinda embarrassed but I've been able to make a lot of gel caps full. I'm going to need more for some mineralized topsoil I've got coming.