Been experimenting with some bentonite I had to plant potted pond
plants in, and it works great. Kitty litter is commonly used to plant
potted plants in ponds, and kitty litter is mainly bentonite.

I have mixed bentonite with regular garden soils and bagged soils with
good results. (Ratio I have had best results with is 3 parts
bentonite to 1 part soil) Any of those little perlite beads etc seem
to get trapped and retained in the mix as the bentonite clay swells
up, and no water staining has been noted. A 50# bag of bentonite is
about $6.00 from feed supply stores or a well drilling company.

I presoak the pot with plant and mixed soil first to allow bentonite
to swell and absorb, before placing in the pond itself. So far it
works good and no leeching of dirt of any significance has been noted.
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