I think what you are going to find is that the type of growing media, from "yard dirt," to a commercial waterlily potting mix is really not all that important given the excellent fertilizers now available.
The real culprit in the formation of what is probably hydrogen sulfide inside the pot. This is due to not only whatever organics are in the soil, but also from the roots of the plant that have reached the bottom of the pont and died in a very much anaerobic environment. Did the smell from the pot remind you of rotten eggs? That would be hydrogen sulfide and it is quite toxic to fish.
In an article in the WGI Online Journal, Vol. 1, Number 3, August 2006, Rich Sacher wrote an article on poting and fetilizing waterlilies. To avoid the problem of a "dead space" in the bottom of the pot, he advocates cutting slits in the sides of the pot, right near the bottom, with a knife.
Yes, roots will eventually sneak out thru these slits, but that really is not a problem. Fish might eat them, but they'll just grow more. I've yet to see any evidence of fertilizer leaching out of those slits, as my unfiltered ponds tend to be quite clear. I just put a layer of dirt in the pot and make sure it comes up over the top of the slits, and then add my granular fertilizer inside that "dirt ring." The slits are only about 3/4 of an inch up the side of the pot, the width of the knife I cut them with.
The slits do allow for some gas exchange, theoretically keeping the soil in the pot from going completely anaerobic. I have been following Mr. Sacher's advice this year and so far, it all seems to work just like he says. My water still stays clear and I've seen no gas build up in the pots with the slits, unlike what I find in the smaller plastic cups I'm using on some plants.
Those cups, so far mostly 18oz ones, all seem to to have the soil in them kind of "dome up" after the plant has been growing a while. If I bounce the cup off the bottom of the pond, a lot of tiny bubble come out of the dirt, and it is a little stinky. In the future, I'll be putting a couple of small knife slits near the bottom of these, as well.
I also remember in the past when repotting plants in everything from one gallon to ten gallon pots, when it came time to repot, the oder sometimes got overpowering. That is, except in the pots that came with holes in them. Tried lining some of those with plastic bags. It worked, and they were smelly when repoting. Others, I used old landscape cloth to keep the soil from coming out the rather large, round holes. These grew just as well as the others, but never developed that smell.
Mike S.
Spring Hill, FL
