I'm planning an expansion of my pond next year to deal with some issues I
created. One of them is poor plant growth in marginals that are in clay
soil but with no additional fertilizer.

One one side of the argument is plants in clay/soil do better, but might not
be able to get at nutrients present in the pond, thus allowing string algae
to outcompete them as the clay "keeps" the nutrient flow away from the
roots. (BTW, I'm not presenting this as fact, just trying to figure things
out!). I have to wonder about this when I see arrowroot growing huge in the
natural ponds around here, yet my in-kitty-litter arrowroot is but a puny
shadow (well, it's also in a pot...). One could fertilize and they would do
better, but why add nutrients here if the pond is already producing SA,
meaning there is excess N and P in the pond already? (I, like many of you,
add K regularly)

On the other side is that plants in gravel or gravel beds, or just tossed
into the water can take up nutrients in the water faster or better, allowing
them to grow better and (hopefully) out-compete the SA.

I suppose pot size per plant would also be a factor here - perhaps my
marginals need bigger pots? But I do know that the WH and other
toss-them-into-the-pond type plants (parrot feather, et al) do fine.

So, how about a poll. Whose marginals are in gravel and whose are in soil
of some type? How well do they grow? Do you fertilize, or add K at all to
your pond? Maybe this could be a useful thread to collect some real life
data.