There are several factors to be considered in answering your question as to pot size. One is, how often do you plan on dividing the plants and repotting? If you will be diciding yearly, then the smaller sized pots might work okay for you. Second consideration- Are your lilies hardy or tropical? Hardy lilies need bigger pots because the tubers get bigger and need room to grow. Generally, a one or two gallon pot will be big enough for hardy lilies. Tropical lilies benefit from the same size pots, but can actually take smaller pots. The larger the pot, the better for the growth and size of your lily. I actually grow most of my tropical lilies in 22oz. tumblers, but I really do not want my plants to grow as large as possible. They are mostly grown in half-barrel sized containes. Only the lilies in the ponds have the bigger containers. This shows, however, that plants can be grown in various sized pots.
As to soil. I use plain top soil out of the ground here and it has some clay in it. I always recommend people to use top soil even if you have to buy it, but do not get potting soil. too many things in it that will float and make your pond look nasty. This is also much cheaper than the aquatic soil that is marketed.
The reason your plants may not have bloomed as much this year, and the leaves turning yellow is probably lack of nutrients. Lilies need to be fed. This year has been much hotter than normal and people who generally use a slow release pellet fertilizer have especially had problems. Because the pellets are there, they think the fertilizer is there. However, the frertilizer is released with heat, and with this years heat, the fertilizer was "dumped" quickly, and so what may have lasted 3 months in the past, only lasted a month this year. Pond fertilizer tablets needed to be fed to the plants for them to continue to bloom and grow. I use fertilizer tablets and feed monthly, or at least every two months. Plants are not burned doing this, nor are they starved.
Happy ponding! Hope this helps.
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