You can use any kind of soil that does not contain light material like perlite,
which will float away. You should choose topsoil if you are buying it in bags.
The best is what ever you can dig up in your backyard.
Wet the soil before placing it in the pot to help compact it. This is important
so that the plant does not float. When the pot is full and compacted, dig a hole
in the center for tropical plants and to one side for hardies. Loop the existing
roots into the hole and cover them with soil.
If you are using topsoil, a layer of sand should be added now to prevent the
heavy soil from escaping into the water. Then add a layer of gravel to thin
rocks to anchor the plant firmly in place. You can also use stones to keep the
plant in place. These can be removed once the plant is established.
When you pot the plants, you can fertilize the lily with pond tabs. You can use
one tab per gallon of soil every four weeks if the water temperature is seventy
or above and every two weeks if the water is eighty and above.
If you want, you can add pea gravel to the soil and this will discourage fish
from digging the dirt. Now you can lower the pot slowly into the pond. Submerge
the plant in shallow water then move it to a depth of ten to eighteen inches to
encourage early growth.