If you are luck enough to have good topsoil in your backyard, by all means, use your topsoil. All you will have to do is add fertilizer. If your are not so lucky and your backyard is sand or heavy red or yellow clay, you can mix up a batch of loam soil.
You can create your own loam soil by mixing 1/2 part topsoil 1/2 part kitty litter (free of dyes and perfumes and clumping properties) and 1 cup of playground sand. Mix together thoroughly. Your soil should clump when squeezed. If your soil is mixed properly, it will not muddy your pond water.
Loam soil is well suited for all aquatic plants (except oxygenators). Oxygenators rarely need to be planted, just anchored in the substrate or in a container filled with sand.
Sand holds little water but does allow for aeration and drainage, clay particles retain more water and silt combines the qualities of sand and clay.
Stay away from commercial potting soils ( as they are too light and will float right out of the pot), do not add too much composted material (as it is too rich in organic matter and it will ferment underwater and destroy the ecology of your pond)
. The texture of loam soil will retain nutrients and water as well as bind to the nutrients in your fertilizer, where they can deliver these nutrients to your plants. Once you have created loam soil for your pond plants, you are ready to plant waterlilies, lotus and marginal plants in your water garden or pond. Enjoy!