RickCorey said: If lilies like acid, maybe sphagnum peat moss would be a good addition. Usually I avoid that to avoid water-logging.
Yes, when in pots as opposed to the ground, one does need to be careful about too much sphagnum for that very reason with lilies. you could add iron sulphate or magnesium sulphate to lower pH. Most commercial mixes found in regular retail outlets already have a large amount of peat (sphagnum peat) incorporated. I wouldn't add any more. Commercial mixes also have their pH adjusted to slightly acid. While more acid ( a lower pH) would be better, slightly acid should be fine, in my opinion.
