What type of potting mix is best for a peace lily?
Here in the UK growing the Spathiphyllum under glass I aim for the following mix and temperatures -
Compost, leaf-mould and peat with a little loam, sand and charcoal.
Water freely during growing season.
Maintain a humid atmosphere throughout the year using the syringe frequently.
Temp., March to September 24-30C, September to March 18-21C.
I use an average potting soil for mine. Nothing particularly special and my peace lillies are all between 15 and 10 years old. I do crush up egg shells and add to the top layer of dirt from time to time for a calcium supplement and this keeps the plants very green.
I don't fertilize very much and I would advise that you avoid any Miracle gro type fertilizers due to the slow salt build up in your soil. A slow release fertilizer or some organic compost added to the pot occasionally works fine.