Check the attached link for all you ever wanted to know about container soil...and then some. Al (tapla)'s post basically encourages the use of a pine bark (rather than peat) based mix. The pine bark is coarse and holds its structure very well--leading to excellent drainage and aeration. I've found this formula to work quite well for vegetables, annuals and perrenials. Pro-Mix is great stuff too. I still use it for seed starting, but have found the pine bark mix to be more economical for large containers as well as longer lasting. Jason Here is a link that might be useful: Container soils and water in containers

One azalea nursery uses straight Soil Conditioner which is mini pine mulch. You can find bags of Soil Conditioner near the potting soil at the big box home improvement stores. I don't grow azaleas in pots but have above ground beds of mostly soil conditioner mixed with some bulk-load topsoil.

For an almost identical question: best potting media

here's the bush they are beating about.. well.. in this case... the tree ... a tree is a tree as far as media goes .. trees in pots want a sip of water .. then near full drainage ... and near drying before the next sip of water .. media with too much peat holds too much water for most trees .. long term ... not to mention houseplant peeps who simply love their plants too much and water their babies too much .. so mix that with the wrong media.. and you have problems .. roots also need air .. especially trees ... and that is why you add all the big chunks of whatever.. to aid in air.. but also increase the drainage ... the problem with the product named is that is is a catchall for all plants ... regardless of plant needs .. and it tends toward the more common houseplants that like growing in a sponge ... so when you balance that out with you wanting to grow trees in pots.. indoors.. there is a big problem ... you dont want sponge bob media pants .. lol .. you can overcome all of this by watering properly .. but it can be much easier if you start with a good media proper to the plant you want to grow ... and.. imo.. moisten all media.. BEFORE use ... start with media that is useful ... even the big chunky stuff .. take a few days in a grocery bag or some such.. and keep adding water .. until everything is moist .. NOT SODDEN ... just moist ... so when you plant the plant ... you water it in to settle the soil .. but you arent wasting weeks trying to wet the media at the same time ... e.g. my garden is all sand.. i dont wait until august and go plant things in dry play sand .. i do it in spring.. when its all moist ... or in fall when its moist .. i dont try to make dry play sand wet.. after i stick a plant in it ... good luck ... ken ps: you can get many things to winter over .. if you put houseplants outside in full shade for summer ... and then you can torture them for the winter .. indoors ... a prime growing season to offset the not so prime winter ...