If you're talking about big commercial growers, I think they usually make their own mixes from pine bark compost, peat and maybe perlite. I've read quite a number of studies of container growing in the nursery trade and noticed how often the mixes they use are bark based. My two favorite garden centers use mixes that are a lot like Al's 5-1-1. It's cheaper and holds up longer than standard peat based mixes.