Nancy,
Dyna-Roc is not available in many stores but can be bought on line @
Dyna-Gro. I have used it in my orchid mixes for years and am currently doing a home experiment using two Phals.(the same variety) One in pure Dyna-Roc(a mix of medium and fine roc) and one in my usual mix. Both plants are healthy & neither are blooming. The only difference I see is that the dyna-roc has settled a lot more & I had to fill it up after a few months.