Over the past two years we have had more rain than I can remember. I believe this has had a very detrimental effect on the pine bark being used as a potting medium. A few rumours are going round such as, that there is a fungus, which in these conditions is breaking down the bark quickly. However, commonsense tells you that any wood type product will decay at an accelerated rate under very constant wet and humid conditions we have had to endure.
From spring two years ago I realised my plants were experiencing abnormal problems from the bark. Very few which were repotted had adequate root systems because of the damage from their sodden bark potting mix. My initial response was to repot them into coarser bark, which had initial better results than the fine to medium grades I had been using. However, after six months or less the root systems quickly deteriorated again.
In fact the weight is probably the only disadvantage but not much of a problem for small plants.
In September last year I repotted nearly all my miscellaneous species and hybrids and nearly all my natives into scoria or pebble. I also for large pots or plants that would grow large used a new product called Absorbastone. Absorbastone weighs in at 18kg for a 50 litre bag (less weight than bark), and about the same price as bark. The progress on these plants in 99% of cases has been great. Having always been keen to grow plants to an absolute premium I am extremely pleased with the results to date.
In summing up, I believe I am still in an experimental stage with the growing mediums but they appear to be an excellent alternative for most epiphytic orchids. Ask this question; what natural orchid normally lives on radiata pine bark? I expect very few if any. Many orchids that are epiphytic however also live as lithophytes. I remember an impressive slide shown by A. B. Porter many years ago showing all sorts of species growing happily on a rock wall.
Rock and pebble type mediums have these properties
I hope this small article gives you another idea, which may help, in growing your orchids. I think you will find in the case of most plants it worth giving a try. Happy orchid growing.