Let's say you have a quart of pudding and it's 80% water and virtually no air. Now mix 30% perlite into the pudding - how much air do you have in the mix? Still none - right? Add 60% perlite to the mix. How much air do you have? STILL none. Will the water drain any faster from the pudding because of the perlite? NO. Is the height of the perched water table reduced by the inclusion of perlite? NO. All the perlite did was reduce the o/a water retention of the mix. It didn't increase aeration or improve drainage. Can we say the same things about peat. Almost. How about sand? Same thing. Essentially, what you guys are doing is adding perlite to pudding and expecting it to do what it can't - improve aeration, reduce the ht of the perched water table, and improve drainage. When you mix 30% perlite and 70% peaty soil, the small particles of peat simply surround the larger particles of perlite and almost nothing changes, except o/a water retention.
However, if you start with a large fraction of a coarse material or combination of coarse materials, like pine bark or other large gritty particles, you can build on that to reduce or eliminate the excess water retention you're talking about, have the excellent aeration in the root zone that all plants grown under conventional container culture appreciate, significantly reduce the ht of the perched water table or eliminate it entirely, and eliminate the concern for the soggy conditions that are killing or significantly limiting your plants.
Water retention and the ht of the perched water table is directly related to the size of the particles in your soils. The smaller the particles, the more water the soil retains and the higher/taller the perched water table. You can't amend a heavy soil appreciably by adding a small fraction of larger particles because you need to have well over 50% larger particles to gain anything significant from them.
Here in MI, we've had an inch of rain today and 2 more days of rain predicted. I have at least 200 trees on the benches right now, and prolly 50 succulents scattered around the gardens & benches. I don't plan on doing anything to protect the plants from rain and I have no concern that 3 days of rain will jeopardize the health of the root systems of any of these plants, including the succulents/pines/junipers - plants that don't tolerate soggy soils well ..... and most of these plants are in shallow containers, which are far less forgiving when using inappropriately heavy soils. If you're growing in a well-aerated soil that holds little perched water, your job is soo much easier and your margin for error soo much greater ....
For more details and a more complete understanding, try this link:
http://forums2.gardenweb.com/forums/load/contain/msg0214580016564.html?96
Al