CULT: ?help - potting mix

No help from you guys when I asked before, & cruising the web didn't turn up much. So I'll pester you folks again:
I'm using Miracle Gro Potting Mix for an emergency planting bed (unexpected bunch of extra plants).
Have any of you used it for growing irises and if so, is there anything I need to add to it to adjust pH or fertility?
It looks like the slow release fertilizer included in the mix that says it will last for 3 months is low on phosphorus.
I found some web info on using it to grow marijuana in pots (!) that said the pH is around 5.5, so figure it needs some lime. And something that implied it might be a bit low on magnesium. Added crushed dolomitic limestone should take care of that. 

If I add fertilizer high in phosphorus, will that be too much 'salt'?
I couldn't find any information on how much phosphorus (or other nutrients, for that matter) are in the mix other than the added time released fertilizer.
Is there another gardening discussion forum with searchable archives where I might be able to find out? 
--
Linda Mann east Tennessee USA zone 7/8
East Tennessee Iris Society <http://www.DiscoverET.org/etis>
Region 7, Kentucky-Tennessee <http://www.aisregion7.org>
American Iris Society web site <http://www.irises.org>
talk archives: <http://www.hort.net/lists/iris-talk/>
photos archives: <http://www.hort.net/lists/iris-photos/>
online R&I <http://www.irisregister.com>

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