I've done more searches on the issue than I can count, but I seem to always come up with conflicting answers. I'm currently growing in a mixture of roughly 75% Promix BX with an additional 25% perlite added for extra drainage. My question is at what pH should my runoff water be? I know that soiless mixtures should be at 5.3-5.8 and mixtures with soil should be at 6.3-6.8, but where would pro-mix be categorized? I've read some things that say it should be treated as a soiless mixture since it is basically peat moss and perlite, while others say it should be treated as a soil mixture. I always thought it should be kept at 6.5, so that's what I've tried to accomplish. But I'm a bit confused by the fact that when I take a fresh pot of promix and run water through it for the first time, the initial runoff water is about 5.3-5.4. This means that I end up having to put gallons upon gallons upon gallons of water through the soil to bring it up to 6.5pH. This takes FOREVER and even with the addition of lime to my mixture, the pH always seems to try to get back to it's original level (5.3-5.4); even when watered with plain, pH adjusted (to 6.5) water (I am NOT confusing this drop in pH with the buildup of salts from nutrient use). I thought it was supposed to be pH balanced already, so should I really be trying to keep the pH at it's initial level? It just seems like a absolutely ridiculous amount of work to flush every single bit of soil from it's initial 5.3 to 6.5, all before I even use it... doesn't it?
I have used the Promix HP for well over a year now......Not once have i ever worried about my Ph of the soil. I watched a crazy old video of some crazy fat mess of man say that promix hp/bx or sunshine mix is the easiest mix to grow in because they already put lime in to it for the peat moss, He specifically said that you can water with tap water and NEVER worry about PH......... I never have and never did.......Promix shouldn't give you this much of a headache.....seriously, put your ph meter away til you start messing with hydro.... otherwise get your water mix your nutes and pour ...easy as pie.
Pro-Mix is marketed and sold as a soil-less mixture. Either way, your runoff should be about 5.8. Nice touch with the perlite, will help with drainage, thereby decreasing time between waterings and some of the subsequent nute accumulation and increase in pH between waterings.
I know it's an old post but here goes, Pro mix is naturally buffered to have ph values of 5.5-5.8 on it's own, You CAN simply just add your nutes to tap water which carries a ph of 8 and the nutes will regulate yor water to about 6.5-7.0 give or take, You can simply just water the plant(s) from this point as the promix will do it's work and regulate the ph to 6.0-6.5 on it's own, the run off will be roughly 6.0 which is ideal for soilless, or if you add ph down to lower ph after adding your nutes this will still be okay but keep in mind that the promix will regulate ph a bit lower and run off even lower, if you go this route keep an eye on the run off after adding ph down to make sure it's never below 6, That's that