Low pH of my Black Gold Seedling Starter soil

About a month ago I was going to use Black Gold seedling starter soil to plant my Gold Leaf seedlings in. Prior to planting, I watered it with tap water pH’d to 6.5pH and got a run off of 5pH. I ran a bunch of water through it and couldn’t get the pH to move. So I abandoned it and planted it in miracle gro hot slow release soil, despite knowing that I’d be in for a bit of nute burn while they established themselves. Long story short, the GLs did fine, got a little nute burn, but are now comfortably in 5 gallon pots with roots organics coco soil.

Anyway, I digress. Fast forward to today, and I have a rooted clone I want to get established. I read that Black Gold seedling starter mix is soilless. Thinking that last time perhaps I had been mistaken with the pH, I gave it another go, watering with tap water pH’d to 5.8 and got a runoff of 4.2, I ran a significant amount of water through it (7.8ph) and it baserly got to 4.8. So again, I abandoned ship and went with my root organics (5.8 in, 5.8 out, on the spot).

Anyway, is this just a bad bag of soil? Has anyone else had issues with BG seedling starter soil having a wicked low pH? Or is this just normal and I was being paranoid?


I use pro mix and it’s soil less and I ph just like I would soil 6.3 - 6.7

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Thanks @hogmaster. Two follow up questions:

When you water your promix for the first time with ~6.5 pH water, would you expect the runoff to have significantly lower pH because its new soil, or would you expect it to be in the 6.3 to 6.7 range you expect? This was the issue I had with the Black Gold seedling mix, it’s runoff pH was ridiculously low at 4.2 when watering for the first time.

Second, during my first grows, @bob31, who is my pot growing hero and unfortunately no longer with the forum, guided me through the noob process and steered me towards using the hydro pH range for soilless. He made me do a slurry test and it came out at 5.9pH just like he told me it would. I bought into it and then as time passed my totally effed plants rebounded based on all his advice. Anyhow, is the slurry test a good indicator of whether folks should be using a soil or hydro scale, or is it really just a matter of preference?