Possibly a salt accumulation in the root zone from salt based fertilizer and not frequently feeding or watering to run off is driving your PH down 
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Just saw the living soil, are you growing this all organic and no salt based fertilizer added?
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I have fox farm trio. Thought it was organic but I noticed when I spilt some bloom when it dried it turned into crystals
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Sorry about that Grow Bro but the FF trio is notorious for salts building up in the root zone. They recommend a periodic flush but if you’re feeding and watering to a good run off, it’ll help rinse those away
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