Florida_Mike
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At the edge of the map, blowing kisses at dragons.
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5.8-6.2 in veg & 6.0 6.4 during flower Is what I run in rockwool. The charts say 5.5-6.5, but 5.5 is really only recommended for pre-soaking the slabs, which tend to be acidic. Below 5.5 Fe becomes highly soluble and is rapidly absorbed by cannabis:
How Far Below 5.5 Before Iron Becomes Toxic?
Iron solubility skyrockets below pH 5.5, and especially dangerous things happen around 5.3 and lower:
pH Level Iron Behavior Risk Level
5.5 Balanced uptake Caution zone begins
5.3 Uptake ↑↑ High risk of Fe toxicity + P lockout
<5.2 Floodgate opens Toxicity likely, deficiency in others guaranteed
<5.0 Game over Complete nutrient chaos, especially in hydro
So yes — 5.5 is not the safe line, it’s the warning siren. Once your res dips below that, especially in low-buffer systems, you're inviting iron toxicity, P starvation, and Ca/Mg imbalance.
How Far Below 5.5 Before Iron Becomes Toxic?
Iron solubility skyrockets below pH 5.5, and especially dangerous things happen around 5.3 and lower:
pH Level Iron Behavior Risk Level
5.5 Balanced uptake Caution zone begins
5.3 Uptake ↑↑ High risk of Fe toxicity + P lockout
<5.2 Floodgate opens Toxicity likely, deficiency in others guaranteed
<5.0 Game over Complete nutrient chaos, especially in hydro
So yes — 5.5 is not the safe line, it’s the warning siren. Once your res dips below that, especially in low-buffer systems, you're inviting iron toxicity, P starvation, and Ca/Mg imbalance.