pH Down Calculator
Front Row Ag · pH Down Calculator
70% Phosphoric Acid · Alkalinity Neutralization
Water Parameters
Starting Alkalinity
ppm
Target Alkalinity
ppm
Which target? Leave 40 ppm for Veg, Ripen, and standard-strength flower — FRA recipes at 2–3 EC neutralize that much on their own and land near pH 5.8–5.9. Use 25–30 ppm for high-strength flower (Stretch, Stack, Swell at 2.4–3.0 EC) or Component Plus flower, which sit at the calcium-phosphate line at 5.8 and should run 5.5–5.7 at the dripper. Always confirm at the dripper with a calibrated meter.
Batch Size
Volume of Water
gal
pH Down Dosing
Rate
0.69 mL/gal
Total for batch
0.69 mL
Neutralizing 100 ppm alkalinity (140 → 40 ppm)
Quick Reference
| Neutralize | mL / gal | mL / L |
|---|
How it works: Each ppm of alkalinity (as CaCO3) requires ~0.00694 mL/gal of 70% phosphoric acid to neutralize.
FRA recipes neutralize roughly 35–45 ppm of residual alkalinity on their own at 3 EC and land at pH 5.8–5.9; leave more and the feed lands higher (60 ppm ≈ 6.1, 90 ppm ≈ 6.4 once the CO2 has gassed off in the lines).
Inject acid ahead of the fertilizer injectors so calcium never meets phosphate at high pH. Phosphoric acid adds ~0.6 ppm P per ppm of alkalinity neutralized — past ~80 ppm of neutralization, consider sulfuric acid or RO blending instead.
Always verify at the dripper with a calibrated meter — a fresh tank reads 0.2–0.3 lower than the lines because its CO2 hasn't left yet.
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