How much acid does this water need? A dose calculated from its alkalinity, the number that carries a dose, not a guess from its pH. Works both directions: acid to bring a feed down, a carbonate to hold one up.
Built for water fresh from the source. Recirculating? See the note in Step 1.
Step 1: Your water test
These figures assume water fresh from the source. In a recirculating system the first adjustment has already neutralized some or all of the alkalinity, while the plants push pH back up (nitrate uptake releases hydroxide), so the numbers below will overshoot.
First adjustment: use the calculator as it stands.
After that: enter whatever alkalinity the calculator said was left. If your source water was under 75 ppm, set alkalinity and hardness to 0. Anything else wants a titration.
Step 2: What are you aiming for?
Leaving some alkalinity is the goal, not stripping it. It is the buffer that keeps root-zone pH steady between irrigations. Aim to knock it down, not out.
Target pH doesn't change the dose (alkalinity does that). It sets what counts as on target, and moves the "too acidic" floor with it, so running deliberately low doesn't get flagged as a problem.
Step 3: Calibrate with a meter (optional, and worth it)
Fertilizer changes your water's pH before any acid does, and by how much cannot be read off an N-P-K label. Mix your feed, measure its pH before adding acid, and enter it here. One reading turns this from an upper bound into a real dose.
Step 4: The dose
pH, alkalinity, and hardness are three different things
Two waters at pH 7.8 can require very different acid doses if one carries 40 ppm alkalinity and the other 300. pH is used here only to estimate the resulting value, never to size the dose.
Safety. Add acid to water, never water to acid, and never mix concentrated acids with anything but water. Sulfuric is the most efficient neutralizer listed and the most hazardous to handle. Use gloves and eye protection, and dose into a stirred or moving volume.