pH Adjustment Calculator

Aim to lower alkalinity, not remove it. What remains is the buffer that keeps root-zone pH steady between irrigations.

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.

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. With that reading, the answer is a real dose instead of an upper limit.

This page treats the feed as your water with its alkalinity lowered. The fertilizer's phosphate also takes up acid, which the Fertigation Calculator counts from your recipe.

• pH is the acidity of the water at the moment of measurement. It says nothing about how much acid the water can absorb.
• Alkalinity is the capacity to resist acidification, mostly bicarbonate, measured in ppm as CaCO₃. This is what the dose is sized against.
• Hardness is dissolved calcium and magnesium. It causes scale and supplies nutrients. It does not buffer pH.

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.

Water pH

How acidic or alkaline the water is right now, from 0 to 14, with 7 neutral. Most tap and well water reads 7 to 8.5. Most greenhouse and nursery crops take up food best from about 5.4 to 6.4 (MSU Extension). pH doesn't tell you how much acid it takes to move it.

Alkalinity (ppm)

How much the water resists being made more acidic, mostly from dissolved bicarbonate. This is the number the dose comes from. Water reports list it as "total alkalinity", often "as CaCO₃". Aquarium KH kits read it too (1 dKH is about 18 ppm). Rainwater and RO water carry almost none.

Volume to treat

How much water gets the dose: a watering can, a bucket or a whole reservoir.

Growing medium

What the roots sit in. Hydro / inert covers rockwool, perlite and water-only systems (NFT, DWC), which need the lowest pH; soil needs the highest. The medium sets the pH to aim for.

Acid

The product that brings pH down. Most "pH Down" bottles sold for gardens are phosphoric acid, with the strength on the label. Sulfuric and nitric are the stronger farm acids. Vinegar, lemon juice and citric acid work once in a pinch, but pH creeps back up over a day or two as microbes eat them.

Concentration (%)

How strong the acid is, as printed on the bottle. A weaker bottle needs more of it for the same job, so this changes the dose a lot.

Product (baking soda, pH Up)

For water with too little alkalinity, like RO or rain, a small amount of a carbonate steadies pH so it doesn't swing between waterings. Baking soda is what most kitchens have. Potassium bicarbonate does the same job and adds potassium instead of sodium.

mL, g

Milliliters for liquids, grams for powders. A teaspoon is about 5 mL. Doses under a few mL are easier to measure with a syringe, or from a diluted bottle.