Short answer: add a small amount, stir, wait a few minutes and test again, repeating until pH reaches about 5.5 to 6.0. UF/IFAS notes that white vinegar typically takes around 2 teaspoons per gallon, depending on the water.
Your options
- White distilled vinegar. Cheap and easy to find. It is a weak acid, so you need more of it than of a commercial product.
- Citric acid. Sold as a powder for canning. Dissolve a little in water first, then add the liquid gradually.
- pH down products. Usually phosphoric acid. Strong and consistent, so a few drops go a long way.
Oklahoma State University Extension lists phosphoric acid, citric acid and vinegar as options for lowering nutrient solution pH.
Step by step
- Mix the nutrient and set the strength first. Nutrient itself changes pH.
- Test pH and note how far it is from your target.
- Add a small dose of acid, about a quarter of what you think you need.
- Stir well and wait several minutes.
- Test again and repeat until you reach the range.
Avoid overshooting
Water with low alkalinity, such as distilled or reverse osmosis water, can drop sharply with a tiny amount of acid. Go slowly. If you overshoot, a pH up product brings it back, but it is easier not to need it.
Safety
Commercial pH down is a concentrated acid. Wear gloves and eye protection, add acid to water rather than water to acid, and store it away from children and pets.
How long it lasts
pH can drift after adjustment, especially with hard water. Recheck a few hours after mixing and again before planting.
Related guide: pH for Kratky nutrient solution.
Sources
- University of Florida IFAS Extension. Set It and Forget It Hydroponics (HS1488).
- Dunn, B. and Singh, H. Oklahoma State University Extension. Electrical Conductivity and pH Guide for Hydroponics (HLA-6722).
