The hydrogen potential of water, or, pH, refers to the balance--or imbalance--between positive hydrogen ions (H⁺) and negative hydroxide ions (OH⁻). When these ions are in balance, pH is neutral, and measures 7. In acids, hydrogen ions outnumber hydroxide ions; pH measures between 0 to 6.9. Bases (alkalines) have more hydroxide ions and pHs of 7.1 to 14. How minerals dissolve is affected by pH. Since plants "drink" these dissolved nutrients, pH affects plant nourishment. To avoid extremes of malnourishment or toxic mineral levels, proper pH is necessary. You don't need to hit 7 pH exactly since plants usually like slightly acidic conditions.
Pour the water you use for plants into a container and note the amount. If it is tap water, let it sit a few days so the chlorine evaporates.
Follow the directions on your pH testing kit or meter to test the water. This will involve dipping the tester into the water.
Read the results according to the kit's directions. Meters will give you a read-out. Paper strips make use of a color chart.
Use the pH adjuster according to package directions. Be exacting. If you aren't using an adjuster, add a few drops of vinegar, citric acid or lemon juice for alkaline water, or about a pinch of baking soda to acidic water.
Stir and let the water sit for a few minutes. Meanwhile, note how much adjuster you added.
Retest the water. The pH may have changed a little or a lot, depending on how much water you started with, how extreme the pH was and the water's alkalinity, which buffers the water from easily changing pH.
Add more adjuster if the pH is not hovering around neutral. Note the amount you add.
Test again. Keep adding adjuster and retesting as necessary.
Tally how much adjuster you had to add per unit of water for future reference.