How to Change the Soil's pH in Potted Houseplants

Measure the pH of the soil to determine its level. You can use a do-it-yourself pH test kit or you can send a soil sample to your local county extension service. The latter is recommended if you have many special plants being grown. The pH test kits sold at garden centers are fine for casual use.

Calculate the amount of acidity you need to increase or decrease based on the plant's requirements. If your soil tests at 5.0 pH and your plant requires 6.0 to 7.0 pH, you need to increase the pH by at least 1 point.

Apply hydrated lime to increase the soil pH or aluminum sulfate to decrease the soil pH. The amount to add is approximate and depends on the size of the pot, the porosity of the soil and the amount of change needed. A 6-inch pot requires about a teaspoon of amendment to alter the pH by a factor of .5. Mix the amendment in the amount of water used to water the plant, stir the mix and then water the plant as usual. The proportion of amendment and water is not important. You do not want to overwater the plant, yet you still want the plant to receive the entire solution.

Retest the soil pH a day or two after you have applied the solutions. Both amendments are quick-acting, but the soil should dry out a little bit before retesting. Reapply the amendment addition at the next watering if necessary based on your test results. It is best to add or reduce in repeated small amounts (.5) over time rather than to double or triple the amendment added at one time.