Commercial potting soil mixtures vary widely in pH levels. The pH scale ranges from 0 to 14; the lower end of the scale is acidic and the higher end represents alkalinity. The middle point---7.0---is neutral. According to SUNY College of Environmental Science, plant nutrients are most available when soil pH is in the range of 6.0 to 7. Many potting soil mixtures contain high levels of acidic peat and may be too acidic for most houseplants.
Determine your plant's pH preferences. Some plants like lower pH--these include Norfolk Island pine, anemones, cacti, orchids, croton, rubber plants and others---but most prefer a pH somewhere above 6.0 but below 7.5.
Amend the soil to raise pH. Agricultural lime neutralizes acidity and provides beneficial nutrients---calcium and magnesium---to your plants. Other choices include calcitic lime and dolomitic lime; both are available in pulverized form that can be mixed into the top layer of potting soil.
Add fireplace ashes to your soil. If you burn wood in a fireplace or fire pit, you can use the ashes to raise soil pH. Use wood ashes sparingly---only about 10 g ashes per square foot of soil. Wood ashes contain potassium which can, in excessive amounts, interfere with plant nutrient uptake.
Lower pH levels if your readings are above 7.5. If the pH is 7.5, use a soil acidifier to lower the pH. Used coffee grounds can be added to your plants as well; your local coffee shop may be happy to supply used grounds. Peat moss will lower pH also and has the advantage of increasing water retention in the soil.