Soil pH for Lilac Bushes

Ideal pH

• Lilacs can grow in a variety of soils ranging from alkaline to slightly acidic. Soil pH ranges from extremely acidic 0 to extremely alkaline 15, with 7 being neutral. Although the ideal pH for lilac bushes is between 6.5 to 7, they can tolerate a wider range of soil types. For example, the common lilac (Syringa vulgaris), hardy in USDA zones 3 through 7, can tolerate soil pH between 5.5 and 8. This versatility makes them easy to grow and adaptable in many locations.

Problems with Acidic Soil

• Lilac bushes encounter problems, such as poor growth, when planted in soil that's highly acidic. This type of soil prevents lilac roots from taking in some of the nutrients they need for growth, including nitrogen, phosphorus, potassium, calcium and magnesium. These nutrients are necessary for healthy lilac growth, flowering and disease prevention. Acidic soil can also inspire a toxic overdose of iron, aluminum and manganese in lilac bushes.

How to Adjust pH

• If your soil is highly alkaline, adding sulfur, compost or peat moss to the soil can help neutralize it. To lower pH to 6.5, you'll typically need to apply 15 pounds of sulfur per 1,000 square feet if the pH is 7.5, 35 pounds if the pH is 8 and 40 pounds if the pH is 8.5. It takes months to over a year before you'll see results. Meanwhile, incorporating a lime source, such as wood ashes, in the soil can raise the pH if it's too acidic, which is usually the type of soil that's most problematic for lilacs. Add 20 pounds for every 1,000 square feet to sandy soil, 25 pounds to loamy soil or 35 pounds to clay soil.

Testing Your Soil's pH

• The best time to adjust the soil pH is before you plant lilac bushes, with period testing every few years afterwards to ensure the soil doesn't become too acidic or alkaline for them. Check your soil's pH with a home soil pH test kit, available at most gardening centers. However, keep in mind that some test kits are specifically intended for soil that's already acidic, which means those types won't work on alkaline soil. Because their measurements of soil pH are rough, they're best for diagnosing soil that's extremely acidic or alkaline, rather than making minor adjustments to the exact pH of soil.