Soil pH: Testing and Modifying Acid or Alkaline Soil

The pH scale measures acidity (below 7) or alkalinity (above 7). Most garden plants thrive in neutral to slightly acid soil (pH 7 or slightly below). Highly acid or alkaline soils hinder nutrient uptake—too high pH blocks absorption; too low causes toxic overload.

Clues from Climate

High-rainfall areas tend to have acid soils—home to azaleas, rhododendrons, camellias, blueberries. Low-rainfall areas often have alkaline soils—suitable for waterwise plants like olive, oleander, pomegranate.

Testing

DIY kits from garden centers reliably indicate acid, neutral, or alkaline. For extreme readings, confirm with a professional lab test, which includes amendment recommendations.

Adjusting pH

• Raise pH (acid to neutral): Lime (ground or powdered). Ground limestone acts slower. Wood ashes and oyster shell also work. Amount depends on soil texture (more for clay).
• Lower pH (alkaline to neutral): Common sulfur (least expensive), ferrous sulfate (adds iron, helps yellowing plants), aluminum sulfate. Organic amendments (compost, manure) lower pH over time.

Follow test result recommendations. If pH change needed exceeds one point, consult a professional for on-site evaluation.

Alternatives

If amending isn't feasible, use raised beds with problem-free topsoil, or choose native plants adapted to your unamended soil.