Gardening glossary
Soil pH
Soil pH is a logarithmic measure of how acidic or alkaline the soil solution is. The scale runs from 0 (extremely acidic) to 14 (extremely alkaline), with 7 being neutral. Each whole number is a tenfold change, so soil at pH 5 is ten times more acidic than soil at pH 6.
Why it matters: pH controls nutrient availability. In strongly acidic soil (below 5.5), phosphorus, calcium, and magnesium become locked up while aluminum and manganese can reach toxic levels. In alkaline soil (above 7.5), iron, manganese, and zinc become unavailable — that's why hydrangeas turn pink in alkaline soil and gardenias yellow with chlorosis.
Sweet spots for common plants: - Most vegetables: 6.0-6.8 - Tomatoes and peppers: 6.2-6.8 - Blueberries and azaleas: 4.5-5.5 (true acid-lovers) - Lavender, rosemary, and Mediterranean herbs: 6.5-7.5 - Most houseplants in peat-based mix: 5.5-6.5
To test pH, use a simple probe meter or a soil-test kit from a garden center — both cost under twenty dollars. For accurate results, send a sample to your local cooperative extension once every couple of years.
To raise pH (make soil more alkaline), add agricultural lime in fall so it has time to react. To lower pH, add elemental sulfur, peat moss, or pine-needle mulch. Both adjustments work slowly — months, not days — so plan ahead rather than chasing dramatic swings. Container plants are easier: just refresh with a fresh, pH-appropriate potting mix when you repot.