pH

Soil pH is a measurement of hydrogen ion concentration expressed on a logarithmic scale from 0 (extremely acidic) to 14 (extremely alkaline), with 7 being neutral. Each unit represents a tenfold change in hydrogen ion concentration: pH 5 is ten times more acidic than pH 6, and 100 times more acidic than pH 7.

For most vegetable gardens, the practical range of concern is pH 5.5-7.5. Outside this range, nutrient availability problems and microbial activity limitations begin to noticeably affect plant growth.

Why pH Matters

pH does not directly damage most plants within the 5.5-7.5 range. What it does is control nutrient availability in the soil solution. Nutrients are held on soil particles and released through chemical reactions; the pH of the surrounding water affects whether those reactions occur and at what rate.

Nitrogen, phosphorus, and potassium (the macronutrients) are most available between pH 6.0-7.5. Below pH 5.5, phosphorus binds strongly to iron and aluminum compounds and becomes largely unavailable even when present in adequate quantities.

Iron, manganese, zinc, and copper (micronutrients) increase in availability as pH decreases. Below pH 5.0-5.5, these metals become available at toxic concentrations. High-pH soils often show micronutrient deficiencies, particularly iron chlorosis (yellowing leaves with green veins) in crops that need high iron availability.

Aluminum and manganese toxicity become relevant below pH 5.0. Free aluminum and manganese at low pH damage root systems directly and reduce uptake of all nutrients.

Soil biology is strongly pH-dependent. Rhizobium bacteria responsible for nitrogen fixation require pH above 6.0 to function. Nitrifying bacteria (which convert ammonium to nitrate) are most active at pH 6.0-7.5. Earthworms prefer pH 6.0-7.5. Overall microbial diversity and activity peaks near neutral pH.

Optimal pH by Crop

Most vegetables prefer slightly acid to neutral soil:

pH range	Crops
4.5-5.5	Blueberry, cranberry
5.0-6.0	Potato, sweet potato, watermelon
5.5-6.5	Tomato, pepper, carrot, parsnip, turnip, radish
6.0-7.0	Most vegetables (optimal range)
6.0-6.8	Brassicas, beans, peas, corn, squash
6.5-7.5	Asparagus, beet, spinach, cabbage

Blueberries are the most common exception to the standard vegetable pH range. They require pH 4.5-5.5 and will fail in typical garden soil at pH 6.5-7.0, showing severe yellowing and poor growth. Preparing blueberry beds with sulfur and acidic organic matter (peat, pine bark) to bring pH into range is a prerequisite for success.

Testing

Soil pH tests are available as:

• At-home test kits: $10-20 for colorimetric pH indicators; results accurate to roughly ±0.5 pH units
• Electronic meters: $15-50; need calibration; useful for ongoing monitoring
• Lab testing: $10-25 per sample through university extension services or commercial labs; most accurate, typically includes recommendations for amendment rates

Test in multiple locations in the garden; pH can vary by 0.5-1.0 units across 20 feet in soils with variable organic matter, drainage, or past amendment history.

Adjusting pH

To lower pH (increase acidity): Apply elemental sulfur. Soil bacteria oxidize sulfur to sulfuric acid over several months. Rates depend on current pH, target pH, and soil type (clay soils require more amendment than sandy soils). A general starting point: 1-2 lbs elemental sulfur per 100 sq ft to drop pH by approximately 0.5-1.0 units in loamy soil. Apply in fall for spring planting; allow the bacterial oxidation process to complete.

To raise pH (increase alkalinity): Apply ground limestone. Calcitic lime (calcium carbonate) or dolomitic lime (calcium-magnesium carbonate) are standard. Rates vary: roughly 5-10 lbs per 100 sq ft to raise pH by 0.5-1.0 units in loamy soil. Dolomitic lime adds magnesium along with calcium - beneficial in magnesium-deficient soils, excessive in soils already high in magnesium.

pH adjustment is not instant and the effects are not permanent. Test annually and amend as needed to maintain the target range.