Soil pH is an indication of the hydrogen ion concentration – the amount of acidity or alkalinity of soil. The pH scale has a range of 0 – 14, with pH 7 as the neutral point. A pH of 0 is the most acidic and pH 14 is the most alkaline or base. Vinegar has a pH of 3, and lime has a pH of 12.
The soil pH affects the solubility of minerals or nutrients and thus its availability to plants. A pH of 6 to 7 is best for many plants. Acid soils in the pH 4-5 range can have high concentrations of aluminum, iron and manganese, which can be toxic to the growth of some plants.
Most tree species live on soils between a 4.0 pH (very acidic) to a 7.5 pH (slightly basic) and most grow best on slightly acid soils. A pH range of 5.5 to 6.5 is considered the ideal pH for tree growth. Some trees show nutrient deficiency symptoms (especially iron) at higher soil pHs.