The pH Scale

Herbs grow best in slightly acid to slightly neutral soil, between pH 6.2 and pH 7.2. The method of measuring soil acidity and alkalinity is measured on a scale from zero to 14, and the ratings are express as "pH ," meaning "parts hydrogen." The concentration of hydrogen ions is an indicator of the acid or alkaline character of a soil sample. Soil pH is strongly influenced by the underlying geological bedrock of the region, and that condition is impossible to change or permanently modify. Lime will temporarily raise pH and sulfur will temporarily lower it, both for short periods of time in one growing season. It's easy to temporarily raise the pH of an acid soil with the addition of lime in spring, but lowering the pH of an alkaline soil with the addition of sulfur is more difficult. The only effective method of moderating the pH of an alkaline soil is by adding generous amounts of organic material to the soil annually, in the form of leaves, garden compost, rotted animal manure, peat moss, pine needles and cones, shredded bark, and crushed twigs. This will create a buffering action and make the alkalinity more acceptable to plants. You can have a small sample of soil analyzed by a soil testing service, but it really isn't necessary to know the pH of your soil. Just do the best thing each year and add organic material to the soil - whether it's acid, neutral, or alkaline - either dug in or laid as mulch on the surface.

• 4.0 - Very acid, garden plants won't grow
• 5.0 - Strongly acid, ericaceous plants such as rhododendrons and azaleas, requiring low pH acidic soil to absorb nutrients
• 6.0 - Moderately acid, most plants will grow
• 6.5 - Slightly acid, ideal for garden plants and roses
• 7.0 - Neutral, most garden plants will grow
• 7.5 - Moderately alkaline, most garden plants will grow
• 8.0 - Strongly alkaline, calcareous plants such as grasses and maples, which prefer high pH alkaline soil
• 9.0 - Very alkaline, plants won't grow