pH scale goes from 0 to 14. 0 = most acidic. 7 = neutral. 14 = most alkaline. Each step represents a 10x change. A pH of 6 is 10 times more acidic than a pH of 7, 100 times more acidic than a pH of 8. The closer to neutral the soil is, the better the plants will grow.
Make sure you know what you’re dealing with; always test the soil pH before attempting to correct it. Don’t try to correct soil pH too quickly, it can “kill” the soil, i.e., destroy all the organisms that allow plant life to grow. It’s better to adjust in spring and fall over multiple years.
pH = potential hydrogen (The number of positively-charged ions contained in the water content of the soil. Soil must be damp to measure pH.)
The vast majority of plants need alkaline soil (8+ pH). Only the following need acidic soil:
Correcting acidic soil (increasing pH):
Add lime to the soil. The hydrated form is the strongest and will work the fastest. Calcitic lime adds calcium to the soil; dolomite lime adds both calcium and magnesium. Sandy or loamy soils require less lime than clay or peaty soil.
Add wood ashes to the soil. It is mostly made up of calcium carbonate and will have a similar effect as lime.
Correcting alkaline soil (decreasing pH)
The most common source of acid is sulfur, which, with the aid of soil bacteria, converts to sulfuric acid. Four factors influence how quickly the conversion happens: the size of the sulfur particles, how moist the soil is, the temperature of the soil, and how much bacteria is contained in it. Thus lowering pH even one unit can take anywhere from several weeks to several months.