The level of acidity in soil is measured on the pH scale. A pH level of 7 is neutral. Soil with a pH of less than 7 is considered acidic, and soil with a pH of more than 7 is considered alkaline. Home gardeners can change the pH of soil by amending it with many different products. Azaleas and blueberries thrive in acidic soil. Hydrangea blooms even change color depending on the soil pH: blooms are blue in acidic soil and pink in alkaline soil.
A combination of elemental sulphur and sulphur may be added to soil to decrease the pH level, making the soil more acidic. The addition of sulphur to soil is not a quick fix; it can take up to six months for pH to change. Elemental sulphur, which is available commercially as granules, should be applied in spring or summer and mixed to a depth of 6 inches below the soil line. Once the elemental sulphur is worked into the soil, the pH level should stay consistent for up to five years. Elemental sulphur, which is the least expensive way to alter pH, is often used in farming or areas where there will be regular plantings.
Organic matter, such as compost, manure or commercially-available organic mulch, will increase soil acidity quickly. These products are high in nitrogen, which will be absorbed by existing soil. Organic matter or mulch may be mixed with soil or spread as a layer of topsoil. Organic matter is best used with fine, sandy soil, as the organic materials will break down more quickly. Thick, clay-like soil usually holds moisture and nutrients well and should be treated with peat moss to make the soil more workable. If you mix organic materials with the soil, it will not only make it more acidic, it will also make it thicker. While mixing organic matter is a quick fix for pH levels, it is not long-lasting. New organic matter should be mixed with soil annually.
Also known as peat moss, sphagnum peat is the decomposed matter from peat bogs. Sphagnum peat moss is a dry material that, when mixed with soil, helps it to better retain water and nutrients. Sphagnum peat should be used on small gardening plots, as it would be cost prohibitive on large areas. If mixed with clay soil, it will help to loosen the soil; if mixed with sandy soil, it will help the soil to stick together. Sphagnum peat is an organic material and will help to quickly increase soil acidity, but should be added annually if you will be planting in the same area. Spread 1 to 2 inches of sphagnum peat over soil and mix in to a depth of 8 to 12 inches.