Ground limestone is the safest and least expensive do-it-yourself soil amendment for reducing soil acidity. Soil acidity is measured on the 14-point pH scale, where pH values below 7 indicate acidic soil, and values above 7 indicate alkaline soil. In vary acidic soil where pH is 5.5 or below, availability of nitrogen, calcium, magnesium, molybdenum and phosphorus is reduced, while aluminum and manganese can increase to toxic levels. The high acidity also inhibits activity by beneficial soil bacteria.
Most cultivated plants do well in soil with a pH between 6 and 7.5. But if your plants are not thriving, you should test your soil pH before applying any type of soil amendment. You can purchase inexpensive soil pH test kits at hardware, home and garden centers. These tests require that you dissolve a soil sample in distilled water, add an indicator chemical and read the pH from a color chart. Alternatively, you can take a soil sample to your county or state agricultural extension service for an inexpensive professional pH test and recommendations on soil amendment.
Correcting Acidic Soil
Adding ground limestone to soil will raise pH by correcting excess acidity. Calcitic limestone is calcium carbonate, which chemically breaks down acids into water and carbon dioxide and leaves calcium in the soil as a plant nutrient. Dolomitic limestone is made from rock containing both calcium carbonate and magnesium carbonate. The magnesium carbonate also breaks down acids but leaves magnesium behind to nourish plants.
Limestone can be applied any time of the year but when you apply it in the early spring or in the fall, it will have a chance to penetrate into the soil before the active growing season. The limestone must be finely ground to have maximum contact with the soil. Finely ground limestone can be hard to spread; some companies sell a granular limestone with small amounts of clays or polymers added to make it easier to spread by hand or with a garden fertilizer spreader.
To raise the pH by one point in a typical sandy loam soil, spread limestone at the rate of about 50 pounds per 1,000 square feet, unless your liming product specifies a different spreading rate. If you need a bigger pH adjustment, divide your liming product into two equal parts. Apply one part in the fall and the other in the early spring. Don’t expect a sudden dramatic change. Limestone works slowly over a period of several months to change soil pH.