Gardeners use lime to increase the pH of soils, making them more alkaline or sweet and less acidic or sour. Alkaline soil is typically found in dry climates, while acidic soil is associated with wet climates. Ground limestone -- calcium carbonate sold in garden supply centers as garden lime -- is most often used to adjust soil pH, but you can also use hydrated lime, or calcium hydroxide, which is more finely ground and more caustic.
Effect of Acid Soil
Humid, wet climates often leach calcium, magnesium, potassium and sodium from soil, making them more acidic. Fertilizers, pesticides and industrial pollutants also contribute to acidic soil.
Acidic soils make it more difficult for plants to take in nitrogen, phosphorus, calcium and magnesium. Conversely, acidic soil may contain toxic amounts of copper, iron, manganese and zinc.
Beneficial soil bacteria and fungi that assist in the ability of plants to take in nutrients and resist disease typically thrive in soil pH between 5.9 and 7.
Hydrated Lime Effectiveness
The effectiveness of lime, its "neutralizing value," is measured against the effectiveness of pure calcium, calcium oxide, also called quick lime, which is too caustic to be used. The neutralizing value of garden lime ranges between 50 percent and 55 percent. The neutralizing value of hydrated lime is 70. When a soil test reveals a lack of magnesium, gardeners use dolomite lime, a ground limestone containing magnesium carbonate in addition to calcium carbonate.
Soil pH is measured on a scale from 1 to 14. Soil below 7 is acidic. Soil above 7 is alkaline. Although a soil pH of 6.5 is the best level for most garden plants, the pH preferences vary by grass or plant species.
Do not apply hydrated lime to your garden without testing the soil to find out how much pH adjustment is required for what you want to grow. You can buy a kit to test your soil pH at most garden supply centers.
You can get a rough idea of your soil pH by adding a few drops of vinegar to dry soil in your garden. If it fizzes, the pH is greater than 7.5. You have alkaline soil and clearly no need to add lime. If your soil fizzes when you add a pinch of baking soda to moist garden soil, the pH is less than 5 and you may need to add lime.
Applying Hydrated Lime
Hydrated lime is applied at two-thirds of the rate of garden lime. Winter is the ideal time to add hydrated lime for annuals planted in spring, although you can add it two weeks before planting. You can apply it one or two days before planting if necessary, but the soil pH will not change for several weeks. Spread the soil evenly throughout your garden then spade or plow it 6 inches deep. A spreader can be used to apply it.
Add 11 4/5 ounces of hydrated lime per square yard of clay soil to raise the pH from 6 to 6.5. Add 19 2/3 ounces per square yard of 5.5 pH soil, 27 1/2 ounces to 5 pH soil and 35 1/3 ounces to 4.5 pH soil.
Add 9 4/5 ounces of hydrated lime per square yard of loamy soil to raise the pH from 6 to 6.5. Add 15 2/3 ounces per square yard to 5.5 pH soil, 23 1/2 ounces to 5 pH soil and 29 1/2 ounces to 4.5 pH soil.
Add 7 4/5 ounces of hydrated lime per square yard of sandy soil to raise the pH from 6 to 6.5. Add 13 2/3 ounce to 5.5 pH soil, 19 2/3 ounces to 5 pH soil and 25 1/2 ounces to 4.5 soil.
Hydrated lime is caustic. Wear rubber gloves and goggles when applying it and do not inhale it. If you do inhale or ingest it, call your physician or 800-222-1222, the national hotline run 24/7 by the American Association of Poison Control Centers.