The ability of a plant to obtain necessary plant nutrients depends largely on the pH of its soil, measured on a scale of 1 to 14. Acidic soil, sometimes called "sour" soil, has a pH below 7, considered neutral. Alkaline soil, sometimes called "sweet" soil, has a pH above 7. Most rose cultivars prefer a pH between 6 and 7. If your soil pH is below 6, you can increase the number by adding lime.
Climate and Soil
There are between 150 and 300 species and 6,000 cultivars in the Rosa genus. Roses will grow in most soils that have a high organic content, in U.S. Department of Agriculture plant hardiness zones 4b to 10b, depending on the species and cultivar. To determine if you need to add lime to your soil you have to know its pH. Test it either in the fall or spring.
Testing Soil pH
Use a spade, spoon or trowel to take a sample from the top 6 to 8 inches of soil. Do not do this when the soil is wet. Spread the sample on plastic wrap and let it dry overnight. When it's completely dry, remove stones and solid matter and put the soil in a plastic bag. It's now ready to test using a home soil testing kit that you can buy from most garden supply centers. Alternatively, you can have it tested by a commercial testing service that will also tell you of any mineral deficiencies in the soil. If your soil pH is too low for roses, add lime to raise the number.
How Much Lime to Apply
Finely ground limestone, calcium carbonate, available at most garden supply centers, is typically recommended for home gardeners. How much lime to apply to raise soil pH to 6.5, ideal for roses, depends on the type of soil. For clay soil: Add 29 1/2 ounces of lime per square yard to raise pH from 5.5 to 6.5, 41 1/4 ounces per square yard to 5 pH clay soil, and 53 ounces per square yard for 4.5 pH, per recommendations from The Royal Horticultural Society. For loam soil: Add 23 1/2 ounces of lime per square yard to raise pH from 5.5 to 6.5, 35 1/4 ounces per square yard on 5 pH loam soil, and 44 1/4 ounces per square yard on 4.5 pH. For sandy soil: Add 20 1/2 ounces of lime per square yard to raise pH from 5.5 to 6.5, 29 1/2 ounces per square yard to 5 pH sandy soil, and 38 1/4 ounces to 4.5 pH.
How to Apply
Winter is a good time to apply lime. If you apply more than 14 3/4 ounces of lime per square yard, dig half of it into the top 6 inches of soil and sprinkle the other half on the surface when you're finished. If you apply less than that amount per square yard, dig all of it into the soil. If digging is impractical, sprinkle all of it on top of the soil. It's best to add lime before you plant roses. If you sprinkle it around plants, it can take years to have any effect.