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Right before springtime, and just after pruning, you may need to make some adjustments to your soil, especially if you are planting new roses. One of the important elements of growing good roses concerns soil pH. Since I’m a chemist by training, I could bore you to tears with a lengthy explanation of what pH is. However, simply put, pH is the amount of acid (H+) or base (OH-) in the material. Numbers 0 to 6.9 are acidic, with 0 being extremely acidic. 7 is neutral (deionized water). Numbers 7.1 to 14 are basic, with 14 being extremely basic (caustic and alkaline are also commonly used terms for basic). Although most references differ, roses generally enjoy a pH of 6.0 through 6.9, with about 6.5 being ideal. In other words, the soil should be just slightly acidic. For pH outside this range, the availability of nutrients to the plant is greatly affected. For example, at a pH of 5.0 or less, phosphorous is ‘trapped’ by aluminum ions and rendered insoluble which cannot be absorbed by the plant. In the 6.0 to 6.9 range, all nutrients are in a form that is available to the plant. The more basic the soil, the less nutrients, such as iron, nitrogen, and manganese, can be absorbed.
So how do you test the soil? There are two methods commonly used for testing soil pH. In both cases, you need to collect a representative soil sample. Do not take a small sample, and don’t take one when there’s a lot of organic material. Scrape off the topsoil and collect a small hand shovel full of soil from ten to twenty places throughout the rose garden. Mix these together and let the soil dry. At this point you can send a portion of the soil to a lab for full or partial analysis, or you can purchase a home pH kit from a local nursery. If your soil is too acidic, add lime. If it is too basic, add either sulphur or aluminum sulfate. (Our Southern California soils tend to be on the basic side.) Don’t be too generous with the application, it doesn’t take much. Nurseries and rose supply catalogs have various pH test kits or meters available, ranging from $5-$15. They should also carry a supply of sulphur, lime, and aluminum sulfate.
Reprinted from Rose Ecstasy, bulletin of the Santa Clarita Valley Rose Society, Kitty Belendez, Editor
