Roses prefer a soil pH of 6.0 to 6.5-slightly acidic soil, but not too acidic. To determine pH and nutrient levels so that corrections can be made if needed. A pH of 6.5 is the point where nitrogen, phosphorous, and potassium (NPK), plus trace minerals are most easily available to your flowers. You can pick up a test at your local gardening center or send a sample to your local extension office.