Do Rose Bushes Need Potassium?

Rose Nutrients

• Roses need 16 different chemical elements, many of which occur naturally in soil and air. But some must be added to the soil in the form of fertilizers and organic amendments, which include compost, bone meal, fish meal and well-rotted manure. The Marin Rose Society says the nutrients can be divided into four groups with the most important group being the essential elements of carbon, hydrogen and oxygen and the macronutrients, which are nitrogen, phosphorus and potassium. Potassium deficiency creates a number of problems, including impeding photosynthesis.

Photosynthesis

• During photosynthesis, potassium controls the opening and closing of leaf pores called stomates, which draw in carbon dioxide and release oxygen into the air. When potassium is scarce, photosynthesis declines. The accumulation of oxygen in potassium-deficient plants inhibits photosynthesis, whereas lack of carbon dioxide causes roses to cannibalize carbon from their various plant parts. Symptoms of a potassium deficiency in roses include stunted growth, deformed flower buds, shorter-than-normal stems that are weak, stems that don't flower and altered color at the tip and margins of leaves due to leaf burn.

Plant Vigor

• Potassium affects overall vigor of rose plants. According to a study about the reproductive growth of the cabbage rose (Rosa centifolia) published in 1999 by Pakistan's University of Agriculture, amendment of the soil with potassium alone can increase foliage, flowering and spread of the cabbage rose. A combined amendment of nitrogen and potassium resulted in the best plant height and flower size. A potassium shortage can make roses more susceptible to bacterial infections that wilt leaves, blacken canes and create root tumors. It may also lead to problems such as leaf mosaic virus -- a yellowing pattern on foliage.

Soil Amendments

• Rose gardeners save money and effort by obtaining a soil test before adding fertilizer or other amendments to soil. Local agricultural extension offices are good resources for these tests, which can help identify what kinds and quantities of nutrients are needed. Chances are that a standard "balanced" rose food will do. A fertilizer containing a "6-12-6" mix of nitrogen, phosporus and potassium (N-P-K) is considered a balanced mix, because it contains the macronutrients roses require in the proportions needed. The University of Missouri Extension says to fertilize roses once every six weeks starting when they first leaf out and concluding at the end of summer. It notes that rose plants don't differentiate between chemical and organic amendments as long as they get what they need.