Our Roses need NPK. What is that?
Cindy Crookston, Consulting Rosarian
Oh, how we love our roses! We love the pretty blooms, and healthy plants. We love how they beautify our gardens and homes. What makes them grow into such beautiful plants? It is simple: adequate sunlight, air, water and mineral nutrients.
The rose is predominately made up of carbon, hydrogen and oxygen, in the form of water, sugar and starches. These three elements are derived from the air and water. All remaining nutrients are obtained from the soil.
The major, or macronutrients found in the soil are:
Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (C), Magnesium (Mg), Sulfur (S)
The nutrients needed by plants in lesser amounts are called micronutrients, and are rarely missing from the soil. They are:
Boron (B), Chlorine (Cl), Copper (Cu), Iron (Fe) , Manganese (Mn), Molybdenum (Mo), Zinc (Zn)
The three main macronutrients are nitrogen, phosphorus and potassium, or NPK. These three nutrients are needed for all plants to live and grow. Roses are especially sensitive to the right amounts of these elements in the soil. If the quantities are not correct, the rose suffers. This is why we pay such special attention to NPK ratios in our soils and in our fertilizers.
Nitrogen is critical for a plant’s growth and survival, or in other words, very simply put, nitrogen makes plants grow. When nitrogen is in balance in the soil the rose will have tall, strong canes, good blooms and rich dark leaves. If there is a lack of nitrogen that is available, the rose will have leaves of light green color, maybe even a yellowing. If there is an excess of nitrogen the rose will show weak, soft canes, small blooms and decreased resistance to disease. The availability of nitrogen to the rose is dependent on soil temperature, moisture and acidity. Adequate drainage will help keep the soil temperature and moisture levels appropriate for a rose to be able to take up the nitrogen. At the same time the correct pH level for roses, 6.0-6.8 will keep the acidity level where it should be for the rose’s ability to take up nitrogen.
Phosphorus stimulates root growth, which in turn produces big blooms and quality plants. It can also hasten plant maturity and winter hardiness. The soil acidity is very important in making phosphorus available to the rose. Here in the western part of the United States, there is generally a high level of phosphorus in the soil. However, with too little phosphorus bloom production will drop. In severe cases the leaves at the bottom of the rose will turn a purplish red. Roses can benefit from bone meal, or superphosphate given at planting time.
Potassium (Potash) promotes root growth, vigor and bloom color. It works a balancing effect on both nitrogen and phosphorus. It is important for starch formation and the development of chlorophyll by encouraging photosynthesis. Soil acidity has little influence on potassium, but certain types of clay soil can have a marked effect on potassium availability. A deficiency in potassium causes brown leaf margins, weak stems and blind shoots.
It is important is if the nutrients will be available to the rose. This issue is highly determined by soil acidity. The pH value of the soil is measured on a logarithmic scale running from 0 to 14. A pH of 0 is very acidic, and a pH of 14 is very alkaline. The pH level 7.0 is considered neutral. The ideal level for roses is 6.0-6.8, with 6.5 being the optimal level. The soil may have sufficient amounts of nutrients, but if the pH level is far off one way or the other, the nutrients will not be available to the rose. The nutrients will be, what we call “locked up” and the rose will not be able to benefit from them. Adjusting the soil pH is necessary to have healthy plants.
Soils that are high in organic matter are less effected by a low pH. Organic matter holds onto nutrients. Soils low in organic matter and with an incorrect pH level will almost always yield a rose that exhibits nutrient deficiency problems. It may be necessary to have the soil tested, if roses are struggling, or before planting new roses. Soil test samples can be sent to Utah State University Extension Services, for a nominal fee. The report of your soil test will reveal the content of current nutrients, which nutrients are lacking and recommendations to improve your soil.
Suggested Home Soil Tests from Utah State University:
Routine Test $25
Basic measurements for the vast majority of home gardening needs
Provides Phosphorus (P205) and Potassium (K2O) fertilizer recommendations
Provides Nitrogen (N) fertilizer recommendations tailored to plants the client indicates are to be grown (please provide this information on the submission form)
Provides soil salinity and pH levels useful in choosing appropriate plants for these important conditions in arid/semi-arid Western US environments
Provides a soil texture classification used to determine water holding capacity and irrigation regiment
Micro Plus Test *$35
Starts out with all the features of the Routine Test
Includes key micronutrient levels that are often deficient in arid/semi-arid Western US soil conditions (Iron, Zinc, Copper and Manganese)
Provides recommendations for deficient nutrients tailored to the plants the client indicates are to be grown (please provide this information on the submission form)
* Choose this test if micronutrient deficiency symptoms are seen or suspected the previous season. Micronutrient fertilizer applications are most effective in the early spring–much less effective mid-season.
The following link will give you instructions on how to gather a soil test from your garden, and then how to send it to USU.
Primary nutrients that are added to the soil are referred to as fertilizers. We will discuss the benefits of fertilizers in a later article. For now, remember that organic matter and the correct pH level will give proper care and nutrition to our roses. With more knowledge of how to care for our roses, the more beautiful our gardens will be. Happy rose growing!
American Rose Society, Consulting Rosarian Manual 3rd Edition Revised, July 2016
Mills, Dan “Soil and Water and Roses”, The Sustainable Rose Garden: A Reader in Rose Culture ,edited by Pat Shanley, et.al. 2010, Newbury Books
Schneider, Peter Right Rose, Right Place, 2009
Stock, Dr. Melaine Assistant Professor/ Extension Urban & Small Farms Specialist USU, Certified Soil Scientist, SSSA, Soil and Water Management for Roses, June 2
Utah State University Analytical Laboratories, “Home Soil Test”, http://www.usual.usu.edu