Many factors affect the shape, branching pattern, quantity and size of blooms on flowering plants. One major factor is the ratio of minerals available to the root zone of the plant. Nitrogen, phosphorous and potassium must be available in sufficient quantities to provide the basic building blocks for plant tissue.
The ratio of nitrogen to potassium affects the morphology of a flowering plant’s roots, stems, leaves and flowers. Numerous scientific studies have been done to explore the effects of the nitrogen-to-potassium ratio on flowering plants.
For many species of flowering plants, it is believed that a nitrogen-to-potassium ratio of about 1:1 is ideal in the vegetative growth phase, and a ratio of about 1:2 is ideal during the reproductive flowering phase. When plants enter the reproductive phase, the way they use minerals changes. Feeding with a nitrogen-to-potassium ratio around 1:2 during the flowering phase can help shorten the stems and increase stem caliper (or thickness). A thicker stem caliper is a prerequisite for large flowers. This nitrogen-to-potassium ratio of 1:2 can also promote more flower sites and increase flower density, quality and dry weight.
The majority of chemical fertilizers sold for flowering are formulated with a nitrogen-to-potassium ratio close to 1:2; however, many organic fertilizers and amendments do not have this ratio. Organic fertilizers and amendments are often high in nitrogen and lower in potassium, because organic fertilizers and amendments are derived from the remains of living organisms, which are often high in protein, making them high in nitrogen (protein is a nitrogen-containing compound). It’s important to make sure there is enough potassium relative to the amount of nitrogen, particularly when flowering with organic nutrients and amendments.
Balancing the nitrogen-to-potassium ratio can be done relatively easily. It is a good idea to do a soil test to determine how much nitrogen and potassium are already in the soil or growing media. The amount of organic fertilizer or amendments to apply should be based on your soil test results. The 1:2 ratio can be easily achieved by using an organic potassium supplement, looking at the fertilizer labels and doing some simple math.
Most organic fertilizers or amendments list the nitrogen, phosphorus and potassium (NPK) ratios on the label. If the NPK ratio is not known, then a fertilizer test can be done by a lab to find out. Care should be taken not to use too much potassium because excess potassium can cause a deficiency in other elements, such as calcium and magnesium.
Ongoing soil or media testing is a valuable tool used by commercial growers to help determine how much additional nitrogen and potassium will be needed to maintain the ideal ratio throughout the growing season.
It should be noted that the 1:2 nitrogen-to-potassium ratio for flowering is a general guideline. Even within one species of plant, many varieties will respond somewhat differently to various ratios of nitrogen and potassium, so experimenting will help determine your ideal ratio for each variety.
Charles Goldwasser is the founder and formulator for West Coast Horticulture LLC, a company that specializes in the production of plant based organic fertilizers. More information about West Coast Horticulture can be found at westcoasthorticulture.com.