Many factors affect the shape, branching pattern, quantity and size of blooms on flowering plants. One major factor is the ratios 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. For many flowering plants the ratio of nitrogen to potassium affects the morphology of the plant roots, stems, leaves, and flowers. Many scientific studies have been done to explore the effects of the Nitrogen to Potassium ratio on flowering plants.
For many kinds of flowering plants it is believed that a Nitrogen to Potassium ratio of around 1:1 is ideal in the vegetative growth phase, and a ratio of around 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. A thicker stem caliper is a prerequisite for large flowers. This nitrogen to potassium 1:2 ratio can also promote more flower sites, increased flower density, quality and dry weight.
Organic Fertilizers and Amendments
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. This is because organic fertilizers and amendments are derived from the remains of living organisms, which are often high in protein making them high in nitrogen since protein is a nitrogen containing compound. When flowering organically is important to make sure there is enough potassium relative to the amount of nitrogen.
A Balancing Act
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. This 1:2 ratio can be easily achieved by using an organic potassium supplement, looking at the fertilizer labels and doing some simple math to make sure the nitrogen to potassium ratio of 1:2 is achieved.
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 to potassium, so experimenting with different ratios will help determine your ideal ratio for each variety.