When you walk down the fertilizer aisle at a garden center or home improvement store, you'll likely notice a wide variety of fertilizer formulations. You’ll see bags, bottles, powders, granules, pellets, sprays, and concentrates. Furthermore, you’ll discover an increasing number of organic and “earth-friendly” fertilizers. Learning more about your options can help you pick the best food for your flowering plants.
For most flower gardeners, a complete fertilizer is necessary to supply plants with the three major elements they require to thrive:
- Nitrogen (N): Promotes lush foliage growth
- Phosphorus or Phosphate (P): Promotes blooming and fruit formation
- Potassium or Potash (K): Develops healthy root systems
The fertilizer packaging will list these three major nutrients in exact order (sometimes referencing them as "NPK") with the numbers representing the percentage of each nutrient compared to other minor nutrients and filler ingredients. For example, a 10-10-10 fertilizer contains 10 percent of each N-P-K nutrient. A 10-30-20 fertilizer contains three times the amount of phosphorus than nitrogen. Filler ingredients are inert materials that add weight and volume, like limestone, sawdust, clay (in powdered formulas), or water (in liquid fertilizers).
To stimulate blooming, choose fertilizers with a higher percentage of phosphorus compared to the percentage of nitrogen and potassium. Healthy flowers start with vigorous root systems, and a potassium boost can ensure your flowers get a good start.
Fertilizer manufacturers create artificial fertilizers by combining inorganic chemicals to form compounds like ammonium nitrate or magnesium sulfate. Some advantages of chemical fertilizers are that they're inexpensive, readily available, and rapid-growing plants like annual flowers take up the nutrients quickly (unless the formula is designed to be a time-released fertilizer). Disadvantages include the risk of over-application, which causes burning, and the absence of any soil-improving qualities.
Chemical fertilizers come in a range of formulations, including pellets, liquid concentrates, and powders. Some products come in pre-measured packets designed to be added to the watering can, making it convenient for gardeners to apply it to containers, houseplants, or their landscape.
Foliar fertilizers are liquid nutrients that plants absorb through their leaves. Not all flowers feed efficiently this way, because the waxes and hairs on some leaves act as a barrier to nutrient uptake. Flowering plants cannot receive all of the nutrients they need through their leaves, but you might be able to address some nutrient deficiencies quickly by using foliar fertilizers. Potassium is one readily absorbed nutrient in foliar feeding applications, so use foliar fertilizers in the flower garden to address potassium deficiencies.
If your flowers are exhibiting signs of chlorosis, a yellowing of the leaves, your plants might have an iron deficiency. Sometimes, the rapid results achieved by foliar fertilizers can help.
Organic fertilizers come from living things, like animal manure, fish emulsion, leaf molds, and non-living things, like rock phosphate or greensand. Fertilizers from organic matter not only supply essential nutrients to flowers, but they also improve soil tilth. There are several advantages to using organic fertilizers:
- Don’t burn plants
- Strengthen plants’ immune systems
- Non-toxic to beneficial insects and wildlife
- Remain active in the soil for long periods
Disadvantages of organic fertilizers include their higher cost, taste appeal to some pets, and limited formulations. Organic fertilizers are not an overnight fix—they won’t correct severe nutrient deficiencies quickly.
If a soil test reveals a deficiency of one major nutrient, you can purchase a simple fertilizer, which contains only nitrogen, phosphorus, or potassium as a standalone ingredient. Simple fertilizers can be chemical or organic in makeup. An example of simple nitrogen fertilizer would be one that lists only urea or ammonium sulfate as the ingredient. A simple phosphorus fertilizer might list superphosphate or ground mineral phosphate as the ingredient. A potassium fertilizer may list muriate of potash on the container.
Technically, all organic fertilizers are slow-release, because it takes time for organic matter to decompose in the presence of soil microorganisms. The slowest acting organic fertilizers include insoluble mineral fertilizers, like rock potash and other rock powders.
Gardeners who prefer to fertilize plants infrequently can use slow-release fertilizers, which are coated or encapsulated to control the release of the fertilizer over a period of several weeks or months. These types of fertilizers are especially popular for houseplant care and outdoor containers.
Here's the Scoop on Chemical and Organic Fertilizers. Oregon State Extension
Foliar Fertilization in the Citrus Industry. University of Florida Extension
Controlled-Release and Slow-Release Fertilizers as Nutrient Management Tools. University of Florida Extension