Like people, plants need regular feeding to obtain nutrition necessary for growth.
The three primary nutrients that plants require are nitrogen (N), phosphorus (P) and potassium (K). These elements are critical to plants which use them in large amounts. Calcium (Ca), magnesium (Mg) and sulfur (S) are secondary nutrients. In addition to the primary and secondary nutrients, plants also need micronutrients. Micronutrients are essential for plant growth, but only needed in very small amounts. When selecting a fertilizer, choose one that contains micronutrients in addition to the three primary nutrients to give your plants the complete nutrition they need.
Every fertilizer is labeled with three numbers that indicate the NPK value—the percentage of each of the primary nutrients in a fertilizer's formulation. Each of the primary nutrients plays a different, but essential role in the health and life cycle of a plant.
Nitrogen is responsible for promoting healthy foliage and rapid growth.
Phosphorus encourages flower, fruit and root development.
Potassium improves disease resistance, winter hardiness and promotes root growth.
While they are in active growth, plants should be fertilized regularly with a water soluble fertilizer. Water soluble fertilizers are less likely to burn plants than granulated fertilizers in the event that too much is applied. For blooming plants, select a fertilizer with an NPK value of 15-30-15 or similar. Roses prefer an 18-24-18 formulation while fruits and vegetables respond best to a 15-10-30 formulation. For acid loving plants such as hydrangeas, azaleas or blueberries, choose a fertilizer with a 21-7-7 NPK value.
In most cases, calcium, magnesium and sulfur are readily available in soil and do not need to be supplemented. If a soil test does indicate that your soil is lacking any of these secondary nutrients, they should be added to the soil as an amendment rather than via fertilizer. Dolomitic lime is available as a powder to add both calcium and magnesium to soil while sulfur is available in a granular form.
Annuals grow quickly and experience their entire life cycle in a single season. They should be fertilized once every 7-10 days after planting to keep them happy and healthy. Young and recently transplanted plants undergo more growth than older, well-established plants. For this reason, they typically require more frequent applications of fertilizer to accommodate their nutritional needs. New transplants should be fertilized once every 2-3 weeks throughout the growing season. Well-established trees and shrubs can be fertilized monthly while in active growth. Remember to stop fertilizing all perennials by September 1st to allow them to harden off in preparation for winter dormancy and never fertilize dormant plants.
By providing your plants with rich, healthy soil and proper nutrition, you are setting them up for success. Regardless of your skill level or soil type, you can have a thriving garden with a little bit of knowledge and hard work!
When feeding your plants, it's very important not to over fertilize.
Don't fertilize a newly transplanted bareroot plant. Wait at least a couple of weeks and until you see signs of growth/budding.
Using a water soluble fertilizer helps eliminate the danger of burning your plants the way a dry granular fertilizer can.
And, just like people and pets, over feeding can be very bad. Fertilizing your plants once every two to three weeks should be plenty to keep your plants healthy. Follow the instructions on the container of your fertilizer to determine the correct amount to use.

Plant Nutrition