Nitrogen

Nitrogen is the first number in the ratio on the bag. Nitrogen is critical for the greening of your plants' leaves and their growth. Some plants that are considered "heavy feeders" may require a fertilizer with a higher proportion of nitrogen. "Sunset" magazine recommends using fertilizers with a higher proportion of nitrogen to the other nutrients during spring growth spurts.

Phosphorous

The growth of the plant's roots, seeds and flowers depends on if it gets enough phosphorous. The percentage of phosphorous in the fertilizer is listed as the second number on the package. A fertilizer with more phosphorous and potassium than nitrogen can benefit new planting beds to promote the growth of a healthy root system.
 

Potassium

Fruit and vegetable plants need potassium for the growth of their edible parts. Potassium also helps plants to resist disease. Have your soil tested to determine the amount of potassium that is naturally in the soil. For instance, Marin County has adequate potassium amounts naturally in the soil, and if you live there, you may not need additional supplementation.