What Does the Number 30-5-5 Mean in Fertilizer?

Fertilizer Numbers

• Plants need three primary nutrients in order to survive: nitrogen, phosphorus and potassium. Not surprisingly, many complete fertilizers contain all three. Likewise, fertilizer numbers express the amounts of each of these nutrients in the fertilizer.

  The first number listed refers to the amount of nitrogen. In 30-5-5 fertilizer, you can expect the product to contain 30 percent nitrogen or 30 pounds of nitrogen per 100 pounds of fertilizer that you apply. The second number tells you the amount of phosphorus. 30-5-5 formulations contain 5 percent phosphorus. Finally, the third number communicates the amount of potassium, so a 30-5-5 fertilizer contains 5 percent potassium.

Choosing Fertilizers

• Each of the three major nutrients provided by fertilizer plays a different role in plant health and growth. Nitrogen is the building block of all life, required for protein synthesis needed for growth, and so nitrogen fertilizers encourage the growth of stems and leaves. Phosphorus plays numerous roles in plant health, but it primarily promotes healthy root, flower and fruit development. Potassium provides plants with protection against environmental stressors, helping them to resist drought, pest damage and disease.

Nitrogen Fertilizers

• 30-5-5 fertilizer provides mostly nitrogen. You can therefore expect that a dose of fertilizer will encourage the leafy growth of your plant. For a foliage plant -- a houseplant or an evergreen shrub, for example -- this type of growth may be ideal. If you're growing a plant for its flowers or fruits, however, a nitrogen fertilizer can promote leafy growth at the expense of flower and fruit development. Applying nitrogen fertilizers too late in the season can delay production in vegetable plants. Also, avoid fertilizing plants with a nitrogen fertilizer in the fall, as the tender new growth will become susceptible to frost damage.

Other Considerations

• Fertilizer packaging contains more information than the fertilizer number. Especially when using a fertilizer high in nitrogen -- like a 30-5-5 fertilizer -- you should also read the packaging carefully to ensure that the product will meet your needs and won't harm your plants. Nitrogen in certain forms can damage plants by creating salts in the soil that pull the water from the plants' roots. Fast-release forms of nitrogen also become susceptible to leaching in some soils, which can cause water pollution.

  Nitrogen sources like nitrate, ammonium and urea tend to become available quickly. While you will get fast results from these, you will have to take care not to damage your plants. Slow-release nitrogen fertilizers -- which include most organics -- will provide long-term feeding and less risk of pollution or plant damage. A 30-5-5 fertilizer with a mixture of fast- and slow-release nitrogen sources will meet most routine fertilizer needs.