Fertilizer Numbers: Learn To Read The Numbers On Fertilizer Bags

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Fertilizer Numbers – What do the numbers on fertilizer label mean? Why are they important?

Here’s the deal.  Fertilizing plants comes at a cost – in time, in money and results!

bag of fertilize slipping over. It is important to understand the fertilizer numbersPin

To get the most from your “plant food” dollar, keep clearly in mind why you are fertilizing.

Each fertilizer product has a definite purpose and the numbers on the label tell the purpose.

The one you should use depends upon:

  • Your soil – Is the soil friable (easily crumbles) or hard packed?
  • Is the soil acid or alkali?
  • Are you growing the plant for its leaves, flowers, or roots?
  • The results you expect.

Did you conduct a soil test to find out its soil pH? The test results tell you if any additional soil amendments are needed.

Do you want quick action from the plant food for a short quick green color or a slow-release action over a longer time period?

These are the things to consider in selecting a fertilizer but what do the numbers on fertilizer mean?

A wrong choice is not only a waste of effort and money but can actually be worse than no fertilizer at all!

In this article you’ll learn:

  • What do fertilizer numbers mean.
  • How to read fertilizer labels.

3 Numbers On Fertilizers

To ensure good plant growth, they require:

  • Nitrogen (first number) – to stimulate the growth of foliage
  • Phosphorus (second number) –  to encourage the development of fruit and blooms
  • Potash (third number) – to stimulate the growth of strong roots
  • plus minute amounts of trace minerals

Nitrogen is the most quickly used up and, therefore, the one that must be replaced most often.

How To Read A Fertilizer Label

Every container or bag you purchase on the market will have a fertilizers label.

The label will have the N-P-K ratio. These tell buyers how much of the plant nutrients each bag contains.

The NPK acronym stands for Nitrogen, Phosphorus and Potassium. These are often referred to as “macro-nutrients.”

The label also provides other information on the make-up and use of the blend.

For example:

  • The nitrogen sources and how many pounds of nitrogen
  • Is the blend a soluble liquid or a slow-release granular

What are the fertilizers blends primary use?

  • For root development and strong, healthy root growth
  • To encourage the growth of foliage
  • To help in fruit production

The “Complete Fertilizer” – Different Plants Use Specific Fertilizers

A so-called “complete fertilizers” is one which contains all three of these N-P-K nutrients.

Meaning a 12-4-8 fertilizer would have 12N-4P-8K.

Since different types of plants require different amounts of each food – no one fertilizer is a “COMPLETE ALL-PURPOSE FERTILIZER” for all plants.

The best grass fertilizers may produce a thriving lawns but using the same lawn fertilizer as a tomato fertilizer will cause the tomatoes to grow only foliage.

The one that makes the wisteria bloom may cause the caladium to wear itself out producing unwanted blooms instead of fancy leaves.

The correct option for the fragrant-leaved geranium plant whose blooms are unimportant is wrong for the zonal geranium from which you expect blossoms.

If you are looking for leafy growth and green leaves with plants such as lettuce, ivy, or grass, select a high nitrogen fertilizer.

If you are growing fruit or flower plants – tomatoes, roses, strawberries, candytuft (Iberis) – use one high in phosphoric acid.

For root crops such as potatoes, carrots, and beets, look for a high potash content.

Learn What Is 10-10-10 Fertilizer Good For?

Two Groups – Chemical and Organic Plant Food

Fertilizers are divided into two groups – chemical and organic fertilizers. Each group has advantages and disadvantages.

Chemical Fertilizers

Chemical fertilizers sometimes called synthetic fertilizers are quick acting and cheap. In addition, they are made according to a definite formula.

When you buy one you know exactly the percentage of nitrogen, phosphoric acid, and potash in each 50-pound bag and whether or not it contains trace minerals.

Under the trade name on each bag or bottle will be three numbers, for instance, 5-10-5.

The first number indicates the percentage of nitrogen, the second number of phosphate acid, and the third number of potash.

Thus, you can easily select fertilizers that is high in whatever your plant particularly needs.

For example, you may select ratios with plant food numbers of:

  • A 5-8-7 formula for growing potatoes
  • Select the formula 8-4-4 for celery
  • For roses a 4-12-4 blend
  • For tomatoes a 4-6-6 mix

A general all-purpose mixture is 5-10-5. The base of a chemical is usually sand.


Chemical applications should be done strictly according to the directions on the label.

An overdose of chemical fertilizers can burn a lawn or green leaves on a plant or even cause them to die.

A teacup holds about one-half a pound of fertilizers. Always water fertilizers in thoroughly, and never let it touch the foliage or stems.

Commercial fertilizers use up the humus in the soil. They are short lasting and often leave a residue in the soil that causes an acid or alkali reaction.

Therefore, you should not use the same chemical mixture on the soil year after year. You must also add large quantities of humus to replace that which is lost.

Organic Fertilizer

Organic plant foods are slow acting, long lasting, and build up the soil in the garden over a long period of time.

Because of their slow action, they may be used more freely with less danger of overfertilizing.

For example, several organic lawn fertilizers include fish emulsion,

Organic fertilizers high in nitrogen are:

The best organic sources of phosphoric acid are:

  • Bone Meal (0-10-0)
  • Rock phosphates

Organic potash is found in:

The value of the “food” in organic fertilizers varies making it difficult to label them as precisely as synthetic fertilizers.

Organic mixtures are packaged, but with the approximate nutrient percentage on the bag.

Any option can be overused, however. Every plant has a saturation point beyond which more food is bad.

Plants seem to be affected by too much food just as human beings are by too rich a diet.

Remember, too, that some plants such as Nasturtiums and certain herbs actually prefer a rather poor soil.

A good rule of thumb in choosing between a chemical and an organic option is this:

If you are primarily interested in the crop that year – annual flowers, tomatoes, etc. – use chemical.

If you are more interested in long-term growth as in the case of trees and shrubs, long-lasting bulbs, and perennials, then use organic.

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