What Do the Numbers in Fertilizer Mean?

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Fertilizer labels tell you not only what active ingredients the fertilizer contains but also how much of three important nutrients are available for your plants.

The NPK Number

  • All fertilizer labels carry a series of three hyphenated numbers collectively known as the NPK number. This number tells you how much available nitrogen (N), phosphorous (P) and potassium (K) is in any given fertilizer. The numbers are standardized, always appearing in the same order. A number that reads 10-20-5 represents a complete fertilizer that contains 10 percent nitrogen, 20 percent phosphorous and 5 percent potassium.

Significance

  • Different types of plants require different nutrients. For instance, if you need a fertilizer for your lawn, look for a fertilizer high in nitrogen, such as 29-2-4. If you want a fertilizer for your vegetable garden, find a product high in phosphorous, such as 10-20-5.

Plants and Nutrition

  • Plants absorb at least 60 different chemical elements, of which 13 are essential. The most important nutrients are nitrogen, phosphorous and potassium. They are also the first to be depleted and the most commonly added back with the use of fertilizers.

    Nitrogen is needed by plants in larger amounts than any other nutrient. It is vital to the production of stems and leaves and is most needed in spring during early rapid growth. Lawn fertilizers contain the most nitrogen. Nitrogen also is depleted during prolonged periods of wet weather.

    Phosphorous is especially needed for root or bulb formation and flower and fruit production. It is most in demand in summer, when flowering and fruiting peaks.

    Potassium is linked to overall plant health. When nitrogen and potassium are in balance, plants suffer less form insect attack, disease and frost damage. Potassium plays a significant role in photosynthesis at low light levels and in internal water regulation. It also improves the flavor and color of fruits and vegetables.

Considerations

  • The NPK number tells you only what is immediately available, not the total nutrient content of the product. It is not a measure of how good a product is. Organic fertilizers usually have lower NPK numbers because their ingredients break down over time, releasing their nutrients slowly. Chemical fertilizers give an immediate boost, but the long-term use of chemical fertilizers has been shown to have a detrimental effect on soil structure and overall soil health. With long-term use, chemical salts build up, acidity changes and the population of soil microorganisms is reduced.

Everything in Balance

  • When it comes to fertilizers, too much is just as bad as too little. Nutrients need to be in balance with each other to work well. Adding a little "extra" is not a good idea. For instance, the availability of nitrogen is closely tied to carbon. With too much or too little carbon, nitrogen becomes unavailable. Too much potassium can block the uptake of magnesium, and sulfur can become unavailable if too much nitrogen is present. Smaller, more frequent feedings are best. An organic soil amendment such as manure or compost will help keep carbon levels up.

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