Fertilizer 101: When and How to Fertilize Your Lawn and Plants

Flowers blooming

Simply put, fertilizers are like vitamin supplements for plants. When used properly, they can treat plant “malnutrition” and promote rapid growth and blooming. When used improperly, however, they can be harmful to both plants and the environment, so it’s helpful to understand the basics of fertilizer makeup and application in order to choose wisely.

Much like supplements for humans, in order to choose the best one you must consider:

  • What nutrients you need for which purpose.
  • The dosage and ratio of nutrients.
  • The source – manufactured, processed, or natural.

Fertilizer Types

Fertilizer can be thought of as:

  • Organic fertilizer should not be confused with the familiar eco-friendly term. “Organic” in this case simply refers to fertilizers composed of plant and animal matter, or naturally-occurring minerals, that have been minimally processed and leave the nutrients in their natural states. Examples include manure, compost, peat, cottonseed meal, and bone meal, as well as some packaged fertilizers.
  • Chemical fertilizers are processed from natural or synthetic chemical sources, from which the nutrients have been industrially extracted and combined with fillers to create pellets, powders, or liquids.

Fertilizer can be made by:

  • Natural processes, such as composting.
  • Chemical processes in manufacturing.
  • Mining and processing of minerals.

Blooming flower

Fertilizer Makeup

In order to grow, thrive, and multiply, plants need a variety of nutrients:

  • Common Elements: Carbon, hydrogen, and oxygen: easily obtained from air and water.
  • Primary Nutrients: Nitrogen, phosphorus, and potassium: sometimes referred to by their chemical symbols N-P-K. They are considered the “primary nutrients” and are usually the main active ingredients in fertilizer, indicated in order by the three numbers on the package label (such as 10-10-10 or 21-7-14).
  • Secondary Nutrients: Calcium, sulfur, and magnesium along with trace elements such as boron, zinc, copper, chlorine, molybdenum, manganese, and iron, are also important to plants and may be present in fertilizer.

Ingredients on fertilizer bag

On the label of most any package of manufactured fertilizer, you’ll see three numbers, such as 10-10-10 or 21-7-14. These numbers refer to the percentage of nitrogen, phosphorus, and potassium in the package. To understand the labeling, you need to look at the numbers in two ways:

  • Percentage: 21-7-14 means that the package contains 21% nitrogen, 7% phosphorus, and 14% potassium. The rest is filler. Packages with higher percentages are stronger, so you can use less.
  • Ratio: 21-7-14 has a ratio of 3-1-2. Both 10-20-10 and 5-10-5 have ratios of 1-2 -1. If you’re not into math, simply look at the numbers as “big, medium, or small” and you’ll do just fine.

Guide to Fertilizer Ratios

  • 1-1-1 (such as 10-10-10): General purpose.
  • 2-1-1: Trees and established lawns, leafy vegetables.
  • 1-2-1: Fruits, vegetables, and seeds, flowers, transplants.
  • 3-1-2 or 2-1-2: Stressed lawns and fall lawn feeding.
  • 1-2-2: New lawns.

Healthy green plants need fertilizer

To choose the correct fertilizer, you’ll want to understand the three primary nutrients and how they work.

Nitrogen (10-10-10)

Nitrogen is what makes plants green and influences growth of leaves and stems. Many “all-purpose” fertilizers have a high nitrogen content for that quick burst of green growth that lets you know it’s working. Nitrogen deficiency is evident by yellowing of leaves or grass blades. If you overdo it on nitrogen, your plants will be huge and lush but not very strong, as that rapid growth is more susceptible to diseases, insects, and frost damage.

In manufactured fertilizers, nitrogen is often processed from fossil fuels, usually natural gas. Other sources of nitrogen fertilizer include blood meal, alfalfa meal, fish emulsion, gelatin, and manure. Also, legumes such as alfalfa, beans, and clover have the ability to “fix” atmospheric nitrogen into the soil. Planting a legume cover crop, then tilling the plants into the soil, is an excellent natural fertilizer when preparing a garden.

Use nitrogen during the growing season, but taper down before fall. You don’t want a lot of new, tender growth as the plant faces freezing weather.


Phosphorus (10-10-10)

Phosphorus primarily influences roots, flowers, and fruits/seeds. Manufactured fertilizer labeled “bloom booster” probably has a heavy dose of phosphorus, as do fertilizers packaged for fruits and vegetables. Phosphorus deficiency is evident in red/purple leaves and stunted growth.

In addition to chemical fertilizers, natural phosphorus sources include rock phosphate, bone meal, and composted fruit. Some states have regulations about how much phosphorus you can apply to your lawn, due to potential pollution from the chemical compounds. Check with your agricultural extension service for more information.

Mix phosphorus (such as 0-19-0 or a natural source) into the planting hole when transplanting larger plants and shrubs, to help establish strong roots. It’s also helpful in containers to encourage flowering.

Roots on plant

Potassium (10-10-10)

Potassium is an “all-over” nutrient, especially good for winterizing and stress-proofing. It works throughout plants at the cellular level. Potassium deficiency results in weak branches, failure to thrive, and dying or curling leaves.

Natural sources of potassium include potassium sulfate, rock sand, granite dust, wood ash, kelp meal and a mineral called Sul-Po-Mag (sulfate of potash-magnesia).

Use potassium in the fall to prepare plants for winter, and also in the spring if you expect drought or other stressful conditions. Potassium is important for new plants and lawns. Although it doesn’t produce as visible an effect as nitrogen, the benefits pay off in the long run with healthy, strong, tough plants.

Further Information


  1. I’m impressed Julie! Such the fertilizer expert. I never knew organic chemistry really applied to anything in real life. Thanks for the information, which will come in handy when I finally get working on the gardening some day.

  2. After reading about the ratios in the fertilizers, I am a little confused what to use. I have clover in the my lawn and I want it gone. Do you have any ideas for me?

  3. I have Elite Tall Fescue grass. Can you tell me which is best for my lawn. Ironite 1-0-1 or all purpose fertilizer 21-7-14. I recently spraid herbicide and it turned yellow.
    Thank You,

  4. I have a centipedegrass lawn. I understand that 15-0-15 in the late Spring is a good fertilizer. When should I use a good ‘weed and feed,’ when should I apply it and what specific name brand should I use? Thanks

  5. I have a centipedegrass lawn. I understand that 15-0-15 in the late Spring is a good fertilizer. When should I use a good ‘weed and feed,’ when should I apply it and what specific name brand should I use? Thanks

  6. Is 19-19-19 a good all purpose product for lawns and plants?
    Thank You
    P.S. How often should it be applied?

  7. Is 21-7-14 Fert. organic? If you put it on your fields in the spring and later pastured you grass fed cows where you applied it would your cows still be considered organic?

    With warmest regards,
    John Gardner

  8. Hello Danny, I just had my soil tested . . . they recommended 16-4-3 Fertilizer per 8#/1000SF two applications . . . and a third application 6#/1000SF with a 21-3-7 fertilizer. I can find at Southern State the 16-4-3 but not the 21-3-7. At what ratio or grade is available to substitute. Cannot find a 21-3-7. thanks, Troy

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