The Numbers on Fertilizer Labels, What They Mean

Please follow the label, more is not better

By JOE LAMP’L

Whenever you see a fertilizer product, it will have three numbers prominently listed on the package, usually on the front. These numbers are very important and tell a great deal about what this fertilizer will do.

Nitrogen, Phosphorus, and Potassium (N,P,K) are what the numbers refer to. They are always listed in this order. It is the percentage within that package of each component. For example, a common type of all-purpose fertilizer is referred to as 10-10-10. This is a balanced blend of equal portions of Nitrogen, Phosphorus, and Potassium. If you purchased a 50-pound bag, five pounds (or 10%) would be Nitrogen, five pounds would be phosphorus, and five pounds would be potassium. The remaining 70% is simply filler, or inert ingredients, which are there mostly to help disperse the chemicals.

Up, Down, and all Around

A common way of describing the purpose behind each chemical is to think “up, down, and all around.” Applying this simple phrase will help you remember that Nitrogen (N), the first chemical listed, helps with plant growth above ground. Nitrogen does a great job of promoting the green leafy growth of foliage, and provides the necessary ingredients to produce lush green lawns. Lawn fertilizers will frequently have a high first number for this purpose.

Phosphorus (P), the middle number, is very effective at establishing growth below ground, in the form of healthy root systems. It is also the component most responsible for flower blooms and fruit production. You’ll notice that fertilizers designed for flower production, or starter-type fertilizers for your lawn, have a high middle number.

Potassium (K), the last number listed, is considered important for overall plant health. This is primarily due to its ability to help build strong cells within the plant tissue. In turn, the plants withstand various stresses, such as heat, cold, pests, and diseases. For example, winterizer fertilizers will have a high component of potassium.

When shopping for fertilizers, be mindful of their intended use. Fertilizers that have equal numbers can generally be used as an all-purpose fertilizer. If you had only one product to work with, 10-10-10 would be my recommendation.

For promoting good fruit or flower production, look for a middle number that is higher than the first. Otherwise, your plants will be stimulated to put out lots of nice green foliage, likely at the expense of fruit or flower production. Instead, you want the energy and nutrition of the plant to go towards the desired result, flowers or fruit, so a higher middle number is a more appropriate choice.

To toughen up your plants or lawn for environmental stresses, then you’ll want a fertilizer that promotes the last number, and middle number. A high first number in this case may not be appropriate, because you are not likely to be promoting new lush foliage when at the same time putting plants or turf to bed for the winter. Instead, your goal should be to promote cell structure and strong roots which continue to grow through winter.

Finally, whenever you apply fertilizers, don’t assume that more is better. You can burn plants by over fertilizing, and damage the surrounding soil as well. Instead, err on the side of less is better. If your soil is rich in organic matter, it should have all the nutrition plants need.

About

Joe Lamp'l is the Host and Executive Producer of the award winning PBS television series Growing A Greener World. Off camera, Joe dedicates his time to promoting sustainability through his popular books, Compost Confidential blog, podcast series, and nationally syndicated newspaper columns. Follow Joe on Google+

Comments

  1. Grace says

    My soil sample gives my garden soil a ph of 5.9 with excessive phosphorus and calcium. What type of organic fertilizer would you recommend for a vegtable garden?

    • says

      Grace, use compost if possible. A good neutral organic amendment such as compost will help bring the pH up some and more over time as you continue to amend with more compost. Organic fertilizer by itself will not fix your pH balance or change the excessive phosphorous and calcium levels. That will take time for these levels to mellow. But the one thing to be sure not to do is add any fertilizer, organic or synthetic that has any measurable levels of P especially since it remains in the soil for long periods of time. Adding more fertilizer with this component will only make your levels higher and you don’t want that. Since you already have a high P number, focus on Nitrogen based fertilizers such as blood meal or fish emulsion as an organic fertilizer high in N and low in C.

  2. Robert Clarke says

    I just planted a plumaria tree in Southern Calif from a 10 gal container. Beautiful plant about 7 feet tall. What is the best fertilizer mix for the transplant from the container. There is no foilage, however spring is here and my other plumarias are starting to produce foilage.

    Best
    Bob

    • says

      My first choice is always compost Robert. Mix about 20% by volume into the planting hole. If you want a true fertilizer mix, look for a balanced organic fertilizer blend such as 2-2–2 or an all-purpose synthetic blend of 10-10-10. Mix according to directions on the package.

    • says

      There isn’t a best lawn fertilizer for zoysia. Zoysia is a heavier feeder so it will be happy with any lawn fertilizer. Having said that, I prefer an organic nitrogen based fertilizer such as Milorganite (www.milorganite.com). However, there are many synthetic products on the market simply labeled as lawn fertilizer. They all share the common ingredient of a high nitrogen component to promote lush green growth. Some include a weed killer as well, usually labeled as “weed and feed”. I think we tend to over-fertilize. At my house, lots of organic compost or Milorganite keeps my lawn looking green and healthy.

    • says

      Fertilizers are mixed by the manufacturer to provide an appropriate amount of nutrients based on the specific need. The numbers on the bag or box provide the important information to aid the user in selecting the most appropriate product. When a user combines two different products, the biggest risk that I can see is over-fertilization. The result can be harmful to the plant, the soil, and the environment by way of the excess runoff or leeching into watersheds. Too much fertilizer is much worse than too little.

  3. Bruce says

    My soil sample suggests that I use 8-4-8 but the store doesn’t carry that. What should I use in its place and should I use more or less of the substitute? Many thanks.

    • says

      If you look at this ratio, it’s 2 – 1 -2 (2 parts Nitrogen to 1 part phosphorus, to 2 parts potassium). Since the numbers represent the percentage by volume or each nutrient, you’ll have to add more or less, depending on what you find as an alternative, IF you find something with the same ratio, such as 4-2-4 or 16-8-16. But I have my doubts that you’ll find that. Most stores sell an all purpose mix, such as 10-10-10. I’d try going to the best garden center or nursery in town and see if they have something that can get you close. If you can’t find anything, I’d go with a product that keeps the middle number low and use that as your base. Then if you need to add more nitrogen (the first number), you can usually find products that have that which you could use to supplement the extra that you need.
      You should also call your county extension agent to ask them what they suggest to convert what you are able to find to what they are suggesting. I think they may have some formulas or conversion charts that will help you get you closer to what you need.
      Good luck Bruce. Let us know what you find out.

  4. Dave S says

    Joe, are the weights elemental (wts of N, P, K alone), or do they reflect amounts of the compounds of these elements used to make up the fertilizer? Thanks.

    • says

      The numbers represent the % by weight of each element. So a 40 pound bag of 10-5-10 40 of fertilizer for example would have 4 pounds of nitrogen, 2 pounds of phosphorus, and 4 pounds of potassium. Make sense?

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