The Numbers on Fertilizer Labels, What They Mean

Please follow the label, more is not better

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. kevin says

    Hi Joe,

    What kind of fertilizer should I use for longan, wax jambu and Chico sapote? Also the amount and frequency of fertilizer applications.

    • says

      Hi Kevin. I have to say here that I’m not sure. Although I grew up in S. Florida where these trees thrive, I left before having the pleasure of growing and caring for them myself. I cannot offer personal experience on what fertilizer to use. I do recall however that longan does will with a balanced fertilizer such as 6-6-6, but even that can change as the tree and fruiting cycle matures.

      I would take time to research this individually Kevin and look for a reliable source, such as a university ag. site that you can trust. University of FL would be my first choice. Good luck.

  2. robert hanna says

    Hi Joe!
    I stumbled across you web site and have a question for you! We live in central Texas and have black claylike soil. I’ve had soil test done and the ph is very high. I’ve also had our well water tested and the ph on it is very high as well. If it gets on your car and dries, the only way to get the dried cloudy residue off is with strong vinegar. I suspect that thirty plus years of using it on our garden has put the ph at this level. I posed the question sat ask.com as to how to lower the soil ph. Their recommendation is elemental sulfur. Over the years I’ve applied a number of trailer loads of mushroom compost, my own compost and ‘all purpose’ fertilizer. Now to my question: Should I just be using of commercial fertilizers with only nitrogen in it, avoiding the other two chemicals?

    • says

      Hi Robert. You can lower your soil pH with Aluminum sulfate. It’s most practical to do this in targeted areas of your beds vs. all of your yard or garden. You can find products in the garden centers for this purpose. Espoma makes a product called “Soil Acidifier”. You add some to each planting hole and mix it into the soil.

      To your question, fertilizer will have little impact on adjusting pH either way. As far as type of fertilizer, if you are using it for your lawn only, then an all nitrogen fertilizer would be ok. Otherwise, look for something with more balance across all three numbers, such as 10-10-10. But again, don’t look to the fertilizer as your solution to fixing the soil pH. I suspect your soil is naturally high in pH and the high water pH is adding to the problem.

  3. Lana says

    Question: I live in CA. I bought last year Plumeria about 5 feet tall with no bloom. This year it has nice big green leaves but absolutely no bloom. I don know why it is not blooming I fertilized with 20-20-20 fertilized about a month ago it did not help me.

    • says

      Lana, Assuming your plumeria is planted in full sun or nearly so, your biggest issue here is the need to feed about twice a month during the growing season with a high phosphorus fertilizer. That’s the number in the middle. So for example, instead of 20-20-20, try 0-50-0 or something like that. There are a lot of fertilizer products on the market that are made to promote blooming. You’ll know you have such a fertilizer because it will have some such name or especially when you see a high middle number compared to the others.

      And don’t overwater. Add about an inch a week. Stems can rot in soil that is too wet. Good luck.

  4. Joe says

    question : I live in SW Florida and have three fairly large, fast growing mango trees that I planted four years ago. A Glenn, a Valencia Pride and a Kent, which is my favorite mango, taste wise. Only sporatic, small harvests from the Glenn and Valencia Pride till this year. Both trees exploded with 40 or so fruits each but the Kent continues to be totally fruitless for the third year in a row, although it looks as healthy as the other two,..lots of brabches & leaves, just no fruit..
    A friend told me I should apply a 0 – 0 – 50 fertilizer to the Kent in the fall, that should bring it around to joining in the fruit fest next summer. What do you say ?

    • says

      Joe, I don’t know how this 0-0-50 idea would generate a fruit fest next summer. Potassium is what is being suggested here. It’s not considered a nutrient to enhance fruit production. However, 0-50-0 is often used by commercial growers and such to promote flower production (which for mango trees is the precursor to fruit production). Could your friend have perhaps confused the order? I can see how 0-50-0 “might” help, but not 0-0-50.
      I’m wondering if you just give it more time. Perhaps the Kent may just need a little more maturity before it too joins the ranks as a heavy fruiter. You might also want to check with your county extension service there. They may know more about the specifics of this variety.
      Having grown up in Miami, I loved my mango trees! The most popular variety then was ‘Haden’. Delicious and abundant!

  5. jason says

    I had a go I had a good healthy Bermuda yard so I decide to over seed for a winner so I scalp an verdacut my yard.So the seed would germinate (pop) quicker. Once I had the Rye grass established I hit it with 2 application of ammonia nitrate 34-0-0 an it looked great I had to cut it twice a week.lol I was scared to spray primo on it because I’m used to spray and larger areas. Anyway summer I’ll let it burn out slowly an I have not had the time or money to take care of it. Weeds took over I have done sprayed all them out. The Bermuda is trying to take back over its just taken to long for me. So I’ve seeded an sprig it.What fertilizer do you recommend I use I’ve already put 29-0-4 on the hea healthy Bermuda an how many times do you think I should topdress an verdacut to i no i have a strong root system? I have only done growin on golf course greens.

  6. Rich says

    Hi Joe- I live in CT and I have been having an issue with brown spots on my lawn from dog urine. It only seems to happen after I fertilize. It would seem through some research that the spots are caused by high concentrations of nitrogen in my dog’s pee mixing with the nitrogen in the fertilizer. Do you think it would be helpful finding a fertilizer that has a very low N number? FYI- the lawn is Kentucky bluegrass. Thanks!

    • says

      Rich, I think backing off on the nitrogen number in your fertilizer might help with the burning, but you need that to promote the deep green color you are seeking. However, you only need to fertilize a couple times of year in my opinion. Once in spring and again in fall. I think the main problem is the urine. Try watering the area as soon after as possible with a hose and spray nozzle. the more you can do to dilute the concentration right after it happens the better.

  7. G.L. Carlson says

    OK. So I go to the store. And a see a 50 lb. bag of granular 21-0-0. Meaning that its just 21% nitrogen. So with those double 0′s, What is the remaining 79%? It can’t be that much micro-nutrients, (or else it wouldn’t be micro!) So after the 21, is the rest inert/filler?

  8. Rachael says

    Hello,
    I live in. San Diego California. My question is, is it ok to change fertilizers in the middle of the growing season? I used 10-1-1 and realized I needed 10-10-10. My plants where growing great until I used the 10-1-1. Now the bottom leaves are turning yellow quickly on tomato, tomatillo plants, help!!! Any advice will be helpful.

    • says

      Hi Rachael. I don’t think the yellowing on your plant leaves is because of your fertilization. This is a common condition of tomatoes and tomatillos, starting with the bottom leaves. It is fine to change fertilization when you’re ready to 10-10-10 but don’t bet all your chips on the fertilizer being the solution to all your problems. In my organic garden, I don’t use any synthetic fertilizer, just compost and some liquid fish emulsion a couple times a season.
      Instead, make sure you mulch the soil around your plants to protect them from diseases that reside in the soil. It will also help hold moisture in and keep weeds down. For now, cut off the leaves and stems / branches that show signs of yellowing or disease. You should be fine. Good luck.

  9. Bill Fibelkorn says

    What is the best fertilizer for Doc Martin Lima Beans? My garden is about 50′ x 50′ . Every year, previous to planting, I haul in horse manure. This manure is at least 1 year old before it is applied, so, it should be composted. I spread out about 2″ of manure all over the garden and till it in so everything is about 7-8 inches deep. Every year it seems we are eating lima beans way before everyone else in the area. My question is if there is a good fertilizer blend after the beans have leafed out to help out the compost? Thanks Bill

    • says

      Wow Bill. Sounds like what you are doing is all you need to do. As they say, if it ain’t broke, don’t fix it. I don’t see a need to amend the soil with anything else to help out the compost. Just be sure to continue to source your horse manure from the same supplier if possible. You may be aware that a lot of horse manure these days is loaded with a persistent herbicide that can last for a couple years in your garden, even after composting! Take if from me, I know all too well. If you want to know more, google “killer compost” or use that term on our website. I’ve written about it and documented the damage tainted horse manure had on my own garden.

  10. Lindsay says

    I live in Dallas, TX and have a potted endless summer hydragena on my porch. I have now learned a 10-10-10 fertilizer would be good to apply in May & July. Is that correct? Using a 24–8-16 (which is what I have) does not sound like a good idea. Again, is that correct? I’d like to get mine looking greener and flourish as much as it can in the pot. I think I’ve done ok with any pruning. I hope! Thank you in advance.

    • says

      Lindsay, a balanced fertilizer is what I would go with based on the above choices. In fact, the 24-8-16 listed above is not a good idea. Ideally the middle number for hydrangeas is the most important and should be no less the first number. The 10-10-10 will be ideal. Good luck.

  11. Roxane Weymouth says

    I live near a lake and I’ve heard that I should put a fertilizer on my lawn that has 0 for the second number. Is that true, and if so, why?

    • says

      The second number represents phosphorus. The chemical can promote excess algae growth in waterways. This excess growth ultimately deprives aquatic life of necessary oxygen as the algae breaks down and decomposes. This throws off fragile ecosystems that depend on healthy watersheds that are ecologically in balance. Nitrogen (the first number in fertilizer mixtures) also contributes to this problem. What isn’t absorbed immediately by your lawn and plants often is washed into the closest watershed.

  12. kay says

    To: Joe Lamp’l— My question is–I have endless summer Hydrangeas and would like the appropriate time to prune them?

    Also I have fed them last week 3rd week in April with 10-10-10—according to your site a number high in the middle (phosphorus) is ideal for flowering and blossoms.

    I would very appreciative for a answer on the pruning.

    • says

      Kay, Endless Summer hydrangeas bloom on “old wood” so prune them right after flowering as the blooms fade. To do it right, take each branch and cut it back by 1/3 or 1/2 to about 1/4 inch above a set of dormant leaf buds. While you’re pruning, go ahead and cut back to the ground any obviously old, dead wood.

      • Troobeesez says

        Um. Actually Endless Summer blooms on new growth, hence the “endless” blooms thru summer.
        Traditional old fashioned hydrangeas need to put on new growth this year, as that growth will produce next year’s blooms. Old fashioned hydrangeas should be dead-headed back to a bud (actually will be two new tips that grow from that point), and the same result is achieved when you cut yourself a big boquet for the house! The more growing tips, the more opportunity for flowers; not every tip will produce one every year, but those that don’t probably will the next year. Remember the Rule of Thumb: (old fashion hydrangeas) “Flowers appear on One Year Old Growth”– which really means, “at least one year old”… those non-blooming tips this year will probably be your first flowers next year.

        With Endless Summer, cutting flowers for your home more often, will get you additional blooms, rather like cut-and-come-again Zinnias. Remember to fertilize at the beginning of the growing season, and put something like True Blue around the plants to make blue and purple blooms from pink and reddish ones. Mulch with acid compost (azalea/rhodo mix), to increase acidity for blue blooms. The acidifying should be done in fall, so it is present the moment the plant thinks of making a flower, won’t work as well if the flower is already on its way (you can’t see them yet).

        Mind you, I live in California (I’m sorry!), so consult local nursery if you live in c-c-cold area. Martha Stewart website might have info on how to winter over your hydrangeas. Worth noting: my DH hacked back a rather scraggly ES a couple years ago (potted) and that thing is currently a Bloomin’ Fool! (as they say) I was sure it was a gonner!

        Good luck!

  13. Donna says

    We accidentally put general purpose fertilizer stakes meant for trees and shrubs around our Sylvester palm trees and Pindo palm trees. These tree/shrub fertilizer stakes are 8-10-10 with FE, MN and Sulfur. Will this harm the Sylvester or Pindo palms? Should we dig up the stakes? The palm stakes we meant to use are 6-0-6 and contains other nutrients to protect from the deficiencies of manganese, magnesium, potassium and iron.

    Conditions: We live on the northeast coast of Florida about a mile from the ocean so its a fairly humid area. Aside from some winter freeze damage this year on the Sylvester lower fronds, the palms appear healthy. The soil is great for growing most anything (except tomatoes), and the lawn around the trees is regularly treated by a lawn service. (fertilizer and insecticides)

    Any advice you can provide is greatly appreciated!

  14. David says

    I love your “Up, down and all around” description of what those numbers represent and their respective affect is on plants; in my case, it’s a tomato deal!

    I purchased Miracle-Gro’s ‘Moisture Control Potting Mix’ which (supposedly) contains 6 months worth of plant food. I’m growing a single Better Boy tomato plant which should be flowering if not already producing fruit in early June if all goes well. Since this plant will likely see a substantial loss of temperature and daylight in less than 6 months, would that get me by or would I be safe in using a 5-10-10 (which I’ve already have) to supplement the current mix, say in a couple months or so?

    Thanks!

    • says

      Your Miracle-gro is a slow release fertilizer so I don’t think you need to supplement it. I don’t think you’ll hurt anything though if you add a little 5-10-10 along the way, especially if you’re growing your tomato in a a container. The nutrients will leach faster in them than in the ground.

  15. says

    I live in central NC and have or had A very nice centipede lawn, late last season I and several of my neighbors got what I will describe as brown spot, large round circles of brown and it seems to be spreading. Any suggestion for a treatment? Also I have been using Scot weed& feed.
    Thanks for any help
    Phil

  16. Moe says

    Hi,
    My soil is high in Nitrogen and very low in phosphorus and potash per soil analysis. Ph is very good at 7.1, what kind of fertizer I should use which is very low in nitrogen but high in P&P

    Thanks
    Moe

    • says

      What you want is a fertilizer product that has a low fist number and higher middle and last number, such as 5-10-10. I’d do a soil test and look at what the lab suggests. They should provide the ideal ratio. You may not be able to find exactly what they are suggesting but getting close to it is ok too.

  17. Larry says

    How do I determine if 30-0-4 or 19-0-7 crabgrass preventer fertilizer is the one I should get? I’ve seen two brands that people recommend but how so I determine which is best for me?

  18. 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.

  19. 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.

  20. 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.

  21. 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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