What is the Best NPK Ratio for Growing Plants?

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Robert Pavlis

Gardeners have been using fertilizer for many years and everyone recommends a particular ratio for each plant type. The rose society suggests, “6-12-6 is considered a Balanced Rose Food, as it supplies the basic ingredients in proportions beneficial to roses on a continual basis.” For tomatoes Burpee recommends, “such as 10-10-10, or where the middle number (phosphorus) is larger than the first number (nitrogen), such as 2-3-1″. Which is it, a 10-10-10 or a 2-3-1? I guess they don’t really know?

If you look at fertilizer supplies at your local nursery you will find numerous products labeled as rose fertilizer and each one has a different ratio. The same for tomato fertilizer – each manufacturer has their onw formula. Fertilizer for house plants or orchids is just as bad.

Here is a fundamental question:

Do different plants need different fertilizer formulations?

Almost everything you have read certainly suggests this is true, but does the science support this? Or is it more correct to say that most plants use exactly the same ratio of nutrients? After all, the biochemistry of all terrestrial plants is very similar.

In this post I will try to answer the question, what is the best NPK fertilizer ratio for plants?

What is the Best NPK Ratio for Growing Plants?
What is the Best NPK Ratio for Growing Plants?, source: SGS

Understanding Fertilizer Ratios

Make sure you fully understand this section before reading more.

What is the difference between a 1-1-1 fertilizer and a 5-5-5? The ratio of nutrients in both are exactly the same, 1-1-1. The difference is that one product has 5 times as much fertilizer as the other.

Microbe Science for Gardeners Book, by Robert Pavlis

What do the numbers in an NPK formula mean?

Many gardeners and garden writers think these are the % amounts of nitrogen, phosphorus and potassium, but that is wrong. It is the % N, % P2O5 and % K2O. To understand this better see Fertilizer NPK Ratios – What Do They Really Mean.

Plants Use More Than Just NPK

I am sure you know that plants use numerous nutrients in addition to the big three; nitrogen, phosphorus and potassium. The discussion in this post applies to all of these nutrients, but to keep things simple I will only focus on the main three macronutrients, N, P and K.

Best NPK Ratio for Plants

It would be nice for gardeners if there was an easy answer, but there’s not. If you want a quick answer, check out the last section. If you want to understand what you are doing in the garden, read on.

The ‘traditional theory’ is that each type of plant grows best with a different NPK ratio. A tomato and a tulip need a different ratio of nutrients. When you get a soil test done, the lab asks you which plant you are growing and they will make a fertilizer recommendation for that specific plant. Why? Because each crop has different needs. Scan a gardening book or the internet and almost every source will provide a different NPK for each type of plant.

But this is not the only viewpoint. There is a growing belief that most terrestrial plants use the same ratio of nutrients. Consider this, the structure and function of a cell in a rose leaf is virtually identical to that of a tomato leaf or even a maple tree. They all use the same mechanism for photosynthesis, most of the proteins are identical, DNA has the same structure and function. If a rose needs more nitrogen to make carbohydrates, so does the tomato and tree. If the functionality is the same, so are the requirements for nutrients which means all plants need nutrients in the same ratio. I’ll call this the ‘contrarian theory’.

Knowing which of these theories is correct, has a dramatic impact on how we use fertilizer, which in turn affects plant growth and the environment.

These two theories were compared in a paper that was part of a thesis, by Joachim Nachmansohn. It contains many references to scientific studies that support both theories and it is used as the basis of this post. I even contacted the author just before posting this and he confirmed that even with the latest research, his conclusion is still valid. The tables below are from this work.

The Traditional Theory – Each Plant Needs a Different NPK

The ‘traditional theory’ states that each plant grows best with a different NPK ratio and I have called it traditional because it is the most widely accepted viewpoint, it’s well established, and accepted in horticulture and agriculture.

In agriculture, soil test results are used to establish recommended fertilizer applications, using yield curves that have been developed for each crop. It is assumed that each crop needs a different ratio of nutrients to produce the most profitable yield. In horticulture production each plant has a preferred NPK ratio.

Extensive research has resulted in detailed fertilizer ratios for different plants as well as for different growth stages.

Recommended fertilizer ratios based on the historical theory
Recommended fertilizer ratios based on the historical theory. The percent numbers are relative to nitrogen.

In commercial horticulture there is agreement that each plant has a different nutritional need, but there is no agreement on what any one specific plant needs. Each professional develops their own “best” formula. Hydroponics does have some ‘universal” formulations, but each grower has their own ‘best universal’ solution.

Some 'universal" formulations based on historical theory
Some ‘universal” formulations for hydroponics based on historical theory. The percent numbers are relative to nitrogen.

Farming communities also alter so-called best solutions to adapt for local conditions, diseases and past experiences. Nutrient adjustments are also made to modify the nutrient density of crops. The excess nutrients may not be needed by the plant, but they might benefit a consumer. Hydroponic production tends to focus on total ion concentrations, so a reduction of nitrogen near harvest, automatically raises the ratio of other nutrients.

Producers of commercial fertilizers also agree that each type of plant has different nutritional needs and so they produce different formulations for different plants. However, they can’t agree on which ratio is best. The image below shows examples of the ‘best orchid’ fertilizer.

Commercial orchid fertilizers showing inconsistent formulations
Commercial orchid fertilizers showing inconsistent formulations

The Contrarian Theory – Each Plant Needs the Same NPK

This theory applies to most terrestrial plants and suggests that all plants require the same NPK ratio. Fast growing plants require more total nutrients than slower growing plants, but the ratio of nutrients is the same. Think of a tomato plant and a tree, both growing 1 gram of new leaves. The physiology of these leaves is almost identical. The chemical process of producing cells, making cell walls, forming chloroplasts, etc. are virtually the same, so it follows that the nutrients required would also be the same.

Plants might require different levels of nutrients. Trees produce most of their growth quickly in spring, while tomatoes put on a lot of growth in summer. Faster growth needs more nitrogen but it also needs the same relative increase in all of the other nutrients. The ratio of nutrients does not change.

Extensive testing has shown that at a maximum growth rate, plants use the same ratio of nutrients, across a broad range of plant types, and that this is true for all nutrients (limited testing for Ni). If one of these nutrients is provided in excess, the plant will absorb a higher amount of that nutrient, but it does not lead to a higher growth rate. A reduction of any one nutrient, below the minimum ideal level, does lead to reduced growth. Testing was done both in labs and in the field.

Nutrient requirements of plants based on modern theory
Nutrient requirements of plants based on the modern theory. The percent numbers are relative to nitrogen.

This theory does have strong supporters, including Torsten Ingestad, who received the Wallenberg Prize in 1989 (known as the small Nobel Prize), for his work in the field of plant nutrition.

Similar testing has also been done in native environments and they found approximately the same ratios at play for coniferous, deciduous and herbaceous plants.

Percent Nutrient vs NPK

Scientists work in terms of % nutrient, as in the above tables. As noted earlier, the values of P and K, in an NPK formula are not percent nutrients, but % P2O5 and % K2O. When the above values are converted to NPK, we get an NPK of 3-1-2.

When I looked at the NPK ratio in houseplants the majority had a ratio of around 3-1-2. The Michigan State University’s (MSU) Magic Orchid Fertilizer has shown some impressive results across orchid genera, but what many don’t realize is that it was developed as an all-purpose general fertilizer and is used on Easter lilies, poinsettias, annual bedding flowers, perennials, ferns, conifers, cacti, succulents and hundreds of tropical plants. It has an NPK of 4-1-5 for RO water.

Which Theory is Right?

Both theories have significant science backing them and both have been used successfully in production, although the historical theory is more prevalent.

The contrarian theory does have exceptions with the primary one being in fruit production which does benefit from a higher potassium level. The extra potassium does not change the nutrient level in most tissue, but the level does increase in the fruit.

The testing done for the historical theory did not always ensure that nutrients were at the ideal minimum level and higher levels will still produce maximum growing plants . Many of these tests also had other goals. For example, it is accepted that potatoes need a higher potassium level, but in fact they don’t. Higher levels do produce potatoes with fewer brown spots and they are less likely to turn brown on peeling. “higher potassium is a requirement of the grower, not the plant“.

Testing a plants true needs is complicated by several factors. Plants grow at different rates and maximum growth rates are not always the goal. They also have different levels of adaptability. Calcium levels in soil vary a lot based on pH and some plants grow well in high calcium situations, but that does not mean they need high calcium. It might mean they can more easily adapt to higher levels. The nutrients absorbed by plants is not the same thing as it’s needs. Plants will absorb more nutrients than they need as a way of preparing for future shortages. All of these factors make it difficult to establish a plants true nutrient needs unless the experiment is specifically designed for this as a goal and most historical research is not. Their focus is usually on growing good plants with a reasonable nutrient input. These complexities could explain some of the support for the historical theory.

The fact that believers of the historical theory can’t agree on which nutrient level is best, for a particular plant, makes the theory suspect. If plants truly had unique NPK ratios, we should have been able to determine those, for some common plants, so that everyone agrees to the values. That’s not the case.

At present both theories have some validity and neither can be ruled out.

Best NPK for Gardeners

What does all of this mean for gardeners?

Most gardeners grow many things and quite honestly, most of the plants they grow have not been adequately tested. Some vegetable crops and lawns are the exception. That means there are no valid traditional theory guidelines for most plants. Even if the ratios were known, and the experts agreed on the best NKP ratio, how does a gardener handle 100 different types of plants, each needing their own “best fertilizer? They can’t. Using the traditional theory is not really an option for gardeners.

The contrarian theory states that all plants have the same NPK needs. This is really the only theory that gardeners can use and based on the current science, this theory is just as valid as the historical theory. You might as well use it.

The best NPK for your garden, containers and houseplants is a 3-1-2 ratio.

Keep in mind that this needs to be adjusted for existing soil nutrients. Many soils have enough phosphate and so you don’t need to add more. Your soil might also have adequate amounts of potassium. Most plants in containers, including houseplants, are in soilless mixes with no nutrients, so just use a 3-1-2 ratio.

Note that this is a ratio. So you can use a 3-1-2, or a 6-2-4 or 9-3-6. The total amount used depends on growth rate. Higher rates of fertilizer are used for faster  growing plants.

What does this mean for users of organic fertilizer like manure and compost?

Many organic sources have a ratio of about 1-1-1. This means that if you add enough nitrogen for plants, you are overfeeding them on P and K. Not only that, but nitrogen is lost quite quickly from soil. Potassium is lost more slowly and phosphorus much more slowly. If you add enough nitrogen for maximum growth, your phosphate levels will accumulate over time and eventually reach toxic levels. Such organic fertilizers should be combined with high nitrogen sources like bloodmeal or even urea.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

25 thoughts on “What is the Best NPK Ratio for Growing Plants?”

  1. Mr. Pavlis, I cannot thank you enough for your efforts to educate and drive home the point that every belief about plant nutrition is a myth unless it is backed up by evidence of the type that will stand up to scientific scrutiny. Too often people jump erroneously to conclusions when they did something and the plant did better, and there is no knowledge of whether the thing that was done is truly the reason the plant did better. As you pointed out, this is the reason it is essential to have a control, where everything is the same except for the one thing that is believed to be the reason the plant did better.

    When the chemistry of a plant is different for different parts of the growing season, or is different for different parts of a plant, or is different for one species vs. another, these differences are reasons for postulating a hypothesis that the plant will be robust in the nutrient supplement matches the chemistry. But only a hypothesis. As you point out, plants generally take from the soil what they need from the soil. If, for example, the chemistry of a particular part of a particular plant at a particular time of year is special in some sense, say, a greater amount of phosphorous is present, it does not genuinely make sense to conclude that increasing the amount of phosphate in the soil will help the plant. It might, but the plant might be perfectly capable of obtaining that extra amount of phosphorous that it needs, from the soil, all on its own. A controlled test with a number of different test subjects/samples are needed to prove that it is beneficial to the plant to be given an extra helping of phosphorous at that time.

    Plants need to be fertilized when, and only when, a proper soil test reveals that a particular nutrient that the plant is known to need is not available to the plant at the level that the plant is known to need for it to be available. Fertilizer makeup, including micronutrients, should always be tailored to the missing nutrients as determined by a soil test. If the soil is not deficient in nitrogen, adding nitrogen to the soil is just as likely to harm a plant as help. If the soil is not deficient in phosphorous, adding phosphorous to the soil is just as likely to harm a plant as help. If the soil is deficient in potassium, adding potassium to the soil is just as likely to be harmful as to be helpful. Pretty much the only scenario where it would make sense to add general purpose fertilizer is the scenario of hydroponic farming where there isn’t going to be any plant nutrients at all unless fertilizer is supplied. There are no doubt other scenarios that are similar, where there is a reason to be certain that the growth medium for a plant is certain to be deficient in a particular nutrient. But these scenarios are exceptional. In non-exceptional scenarios, fertilizer should be added only when a soil test says that the soil is deficient in one or more specific nutrients.

    Reply
  2. If the ratio remains the same how does one know how often and how much (amt/volume) to apply the fertilizer of the same ratio…in spring the plant is growing fast so often we are told they need more nitrogen, but if they do, then they also need more P&K, right? But later in flower or fruit stage we have been taught then need more P or K….would the N then be in excess? Or does it ‘level out’ and still needs that? So if I have a vegetable, like a pepper, and my soil is 0-0-0; I would add 3-1-2 to the soil? Does that ratio account for the difference between NPK and % P2O5 and % K2O; which would be less than the 3-1-2 indicates? Finally, I start with a pepper seedling. How do I know how often to add 3-1-2? Weekly, monthly, quarterly during various stages of growth and size? Do I need to add more volume (and how much as a percentage of plant size) between a start and a fruiting pepper which is much bigger? I still feel like this is a frustrating guessing game. Amount and frequency of the 3-1-2 fertilizer. And why would you buy 6-2-4 if you cut serving size in 1/2 to be 3-1-2?

    Reply
    • You do not feed plants. You replace the nutrients missing in soil. The goal is to have enough nutrients all of the time – you don’t try to match the growth of the plant.

      The amount you add depends on how low the level is in the soil. So if the soil is low on nitrogen, you only add the nitrogen.

      Ground soil generally has all the nutrients. For containers – assume no nutrients and fertilize weekly.

      “why would you buy 6-2-4 if you cut serving size in 1/2 to be 3-1-2” – the higher nutrient level is usually cheaper on a per nutrient basis.

      Reply
      • o.k. Thanks for the quick reply. I understand, replace what the soil is missing, not feed the plants. Here is the issue; if I test my soil and the soil report shows high phosphorus, near zero nitrogen, low potassium, I need to add nitrogen and potassium. The problem is I don’t know HOW MUCH the plant will need available in the soil over the course of the growing season. It seems to me that 3-1-2 is what they need available in the soil the whole time if I understand what you wrote. And, it would seem from your reply that I would add an appropriate amount at the beginning of the growing cycle to last through all stages rather than adding applications throughout the season as some sites with complicated ratios suggest. But, I still don’t know how much per plant to add…..I don’t farm in acres…I have small garden beds. Do I need 1 cup per plant for the whole cycle or a quart or a tablespoon? I have a 5 lb box of 3-1-2; I have 10 pepper plants, 5 per row. If I have zero nitrogen and zero potassium from the soil test (just for example), how do I determine how much I actually need to add. That is the question. How much do I need to meet those parameters (3-1-2 available) per plant from transplant to harvest.

        Reply
        • “I don’t know HOW MUCH the plant will need available in the soil over the course of the growing season” – along with the soil test, the lab should have provided amounts that you should add.
          A 3-1-2 will be adding too much P. You may need a 3-0-2 – or whatever the lab recommends.

          Reply
  3. I think 12-4-8 is available, which is the same as the 3-1-2. I am wondering if anyone has tested soil sample before and after applying any particular fertilizer. For example, if the soil test reports th availabe N:P:K in ppm and you apply a certain amount of 3-1-2 and water it in and re-test the soil sample, how much would it change the results in terms of NPK in ppm? Just wondering.

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  4. In your book ‘Soil Science for Gardeners’ you write “The amount of NPK used by plants varies by species, but an average value is 7-1-6.” The book was written a year before this article so is 3-1-2 the ratio with more evidence?

    Reply
    • A couple of points.
      1) the 7-1-6 came from another source and was my best number at the time of writing – 2 years ago.
      2) Nobody makes a 7-1-6 fertilizer, but they do make 3-1-2 fertilizer so this latter ratio is more practical.
      3) The two ratios are not that different, and it is just an approximate value – each plant, if studied would be a bit different.
      4) 3-1-2 is easier to remember – which sounds silly, but it is important if I want people to remember the ratio and use it.

      Reply
  5. 3:1:2 Is a good workable conclusion as a basis. However I would have thought that this basis would vary, mostly, with the stage of growth – Spring green shoots (Nitrogen bias), Summer Flowering (P and K Bias), and autumn fruiting (a different P and K bias). An update including reference to plant stage would be very informative on this. Otherwise a really excellent article. Appreciated.

    Reply
    • Do you have some evidence that NPK needs change with growth? There might be some benefit in large crop production where they can fine tune nutrients, but I found no evidence that this would apply to gardening.

      Reply
  6. I use a small amount of general purpose balanced fertilizer on the notion that garden vegetables are fast growing.
    So I add a small amount when planting “should it be needed,” we might say.
    But, I use mostly compost and composted manure – the fertilizer is just a booster.
    It’s probably not needed – but I don’t think a pinch does much harm

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  7. That means alfalfa meal is pretty darn close to being a generally recommended fertilizer. It’s usually 2.5-0.5-2.5 but sometimes I have seen 3-1-2.

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  8. Here in northern Utah, our soil is naturally high in potassium and rarely needs to be supplemented. I have my garden soil tested every three years. The test results almost always tells me to add only nitrogen

    I have two vegetable gardens, one is 7,500 square feet and the other is 6,000 square feet. Twelve years ago, I added 50 pound of 0-54-0 to each garden and tilled it in. Both gardens have tested excess in phosphorus ever since. Since then I’ve only added ammonium nitrate (34-0-0).

    When people ask me what fertilizer to use, my advise is to only use ammonium nitrate until they get their soil tested; then only add what the test report tells them to use.

    Reply
  9. I am again pleased to read your article. I actually read this one with more pleasure than most at this particular time as I’m juggling fertilizer jugs! I have 0-10-10 for Fall application, 3-1-2 as a fav, arguing with my hubby that insists on slow release, and also the 15-30-15, then there’s… Oh you get it! I have a shed full! Thank you again for a a comprehensive article!

    Reply
    • It still applies. If one nutrient is below the ideal level, plants grow less.

      The two theories have different ideas of what is the “ideal level”.

      Reply
  10. Excellent, as always. A “general” recommendation as to how much fertilizer should be applied, how many times in a growing season, per 1000 square ft of a general “garden” (assuming there is a soil test which shows average conditions) would make most gardeners happy and ecstatic! That is where most gardeners are challenged. How to make practical use of the knowledge provided by this article in my garden.

    Reply
    • The answer depends on what you already have in the soil – that is why there is no general answer to your question.

      Reply
  11. What plants need & what fertiliser to use to enable the plant to produce the best outcome for the grower might well be two different things. After all, we’ve been modifying plant behaviour away from its natural state for millennia, thus it’s highly likely these modified plants will require a different nutritional balance from the “wild” parent.
    For example: the aim of a plant is to reproduce but in the case of say, lettuce, the grower’s aim is to maximise leaf growth rather than seeds.
    Do these need a different nutritional balance?
    I’d suggest they do, with nitrogen of greater importance in the latter case.

    Reply
    • It is natural to think that these two plants need different nutrients, but most of the parts of a plant have the same nutrients, because the biochemistry of these various parts are similar. Growing a leaf or a flower stem are essentially the same on a molecular level.

      The one exception that has been identified and mentioned in the post – production of fruit – seeds are fruits.

      Reply

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