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?
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.
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.
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.
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.
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.
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.
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?
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.
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.
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.
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
What about the 14-4-14 ratio?
Plant Food for Space Grows Crops on Earth
https://spinoff.nasa.gov/Spinoff2018/ee_3.html
Method of hydroponically growing plants
https://patents.google.com/patent/US9527778B2
Another interesting issue, does the nutritional needs of plants change under stress?
So this formula has a slightly higher K. There is no data to compare it to other formulas.
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.
I’m reading through this article and the idea of alfalfa meal/pellets being very close to the 3-1-2 ratio also came to mind, too. Maybe bump the N a tad with some blood meal or urea?
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.
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!
What’s happened to Liebig’s Law of the Minimum? – https://en.wikipedia.org/wiki/Liebig%27s_law_of_the_minimum – this is what dominated this kind of discussion when I took plant nutrition.
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”.
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.
The answer depends on what you already have in the soil – that is why there is no general answer to your question.
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.
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.
Thought provoking