Fertilizer – Selecting The Right NPK Ratio
We eat 3 times a day and the food we eat provides all of the nutrients we need to live. The chemicals in food are used by our bodies to grow and maintain our body parts. The protein you eat helps build muscles. Carbohydrates provide energy that is used for all kinds of things such as moving, breathing, thinking and eating more food.
Most of the food we eat is in the form of large molecules such as protein, carbohydrates, DNA, and fats. Our digestive system takes these large molecules and breaks them down into smaller molecules like simple sugars and nitrates. Our bodies then use these small molecules as building blocks to build new large molecules. Our bodies are actually a very efficient recycling plant. We take food in, break it down into basic building blocks, and then use these building blocks to create complex body parts.
Plants differ from humans and other animals in that they do not have a digestive system. Therefore, they are not able to break down large molecules. Plants must start with small molecules like nitrate and phosphate and build the large molecules they need.
Plants Use Fertilizer to Build Large Molecules
Understanding NPK ratio is fundamental for anyone looking to optimize plant growth, and thorough fertilizer analysis (N-P-K) can guide you in selecting the right nutrients for your garden. Many experts recommend exploring 3-1-2 ratio fertilizer brands, as they offer a balanced blend that promotes healthy foliage and robust root development. By integrating these insights into your fertilization strategy, you can ensure that your plants receive the precise nutrients they need to thrive while maintaining soil vitality.
Plants take small molecules such as carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium and dozens of other minor nutrients, and use them to build large molecules such as sugars, carbohydrates, oils, protein, and DNA. These large molecules are used for everything that happens in the plant. Carbohydrates are used to build cell walls, which in trees, eventually turn into wood. Enzymes are proteins that make all of the chemical reactions in a plant work. Sugars and carbohydrates are the energy source that allows the plant to grow. The production of flowers and fruit require many different types of large molecules and all of these are made by the plant using the small molecules we call nutrients.
The figure below shows a list of some of the large molecules found in plants along with the nutrients used to make the molecules. All of the molecules contain carbon, hydrogen and oxygen. Nitrogen and phosphorus are also common. Other nutrients are used less often, and in smaller amounts, but they are critical for building the large molecules.
If a single nutrient is missing the plant will not be able to produce all of the large molecules it needs, and plant growth will slow down or even stop. A gardener’s job is to make sure that plants always have access to the nutrients they need. That seems like a daunting task, but it is easier than you think. There are two important concepts to understand:
Plants need much more than just the nitrogen, phosphorous and potassium found on fertilizer labels.
They need to eat all of the time—not just once or twice a year when you fertilize.
What is NPK?
It is time to feed your plants and you go to the store to pick up some plant food which is called fertilizer. Most packages of fertilizer show the letters NPK followed by some numbers, for example: NPK 10-5-5. NPK stands for nitrogen, phosphorous and potassium which are three of the most important nutrients required by plants. The numbers following NPK are the percent amounts of each nutrient. An NPK value of 10-5-5 means that the fertilizer contains 10% nitrogen, 5% phosphorus and 5% potassium.
NPK is the common way to describe fertilizer in North America but some countries use an NPKS value where the S stands for the amount of sulphur. Sulphur can be as important as the other three nutrients.
For my techi readers: The above statement “10-5-5 means that the fertilizer contains 10% nitrogen, 5% phosphorus and 5% potassium” is not totally correct. For a detailed explanation see my Garden Myths blog post called Fertilizer NPK Ratios – What Do They Really Mean?
The letters N, P and K are the symbols used by chemists as a short hand to describe the elements. N is used for nitrogen and P for phosphorus. The letter K is used for potassium and stands for kalium, the original Latin name for potassium. If you have trouble remembering whether P is for phosphorus or potassium, remember that the three nutrients are listed in alphabetical order. Phosphorus comes before potassium alphabetically and so the last letter in the list, K, is short for potassium.
A bag of plant food that contains 10% nitrogen, 5% phosphorus and 5% potassium, also has 80% of some other material, but you can ignore it since it has no effect on your plants. If the fertilizer is in a dry form like lawn fertilizer the 80% may be small stones or other dry inert material. If the fertilizer is in liquid form the extra material is water.
The History of NPK Ratios
To understand fertilizer it is worth while looking at the history of the NPK ratios. How did they develop? Who decided that a formula of 10-5-5 was the right formula? Is it the right formula for every plant?
Quite a few years ago a company providing fertilizer decided to market the concept of a balanced fertilizer and decided, arbitrarily, that a 10-10-10 formula would work well. This was a marketing tactic that was not based on any kind of science. Customers bought into the idea and so you now see a lot of references using the term balanced fertilizer and recommending a 10-10-10 formulation. On the surface it seems to make sense. Plants need nitrogen, phosphorus and potassium so why not give it to them in equal amounts.
To be competitive, other companies decided that they would start promoting a fertilizer that was based on the actual nutrients required by plants. Scientists took various plants and analyzed their content. For example, lawn grass is high in nitrogen relative to P and K, and the company developed an 18-10-10 fertilizer for grass. A new marketing approach was born. Make fertilizer specific to the needs of the plant.
Over time, more research showed that plants used a lot of phosphorous to make blooms and roots, and so manufacturers developed bloom boosters and starter fertilizers with formulas like 10-52-10. This idea also seemed to make sense. Give plants the fertilizer they need, at the time they need it. Starter fertilizer is used when transplanting a new plant so that new roots grow quickly. Once established, a plant could be given a fertilizer higher in nitrogen since the plant is now focused on growing new leaves. At bloom time go back to a 10-52-10 to give it another phosphorous boost.
These advances in fertilizer development resulted in an avalanche of different fertilizer formulations and gardening experts were eager to buy into the idea. They started recommending the right fertilizer for each type of plant and for various times of year. These recommendations are still popular in today’s books and websites, and they are extremely confusing.
Don’t worry – I will make fertilizer simple for you in this and the next couple of posts.
How Does a 10-10-10 Fertilizer Compare to a 5-5-5 Fertilizer?
This is a very common question. Should I use a 10-10-10 fertilizer or a 5-5-5? What is the difference? The answer is simple. It doesn’t matter which you use because they provide the same relative amount of nutrients. It is however important that you use the correct amount of fertilizer.
When dealing with fertilizer there are two things that are important: the actual amount (the weight) and the ratio of nutrients.
The ratio is the relative amount of each nutrient. In the two examples above, there is an equal amount of each nutrient and so the ratio is 1:1:1 for both of them. In comparison, an NPK value of 20-10-10 has twice as much nitrogen as phosphorus or potassium and so the ratio is 2:1:1.
A 10-10-10 fertilizer has 10% of each nutrient and a 5-5-5 has 5% of each nutrient so a bag of 10-10-10 contains twice as much fertilizer as the same sized bag of 5-5-5. The 10-10-10 is more concentrated, but both have the same ratio.
Which one is best for your garden? It doesn’t matter since they have the same ratio. If you need to add equal amounts of N, P and K, either one works just as well. However, you will have to use twice as much of the 5-5-5 to provide the same level of nutrients as the 10-10-10.
The most important thing when buying fertilizer is to buy the correct ratio so that you get the correct relative amounts of nutrients. In general, a fertilizer with higher numbers is cheaper. Many of the liquid fertilizers on the market are very dilute, in the range of 1-1-1, and they are also some of the most expensive fertilizers you can buy. From a price point of view, always buy the one with the higher numbers, provided it has the correct ratio.
What is the Correct NPK Ratio For My Plant?
Fertilizer is now available in many different NPK ratios and knowing which one to use can be very confusing. As you begin to understand fertilizers and plants you will start to realize that the answer is actually very simple. It is however very instructive to look at the question in more detail.
A million web sites and thousands of books give you recommendations for fertilizer. Lawns need one formula, tomatoes need a different one. Tress and perennials are different again. It all gets very confusing and it is all bad advice.
Statements such as the following are always wrong:
Use 5-10-5 fertilizer for tulips.
Use 34-10-10 on grass in spring and a 15-5-5 in fall.
Use 5-7-3 for vegetables.
Let me repeat the last statement—the above advice is always wrong. The advice is wrong for several reasons.
Tulips might in fact need more phosphorus—the middle number—but it is just as likely that this advice is based on old myths and not on actual science. At best these recommendations are based on plant tissue analysis and not on nutrient levels in your soil.
Link to Lvwang Ecological Fertilizer
Additional resources:Key Questions to Ask When Ordering Aventics Cylinder
Hardware - GNU Radio WikiThe main reason the above recommendations are wrong is because you don’t add fertilizer to plants – you add it to soil. Read on and this will make sense shortly.
You Don’t Add Fertilizer to Plants
The idea that we feed the plant what they need seems to make perfect sense but it ignores one very important point. Plants get their food from the soil. You don’t add fertilizer to plants—you add it to soil. This is an extremely important concept that is not well understood and is best explained by a simple example. Assume that your soil is naturally very high in phosphorous. The fertilizer you use at transplant time, or for tulips, or to get more blooms does not need a high amount of phosphorous because your soil already has more than your plants can use. The fertilizer you add to the garden should be a supplement to what is lacking in your soil.
If your soil has high phosphorus levels, any fertilizer you add – for any type of plant – should contain no phosphorus because you already have too much.
Plants absorb the nutrients they need from the soil. If grass needs more nitrogen, it takes more nitrogen from the soil than a plant that needs less nitrogen. If a plant is ready to make flowers and it needs more phosphorus, it takes more phosphorus from the soil.
Matching fertilizer to what a plant needs does not make sense. Instead you should match the fertilizer to what the soil needs. If your soil is low in nitrogen and high in phosphorus you should use something like a 20-0-5 fertilizer for all of your plants no matter what type they are.
The author that recommends a 5-10-5 fertilizer for tulips has no knowledge about your soil. So they can’t make correct fertilizer recommendations for your soil.
Consider this example. For years the common lawn fertilizer recommended for North America has been some thing like 34-10-10, high nitrogen and fairly high P and K. Over the years we learned to understand two facts. First, most urban garden soil in North America has lots of phosphorus – it does not need any more to grow grass. Secondly, the excess phosphorus ends up polluting rivers and lakes. So now many states have banned phosphorus in lawn fertilizer. In Ontario, I noticed 2-3 years ago, that many brands have now eliminated P from lawn fertilizer.
But people still recommend adding extra phosphorus for their bulbs! That is dumb. If our soil has lots of phosphorus for grass, then it also has lots of phosphorus for bulbs. I grow over 200 different kinds of spring bulbs and I have never added phosphorus – they grow and flower just fine.
NPK Ratio for Vegetables - Fertilizer 101
What is Fertilizer and Why Do I Need It?
Plants need nutrients to grow. The major nutrients they need are carbon, hydrogen, oxygen, nitrogen, phosphorous and potassium. They also need magnesium, sulfur and calcium. Without all of these nutrients, plants may grow slowly or fail to grow at all.
Plants need nutrients to grow. The major nutrients they need are carbon, hydrogen, oxygen, nitrogen, phosphorous and potassium. They also need magnesium, sulfur and calcium. Without all of these nutrients, plants may grow slowly or fail to grow at all.
Plants absorb the nutrients they need from the soil. Over time, the soil will have less nutrients than before. Gardeners add fertilizer to the soil to replenish the soil with the nutrients plants need. A soil test will show you what nutrients your soil needs.
What Is NPK and Why Is It Important?
Most fertilizers or plant foods list the NPK fertilizer ratio on the bag. NPK stands for nitrogen, phosphorous and potassium, the three major macronutrients plants need. Nitrogen is mostly responsible for plant leaf growth; phosphorous is needed for the growth of plant roots, fruits and flowers; potassium is responsible for the plant's overall functioning. At different times in the plant's growth cycle, it may need more of one macronutrient than another.
Most fertilizers or plant foods list the NPK fertilizer ratio on the bag. NPK stands for nitrogen, phosphorous and potassium, the three major macronutrients plants need. Nitrogen is mostly responsible for plant leaf growth; phosphorous is needed for the growth of plant roots, fruits and flowers; potassium is responsible for the plant's overall functioning. At different times in the plant's growth cycle, it may need more of one macronutrient than another.
When looking at NPK fertilizer ratios, it may be listed as 6-3-3. This means that it has six parts nitrogen, three parts phosphorous and three parts potassium. Often the NPK ratio for vegetables like corn and lawns have higher percentages of nitrogen. Other NPK fertilizer ratios may be listed as 10-10-10. This mean the fertilizer has equal parts nitrogen, phosphorous and potassium. These are sometimes called balanced fertilizers.
What Is the Best NPK Fertilizer Ratio for My Plants?
To determine the best NPK fertilizer ratio for your plants, start with a soil test. This will help you determine what macronutrients are in your soil and what nutrients should be added. For instance, many lawns and gardens have plenty of phosphorous, but may need more nitrogen. If that is the case with your garden, look for fertilizer ratios where the first number, nitrogen, is higher than the middle number, phosphorous. When adding fertilizer to your soil, you should add nutrients that are lacking in the soil. Remember, plants take the nutrients they need from the soil.
Expert Advice
To determine the best NPK fertilizer ratio for your plants, start with a soil test. This will help you determine what macronutrients are in your soil and what nutrients should be added. For instance, many lawns and gardens have plenty of phosphorous, but may need more nitrogen. If that is the case with your garden, look for fertilizer ratios where the first number, nitrogen, is higher than the middle number, phosphorous. When adding fertilizer to your soil, you should add nutrients that are lacking in the soil. Remember, plants take the nutrients they need from the soil.
Q. Twice this year friends have come to me and said, "My tomatoes are all vines and no fruit." I asked what they were fertilizing with (thinking they were applying too much nitrogen), and to my surprise both replied: "10-10-10". I said, "That'll do it." What I don't know is why anyone would think 10-10-10 is fine for their garden—or how to help them fix the problem they've caused. Maybe by next year the crappy chemical fertilizer will be flushed out by rain and winter?,
A. I have mixed feelings about you, Michael. On one hand, you're my new favorite listener for knowing how bogus so-called "balanced" fertilizers like 10-10-10 are. On the other hand, if more people DID know things like that I'd have to go out and find a real job.
Seriously, you're absolutely correct: 10% nitrogen is only appropriate for non-flowering plants like sweet corn and lawns; it's way too much 'N' for plants that flower, like tomatoes,squash, beans,peppers, melons, eggplant, and—oh yeah, flowers! Bogus 'even number' fertilizers like 10-10-10 and 20-20-20 are always composed of concentrated chemical salts; and the super-fast growth they cause makes plants extremely attractive to pests and diseases. And those salts—originally designed to be used as high explosives—ruin the soil, and kill the soil life that keeps plants naturally healthy. And finally, despite their arithmetic rhythm, fertilizer ratios like 10-10-10 are also unbalanced. No plant uses those three nutrients in equal amounts.
A little background: The three numbers (commonly referred to as "N-P-K") that appear on the label of every packaged fertilizer represent the three main plant nutrients: Nitrogen, Phosphorus and Potassium (which is sometimes called 'Potash'). Studies have found that the ideal NPK fertilizer ratio of those nutrients for flowering plants is 3-1-2. (That's 3% Nitrogen, 1% phosphorus & 2% potassium.) So look for that ratio on the label of packaged fertilizers; anything close to a 3-1-2, a 6-2-4 or a 9-3-6 should be ideal. (Beware higher numbers—that's the realm of chemical salts.)
My advice to folks like Michael's friends is to water on the heavy side to wash those salts out of the soil as quickly as possible, feed with compost alone for the rest of the season, and then move to organic fertilizers and/or compost in the future and sin no more!
Q. I've been growing heirloom tomatoes for a few years with reasonable success. But I would like to know how to build on that success. I understand that some fertilizers promote root growth, some promote leaves, and some encourage flowering. I have fish emulsion and a seaweed solution. Which should I use and when to maximize my tomato yield this year?
I'm curious about the difference between bone meal and blood meal. Would I use either for new plantings?
A.Nitrogen—the first number of an NPK rating—grows big plants with lots of leaves. But too much nitrogen, especially combined with a lack of other nutrients, will inhibit flowering and fruiting. The plants that thrive with this nutrient are the non-flowering grasses and grains (i. e. lawns and sweet corn). Blood meal is a high nitrogen fertilizer (it rates a 12-2-0; a very high number for a natural product), as are fish meal (and fish emulsion), horse and poultry manure and corn gluten meal (which also prevents seed germination, making it the only natural springtime weed and feed for lawns).
Phosphorus—the middle number—is best known as the nutrient that produces more flowers and fruits, but it's also essential to strong root growth early in the season. Bone meal (1-11-0) is the organic source that becomes available the fastest. Many growers prefer rock phosphate or colloidal rock phosphate, which release the nutrient slowly, and for a long time after application—three to five years. But that slowness means you should try and apply rock phosphates the season BEFORE you want your blooms boosted, to give it time to get ready to work.
Potassium—the third number—helps plants process all nutrients more efficiently, improves the quality of fruits, and helps plants resist stress. The best single-ingredient source is green sand. Also known as glauconite, this mined mineral formed in prehistoric oceans also contains lots of important trace elements and minerals. It releases its nutrients the slowest of all—over the course of a decade; so, like the rock phosphates, always try to spread green sand in the fall in preparation for the following season.
But it's important to remember that all fertilizers—chemical and organic —rely on soil life to make their nutrients available to plants, and that high levels of organic matter in your soil are vital to the healthy growth of all plants. So all fertilization plans should begin with an inch of high-quality compost applied to the soil (preferably on top as opposed to tilled in). Then as the season progresses, you can give your plants a little boost by adding more compost or using a well-balanced organic fertilizer (remember, the NPK ratio you want to come close to achieving is 3-1-2).
I like to use a liquid fish and seaweed mix for that boost, as these products provide a nice balance of the basic nutrients, and lots of essential trace elements from the seaweed component. Just pour some into a watering can, dilute it as directed and water it into the soil around the root zone. If you prefer granular fertilizers, spread the material evenly over the soil beginning about six inches away from the plant stem and then cover it with some soil or compost to help it get to the plants faste
What is an ideal N-P-K ratio for plant fertilizers?
Researching suitable N-P-K ratios for indoor plants recently, I was reminded that product labelling is not always consistent across the globe. Nitrogen, phosphorus and potassium are three essential elements required by plants to support their growth and it is essential that interior plantscapers understand just how much they are applying to their plants. Deficiency of these elements usually shows up in older leaves, a useful diagnostic tip especially for those interior plantscapers just starting out.
An N-P-K ratio essentially refers to the percentage of nitrogen (N), phosphorus (P) and potassium (K) found in a fertilizer product. American interior plantscaper (retired) and IPA Honorary Member, Kathy Fediw says that most professionals in her country recommend a fertilizer with a 24-8-16 N-P-K ratio for indoor plants (or one with a 3-1-2)1. She also says that a 20-20-20 fertilizer, or a 15-30-15 fertilizer are considered good all-purpose fertilizers and can also be used on indoor plants1.
However, Dr Samuel Stacey, a Technical Manager with ICL Specialty Fertilizer, said
“That those ratios appear to be based on oxide analysis, rather than elemental analysis which is used in Australia. Fertilizer labelling regulations differ here to those used in the United States and the European Union, which means that the N-P-K numbers quoted on labels are different. The American ratios are therefore different to what an Australian grower would see.
The Association of American Plant Food Control Officials’ AAPFCO Product Label Guide states that the grade statement on a fertilizer product in the United States must refer to the percentage of Total Nitrogen (N), Available Phosphate (P2O5) and Soluble Potash (K2O) in the same terms, order, and percentages as in the guaranteed analysis2.
However, here in Australia our fertilizer product labels must include the concentration for all elements in the elemental form (e.g. P, K) and not the oxide form as is the case in some countries (e.g. P2O, K2O). For solid fertilizers, percentage or grams per kilogram on a weight for weight basis as supplied. (e.g. 4.1% w/w 41g/kg w/w). For liquid fertilizers, on a weight for volume basis, as percentage weight for volume, with the method to be stated. (e.g. 4.1% w/v). The concentration may in addition be stated as grams per litre or milligrams per litre in the same manner (e.g. 4.1% w/v and (optionally) 41g/L)3. “
What is a suitable N-P-K ratio for indoor plants and why?
Dr Stacey says that plants normally require higher levels of nitrogen and potassium than phosphorus. His colleague Robert Megier, ICL’s Turf & Ornamental Regional Manager says that an N-P-K ratio of 4-1-3 or 6-1-4 would work well for most indoor plants.
So nitrogen, phosphorus, and potassium are obviously important in maintaining plant health? But what is it they do exactly? And why do symptoms of their deficiency appear in older leaves first?
Speaking with Michael Harding, EcoGro Director and IPA member said:
“Nitrogen is essential in the formation of protein, and protein makes up much of the tissues of most living things. Phosphorus stimulates root growth, helps plants set buds and flowers, and produce seeds. It also helps plants use other nutrients more efficiently and helps turn energy from the sun into usable energy for your plants.
Potassium helps plants grow strong stems and keep growing fast. Plants lacking in potassium do not have enough energy to grow properly grow, their roots are not well formed, and they have weak stems, the edges of older plant leaves appear burned, and potassium deficient plants cannot regulate and use water efficiently, Michael adds.
In addition to nitrogen, phosphorus and potassium, plants require additional elements, including oxygen (0), carbon (C) and hydrogen (H). Elements taken up from the soil or growing media are generally categorised as either macro or micro-nutrients. Macronutrients include nitrogen (N), phosphorus (P), potassium (K), as well as calcium (Ca), magnesium (Mg) and sulphur (S). Micronutrients (or trace elements) include boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), silicon (Si) and zinc (Zn). These elements are required by plants in smaller amounts than macronutrients.
Movement of these macro and micronutrients through a plant occurs through its conductive tissues, i.e. its xylem and phloem. Whilst most movement from the roots occurs in the xylem, lateral movement and redistribution of nutrients in plants is largely via the phloem.
Xylem (blue) transports water and minerals from the roots upwards, whilst the products of photosynthesis are transported from the leaves throughout the plant via phloem (yellow) (Image: Nefronus , CC BY-SA 4.0)
So called phloem-mobile elements are present in soluble forms in plants. These elements include magnesium (Mg), nitrogen (N), phosphorus (P) and potassium (K). Their deficiency symptoms usually first appear in recently matured and older leaves.
Phloem-immobile elements are present in plants in insoluble forms. These elements include calcium (Ca), iron (Fe) and boron (B). Their deficiency symptoms appear in youngest leaves first.
In between, we have those elements with intermediate phloem mobility: copper (Cu), manganese (Mn), molybdenum (Mo), sulphur (S) and zinc (Zn).
So if you are trying to determine why a plant’s health is poorly, look not only at what colour its leaves are compared to its healthy counterpart, but also the location of those leaves on the sickly plant.
Element (macronutrient)FunctionsDeficiency symptomsNitrogen (N)Component of amino acids, proteins, nucleotides, nucleic acids, chlorophylls, and coenzymesGeneral chlorosis, especially in older leaves; in severe cases, leaves become completely yellow and then become tan as they die; some plants exhibit purple coloration due to accumulation of anthocyaninsPhosphorus (P)Component of energy-carrying phosphate compounds (ATP and ADP), nucleic acids several coenzymes, phospholipidsPlants dark green, often accumulating anthocyanins and becoming red or purple; in later stages of growth, stems stunted; older leaves become dark brown and diePotassium (K)Involved in osmosis and ionic balance and in opening and closing of stomata; activator of many enzymesMottled or chlorotic leaves with small spots of necrotic tissue at tips and margins; weak, narrow stems; mostly older leaves affectedSulphur (S)Component of some amino acids and proteins and of coenzyme AYoung leaves with light green veins and interveinal areasMagnesium (Mg)Component of the chlorophyll molecule; activator of many enzymesMottled or chlorotic leaves; may redden; sometimes with necrotic spots; leaf tips and margins turned upward; mostly older leaves affected; stems slenderCalcium (Ca)Component of cell walls; enzyme cofactor; involved in cellular membrane permeability; component of calmodulin, a regulator of membrane and enzyme activitiesShoot and root tips die; young leaves at first hooked, then die back at tips and margins, developing cut-out appearance at these sites
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