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.
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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.
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.
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.
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.
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.
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.
The 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.
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.
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.
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.
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.
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
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.
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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
To balance your growing media pH, select a water-soluble fertilizer that works with your irrigation water.
There are two primary goals to keep in mind when selecting a water-soluble fertilizer for your facility:
1. Maintaining the growing media pH within an acceptable range, and
2. Supplying your plants with a sufficient amount of essential nutrients for good growth and flowering.
The “best” fertilizer to use on your plants is the one that not only supplies nutrients, but also complements the alkalinity and nutrient content of your irrigation water.
When discussing how water-soluble fertilizer affects growing media pH, it is important to understand that water-soluble fertilizer cannot be applied without irrigation water. The best guide when selecting an appropriate water-soluble fertilizer is to balance the proportion of nitrogen in the ammoniacal form (acid) against the irrigation water alkalinity (base). Although other factors affect growing media pH, research has shown that it is the balance between the ammoniacal nitrogen in the fertilizer and water alkalinity that has the greatest effect on growing media pH of long-term crops.
To understand how the alkalinity concentration in the water and the percentage of ammoniacal nitrogen in the fertilizer interact, picture a balance with water alkalinity on one side pushing the pH up, and on the other side, with the ammoniacal nitrogen pushing the pH down.
If either of these factors is out of balance, the growing media pH will be affected. For example, using a fertilizer very high in ammoniacal nitrogen (like 21-7-7 acid) with low alkalinity water (like RO or rain water) is very effective at driving the growing media pH down, because there is nothing to neutralize all the acidic hydrogens (H+) being produced through nitrification or plant uptake.
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Applying the right water soluble fertilizers can work wonders on field crops, in home gardens, and even for houseplants. Just as the fruits, flowers, and vegetables you grow need adequate light and water to thrive, they need essential nutrients, too. Providing macronutrients and micronutrients that plants can quickly and easily access can help you net better results overall. Water soluble fertilizers can boost both crop growth and the quality of produce. In fact, which nutrients you provide—and how and when you provide them—can affect everything from the health and size of cut-flower blossoms to the thickness of your lawn and the taste of fruit and veggies.
To understand what water-soluble fertilizers are and how they work, you first need to understand how plants access nutrients. Plants acquire nitrogen, phosphorus, potassium, and other essential soil-based elements through their roots. But it isn’t until you fully water plants—or they get a good, soaking rain—that soil-based elements become accessible to your plants’ roots. Once watered, your plants’ roots draw in much-needed moisture and the nutrients present in the resulting soil solution.
Typically available in pellets or granules, water-insoluble fertilizers don’t readily dissolve in water. Instead, these “slow-release” dry fertilizers offer nutrients very gradually. Slow-release formulas are worked into the soil near the plants they’re intended to feed. When it rains or when you water your plants, some of the dry fertilizer’s nutrients reach your plants’ roots.
Unlike their slow-release counterparts, water soluble fertilizers easily dissolve in water and allow for the instant uptake of nutrients. Some water soluble fertilizers are commercially available as concentrated liquids. Others are dry preparations. To use, you measure out some of the concentrated liquid or the dry ingredients and mix with water. Then, you simply water with the quick-release fertilizer mixture. Because the included nutrients are already in solution, they’re immediately available to plants.
Of course, the nutrient sources in fertilizer products vary widely. Some ingredients come from natural, organic sources. Others come from artificial, inorganic sources. Although such chemical-based liquid fertilizers often contain higher percentages of essential nutrients, it’s possible to have too much of a good thing.
From a plant’s vantage point, nitrogen synthesized from a nitrate form like ammoniacal nitrogen or calcium nitrate is just as useful as nitrogen derived from natural ingredients like bat guano or blood meal. The same goes for a potassium-containing potash (which is produced from potassium chloride) and potassium derived from natural sources like sea kelp. Nevertheless it’s best to avoid chemical-based liquid fertilizers.
Synthesized from chemical salts, inorganic fertilizers can negatively impact soil health and structure. With their continued use, accumulating sodium residues contribute to higher acid pH levels. This, in turn, repels worms and other beneficial soil-dwellers and can “lock up” your plants’ ability to uptake nutrients. Excess fertilizer salts also draw water away from plant roots—the cause of fertilizer “burn” injuries. In container gardens, excess fertilizer salts can lead to scale formation on the outside of the pots or the top of the soil. Eventually, the soil’s capacity to retain water also declines. Excess nutrients, such as soluble phosphates, can leach into area waterways, contributing to algae blooms and other harmful environmental effects.
Liquid fertilizers derived from natural sources are better for plants and the soil. They generally have a low salt index, meaning they’re less likely to cause fertilizer burn injury, alter soil pH, or disrupt soil microbial activity. Because they’re derived form natural rather than inorganic ingredients, they may also be chloride-free and include bioactive extras like amino acids, enzymes, and beneficial bacteria. These work to feed and support the soil itself.
You can use water soluble fertilizers on new seedlings, in well-established gardens, and everywhere in between. Want to kick-start very young plants? Start with a relatively weak nutrient solution. Want to induce early flowering or early fruit formation? Incorporate phosphorus and micronutrients like zinc and manganese into the fertilizer you apply. As all of your plants grow larger, you can provide a stronger, all-purpose mixture every couple of weeks. Ditto for routine feedings of houseplants and container gardens.
Water soluble fertilizers have many benefits—and some drawbacks. Their uniformity of application is one of their biggest strengths. Plants take in dry, slow-release fertilizers only when water is present in the soil. Unless this fertilizer remains evenly distributed, you may end up with some nutrient-rich pockets and other nutrient-poor areas. Plants near heavier concentrations of fertilizer salts are at greater risk of burning.
Conversely, water soluble nutrients are immediately available to plants wherever they’ve been applied. They are fast-acting but also short-lived. As a result, water soluble fertilizers are less likely to injure your plants, but they must be reapplied more frequently. Also, some of these cost a little more than dry, slow-release products. Still, being able to deliver specific nutrients exactly where and when you want to just might be worth it.
You can tell how much of each of the “big three” essential nutrients a liquid fertilizer contains by checking its label for three numbers, separated by hyphens. This is known as the NPK ratio. (Nitrogen, phosphorus, and potassium are represented as N, P, and K, respectively.) Say a product label shows a ratio of 3-2-6. That means the product contains 3% nitrogen, 2% phosphorus, and 6% potassium by weight. Just what do nitrogen, phosphorus, and potassium do for plants?
Nitrogen (N)—important for green, leafy growth and new shoot development
Phosphorus (P)—stimulates blossoming and fruiting; aids in new root development and prompts root growth
Potassium (K)—essential for critical functions like plant rooting and cell wall formation
Other important nutrients include but aren’t limited to:
Calcium (Ca)—plant building block bolsters strength of cell wall; neutralizes certain plant acids; aids in protein manufacture
Magnesium (Mg)—important chlorophyll component; helps make plant fats, starches, and more
Zinc (Zn)—needed for chlorophyll production as well as certain plant enzymes and hormones; helps plants set seed
Boron (B)—regulates cell growth and metabolic processes
Molybdenum (Mo)—essential for plants’ nitrogen uptake and use; helps plants make proteins
Manganese (Mn)—another chlorophyll component; assists with uptake of other nutrients
Liquid fertilizers may contain different combinations of these additional elements. Check your product’s ingredients list for references to secondary nutrients, micronutrients, or trace elements.
You can buy organic water soluble fertilizers in either dry or liquid forms. Often available as a powder or in pellets, dry preparations are intended to be measured out, mixed with water, and then applied to plants. Similarly, the concentrated liquid formulas also require some measuring, dilution into water, and mixing before use. Depending on the age and size of your plants, you may want to adjust your fertilizer-to-water ratio. Carefully read the manufacturer’s recommendation on the product label before you mix. Some commonly used organic nutrient sources follow.
Liquid kelp and seaweed preparations can really pack a punch since they often contain some nitrogen, potassium, and many of the harder-to-find trace elements all in one place. Because it prompts roots to grow, some gardeners use very diluted liquid kelp/seaweed solutions as pre-germination seed soaks. Whether you’re growing cherries or cherry tomatoes, the nutrients found in most liquid kelp preparations also can improve the quality of the produce. When applied during fruit development stages, liquid kelp/seaweed may increase fruit size and boost sugar content.
Most fish-based fertilizers are derived from whole fish or fish parts which have been ground up and liquified. These products are usually richest in phosphorus and nitrogen and also may contain some trace elements, beneficial enzymes, and amino acids. Because they may be lacking, some manufacturers add extra sources of potassium to their fish-based preparations.
Although it’s possible to make your own fertilizer teas from finished compost or vermicompost, you won’t necessarily know which nutrients and microorganisms are in your DIY concoctions. Commercial manufacturers offer at least some specifics about the myriad ingredients in liquid compost/earthworm castings tea. These products often contain important trace elements, beneficial bacteria, and fungi which can improve soil structure and health.
Created from a mixture of different nutrient-rich ingredients, combination liquid fertilizers usually contain staples like fish or bone meal, animal manures, and seaweed or kelp extracts. Sometimes marketed as fertilizer supplements, there’s a combination liquid fertilizer for nearly every use case. For example, calcium deficiencies contribute to flower drop, fruit dropping, and tomato blossom end rot, and calcium-rich combination liquid fertilizers exist to mitigate (or, better still, to prevent!) these issues.
You can apply water soluble nutrients with a simple watering can or even an elaborate irrigation system. Want to use water soluble fertilizers with drip irrigation? Just be sure that your chosen fertilizer is thoroughly mixed first. (You might also want to strain it to remove any potentially clogging particulates.)
Besides directing liquid nutrients to your plants’ root zones, you can also apply them to plant leaves as a foliar spray. For this use, check your product label for foliar application instructions and dilute water soluble fertilizers accordingly. Then, decant your mixture into a clean spray bottle. Foliar feeding is particularly helpful if you need to take quick, corrective action. (To avoid burning your plants, lightly mist foliage when the ambient heat and humidity are low—early mornings or evenings are best.)
Water soluble fertilizers provide a greater degree of control in terms of the strength of the nutrient solution you provide and the frequency with which you provide it. What’s more, because organic water soluble fertilizers also contain many essential micronutrients and even beneficial microorganisms, they feed your plants and the soil. That puts you in a better position to combat common issues like insect pests, plant pathogens, and adverse weather events like record heat and drought. Best of all, no matter what you grow, you can choose liquid formulas containing combinations of essential nutrients to meet any special requirement of crops.
Ah, the memories of those science classes! Remember sitting there, soaking up knowledge about photosynthesis? It was like learning the ultimate life hack of the plant world – turning sunlight into food.
But wait, did your teacher delve into the world of fertilizers? You know, those nutrient boosters that often get mistaken for plant grub? The official definition is “any material, organic or inorganic, natural or synthetic, which supplies one or more of the chemical elements required for the plant growth.”
Okay, that’s a little formal. Simply put, fertilizer is not plant food (they make their food!); its plant nutrients – like sulfur, calcium, nitrogen, etc. – required for the plant to grow, develop, and reproduce. There are different sources of nutrients that plants can use. It’s sometimes hard to know what to apply since there are so many options. Let’s go a bit deeper.
Types of Fertilizer
There are two main types of fertilizers: organic and inorganic. Organic ones are naturally derived, meaning they aren’t man-made. Inorganic fertilizers, on the other hand, are the factory-made, water-soluble counterparts.
While both have their merits, it’s important not to overdo the use of either type. Applying too much, or applying them when plants aren’t actively growing, can spell trouble for both your plants and the environment. Think nutrient runoff wreaking havoc on water quality and contributing to pesky problems like algal blooms. 
Regardless of the type of fertilizer, they can do damage to plants and the environment. Luckily, the same basic principles of selection and application apply to both organic and inorganic, or synthetic, fertilizers.
In the wild, plants get their nutrients from natural cycles. In developed areas, we use fertilizers to mimic the nutrients these cycles provide to wild plants. Developed, in this instance, refers to lawns, landscapes, and potted plants or raised beds. Fertilizers should be applied when the essential nutrients for plant growth are depleted, which is common in “developed” situations.
Fertilizers are similar in concept to a nutrient cocktail that plants use to support their growth and survival. Think of it as their vitamin regimen. The main players in this nutrient mix are nitrogen, phosphorus, and potassium – the trio responsible for keeping plants thriving. These are often referred to a macro nutrient – not because they are the most important, but they are typically required in the highest amounts when compared to the micronutrients (think calcium, iron, sulfur, etc.). Nitrogen (N) beefs up those leafy greens, phosphorus (P) digs deep for root health and flowering, while potassium (K) is like a plant’s immune system booster. Some fertilizers also contain micronutrients to support plant growth as well. Match those numbers to your plant’s needs and voila!
The Impacts of Fertilizer
But before you go wild with the fertilizer, hold your gardening gloves! Timing is everything. Just like sticking to a skincare routine, plants need a fertilizing schedule to keep them glowing. Applying fertilizers appropriately will give you the best version of your garden. Overapplication is financially wasteful and can damage your plants. The extra nutrients get washed away and enter water sources, which directly impacts public health and aquatic ecosystems, and not in a good way.
Picture this: Florida, a sunshine paradise, plagued by issues like red-tide and blue-green algae. Sound familiar? Yep, blame it on eutrophication – the fallout from excess nutrients in the water, courtesy of nutrient runoff. When aquatic plants overgrow from the nutrients in the water due to fertilizers, oxygen levels plummet, leaving marine life gasping for breath. Hence the harmful algal blooms.
Fertilizers are regulated materials that contain plant nutrients. On the label, there must be a nutrient analysis. This analysis is the representation of the nutrients as a percentage of weight. These materials are listed on the nutrient analysis, as well as any additional nutrients. So, it is important to be able to read and understand the label to choose the right one. Let’s learn how:
Nutrient Analysis – you can identify this by looking for the 3 numbers on the label. These numbers give the percentage of the three main nutrients, nitrogen (N), phosphorus (P), and potassium (K), by weight. So, think N-P-K when you see these 3 numbers. For example, a 100lb bag has 12-3-4, this means that there are 12lbs of N, 3 lbs. of P, and 4 lbs. of K.
Guaranteed analysis: this is the analysis by weight of each element present in the fertilizer, warranted by the manufacturer.
Total Nitrogen: the total amount of nitrogen expressed as a percentage.
Additional Macro-nutrients: additional nutrients displayed as a percentage by weight of the total amount of fertilizer.
Additional Micro-nutrients: micronutrients displayed as percentage by weight of the total amount of fertilizer. In Florida, our sandy soil may tend to have low levels of micronutrients, but your soil content should be determined by a soil test. Excess amounts of these nutrients can damage and kill plants.
Nutrient Sources: details where the fertilizer was derived from.
How do you know which fertilizer to pick, you ask? Start with a soil test. It’s like giving your plants a doctor’s check-up.
You can easily take a soil sample at home with a kit, or that can be sent to the UF/IFAS soil laboratory, where inexpensive testing is available to the public. Or you can pick up an online based soil test kit at your local Extension Office. Either kit is simple, provides specific instructions, and provides recommendations based on the specific plant and situation you note on the form. Based on the results, choose a fertilizer that caters to the specific nutritional needs of your plants.
Choosing the right fertilizer depends on a lot of factors. The type of plant, soil, and time of year are just a few considerations. Lawn fertilizers would burn garden plants because they contain too much nitrogen and sometimes other chemicals that are meant to control weeds, so it is important to do your research and pick the best fertilizer for your type of plant. One size does not fit all when it comes to fertilizing your plants.
Complete fertilizers: this type of fertilizer contains all three of the primary macronutrients – nitrogen (N), phosphorous (P), and potassium (K).
A balanced fertilizer contains equal amounts of nitrogen, phosphate, and potassium – like a 10-10-10 fertilizer.
It may be beneficial to apply this type of fertilizer if your soil lacks all three nutrients, but if your soil is rich in 2, applying all 3 nutrients could burn your plants and lead to toxicity as well as waste your money!
Incomplete fertilizer: a fertilizer that lacks one or two of the three major plant nutrients.
Slow & Controlled release: provide nutrients to plants over an extended period of time. The idea is to save money by fertilizing less frequently and prevent excess nutrients from entering waterways.
Read the label to find the percentage of slow – or controlled-release nutrients in the fertilizer, paying particular attention to the higher amounts of slow-release nitrogen.
In Florida, any fertilizer that is labeled “slow-release” or “controlled-release” must contain at least 30% or more slow- or controlled-release nitrogen.
Organic fertilizer: made from materials that are derived from plants and animals, like manure.
Inorganic fertilizer: materials that are mined or synthesized from non-living materials.
Dry fertilizer: made of powder or granular materials that dissolve in water.
Water-soluble/liquid fertilizer: water soluble powders or liquid concentrates that mix with water to form a solution.
Check out Types of Fertilizer – Gardening Solutions – University of Florida for more information.
By understanding the delicate dance between photosynthesis and fertilization, you can be a gardening guru and Florida Friendly. Remember, photosynthesis is the plant’s superpower, while fertilizer is its trusty sidekick, ensuring a healthy and flourishing garden. So, next time you’re out amongst the blooms, armed with your watering can and fertilizer in hand, channel your inner botanist and let the gardening magic begin! Happy gardening, folks!
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