What Is NPK and Why Is It Important?

Timing is everything with fertilizer

By Heather Kirk-Ballard

LSU AgCenter Horticulturist

Spring is here and the plants are booming. Plants perform best when they are getting the optimum amount of sunlight, appropriate amounts of water and plenty of essential nutrients for good growth and production of flowers and fruit. Optimize plant growth by knowing how and when to fertilize.

Fertilizers provide the nutrients needed by plants to grow, form flowers and healthy foliage, produce more fruit, establish a strong root system and fight diseases and insect pressures. Plants benefit from fertilizer applications most when they are in their peak growth, and spring is one of those times!

The best time to fertilize is when many plants come out of dormancy in the springtime. In spring, deciduous plants leaf out, flowering plant buds begin to burst, stems and branches elongate and new roots are formed. Nutrients will aid in all of this growth, so the rule of thumb is to make an annual application of fertilizer in the early spring.

There are several ways to fertilize plants, including slow-release granular fertilizers, liquid feed applied to soil as a drench, foliar sprays and fertilizer stakes. In addition, using natural fertilizers such as compost can add nutrients to your soil.

Compost is decayed organic material from other plant materials and animal waste from livestock that has a diet mostly made up from plants. Those include chickens, cows and horses. Composts can be found already bagged at many retail garden centers or available in bulk delivery form from local sources.

Compost also can be made from waste such as grass clippings and fallen leaves in your yard, in addition to kitchen scraps such as coffee grounds, eggshells and fruit and vegetable peelings. Worm castings and mushroom composts are two additional composts that add nutrients to your soil.

Plants can have individual needs when it comes to fertilizers. Acid-loving plants such as azaleas, blueberries, camellias, hydrangeas and rhododendrons prefer more of an acidic soil pH to take up optimal nutrients. Flowering plants need more phosphorus to help stimulate stronger bud, fruit and flower development.

Vegetable plants benefit and require more frequent fertilizer applications throughout the season because they are producing and that makes them heavy feeders. Slow-release fertilizers can be incorporated at planting time for extended nutrient release. Side dress with calcium nitrate at the first and third bloom set. Additionally, some growers utilize liquid fertilizers every other week.

Nitrogen is very important for all plants but especially important for lawns and turfgrasses. Woody plants such as shrubs and trees can benefit from an application of three pounds of nitrogen per thousand square feet and perennials benefit from one pound of nitrogen per 1000 square feet in the spring.

Indoor potted plants may be fertilized with a liquid plant food every month for most varieties. Do this once each season in spring, summer and fall, but skip winter for plants that go into dormancy. No matter the plants you are trying to feed, scheduling the correct timing is important.

Applying fertilizer in the wrong season can cause increased tender new growth that can be damaged, especially if applied in cold weather in late fall or in winter. So it is best to stop fertilizing during dormant seasons. Additionally, in zones that can be in danger of a late freeze, it’s best to wait until mid-spring. Everyone could benefit from putting fertilizer application schedules in their calendars.

Here are some general rules to follow when fertilizing:

Avoid fertilizing new plants until they are fully established, because it can cause an increase in tender new growth that can make the plant weak and leggy.

Always follow the manufacturers recommended rates and application methods.

Fertilize outdoor plants in the coolest part of the day, such as early morning or late evening, especially in hot summer weather.

Always water in fertilizers well to distribute nutrients throughout the soil profile and to prevent burning.

Lastly, avoid excessive fertilizing and perform soil tests annually to see what nutrients your plants need.

With fertilizer prices doubling as they have, waste not!

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 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

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?,

---Michael in Albemarle, NC

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?

---Nina in Central NJ

I'm curious about the difference between bone meal and blood meal. Would I use either for new plantings?

---Ray from Front Royal, VA

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

Fertilizer Basics: Organic Fertilizer, NPK Ratio

As a founding employee of Gardener's Supply, I wore many different hats over the years. Currently, I have my own company called Johnnie Brook Creative. The gardens around my home in Richmond, VT, include a large vegetable garden, seasonal greenhouse, cutting garden, perennial gardens, rock garden, shade garden, berry plantings, lots of container plants and a meadow garden. There's no place I'd rather be than in the garden. If you'd like to learn more, check out this January 2021 video interview by Garden Gate magazine.

According to the Gallup Gardening Survey, less than half of the country's home gardeners use any kind of fertilizer on their lawns or gardens. What's unfortunate about this statistic is that it means gardeners aren't getting as many flowers or as much produce as they should. And they're probably struggling with disease and insect problems that could be avoided.

Well-fed plants are healthier, more productive and more beautiful. This article covers the basics of why and how to fertilize your garden.

Plant Nutrients 101

Remember those tasty tomatoes and beautiful roses you grew last year? It took nutrients from the soil to build those plant tissues. By fertilizing your garden, you replenish lost nutrients and ensure that this year's plants have the food they need to flourish.

Nutrients Required by Plants

Macronutrients

Structural nutrients are the building blocks of most biomolecules. Plants primarily get these from CO2 (carbon dioxide) and H2O (water).

Carbon

Hydrogen

Oxygen

Primary nutrients are needed in large quantities by plants, and therefore tend to get depleted from soils first. These are the major components of most fertilizers.

Nitrogen is the base of all cells, proteins, and chlorophylls. It helps plants make the proteins they need to produce new tissues and promotes lots of vegetative growth. In nature, nitrogen is often in short supply so plants have evolved to take up as much nitrogen as possible, even if it means not taking up other necessary elements. If too much nitrogen is available, the plant may grow abundant foliage but not produce fruit or flowers.

Phosphorus stimulates root growth, helps the plant set buds and flowers, improves vitality, and increases seed size. A phosphorus deficiency leads to slow growth and weak plants.

Potassium improves overall vigor of the plant. It helps the plants make carbohydrates and provides disease resistance. It also helps regulate metabolic activities.

Secondary nutrients are also required by plants, but but in much smaller amounts

Calcium is used by plants in cell membranes, at their growing points and to neutralize toxic materials. In addition, calcium improves soil structure and helps bind organic and inorganic particles together.

Magnesium is the only metallic component of chlorophyll. Without it, plants can't process sunlight.

Sulfur is a component of many proteins.

Micronutrients

Finally, there are seven elements that plants need in tiny amounts. Healthy soil that is high in organic matter usually contains adequate amounts of each of these micronutrients:

boron

copper

iron

molybdenum

zinc

chlorine

manganese

Shop Fertilizers

To look their best, most perennials like this delphinium require regular fertilization.

Organic vs. Synthetic

Do plants really care where they get their nutrients? Yes, because organic and synthetic fertilizers provide nutrients in different ways. Organic fertilizers are made from naturally occurring mineral deposits and organic material, such as bone or plant meal or composted manure. Synthetic fertilizers are made by chemically processing raw materials.

In general, the nutrients in organic fertilizers are not water-soluble and are released to the plants slowly over a period of months or even years. For this reason, organic fertilizers are best applied in the fall so the nutrients will be available in the spring. These organic fertilizers stimulate beneficial soil microorganisms and improve the structure of the soil. Soil microbes play an important role in converting organic fertilizers into soluble nutrients that can be absorbed by your plants. In most cases, organic fertilizers and compost will provide all the secondary and micronutrients your plants need.

Synthetic fertilizers are water-soluble and can be taken up by the plant almost immediately. In fact applying too much synthetic fertilizer can "burn" foliage and damage your plants. Synthetic fertilizers give plants a quick boost but do little to improve soil texture, stimulate soil life, or improve your soil's long-term fertility. Because synthetic fertilizers are highly water-soluble, they can also leach out into streams and ponds. Synthetic fertilizers do have some advantages in early spring. Because they are water-soluble, they are available to plants even when the soil is still cold and soil microbes are inactive. For this reason, some organically-based fertilizers, such as PHC All-Purpose Fertilizer, also contain small amounts of synthetic fertilizers to ensure the availability of nutrients.

For the long-term health of your garden, feeding your plants by building the soil with organic fertilizers and compost is best. This will give you soil that is rich in organic matter and teeming with microbial life.

Foliar Feeding?

Plants can absorb nutrients eight to 20 times more efficiently through their leaf surfaces than through their roots. As a result, spraying foliage with liquid nutrients can produce remarkable yields. For best results, spray plants during their critical growth stages such as transplanting time, blooming time and just after fruit sets.

What About pH?

Even if proper nutrients are present in the soil, some nutrients cannot be absorbed by plants if the soil pH is too high or too low. For most plants, soil pH should be between 6.0 and 7.0. A soil test will measure the pH of your soil. You can send a sample to a lab (contact your local extension service for a low-cost kit) or buy a home kit and do it yourself. Lime or wood ash can be used to raise pH; sulfur or aluminum sulfate can lower pH. Keep in mind that it's best to raise or lower soil pH slowly over the course of a year or two. Dramatic adjustments can result in the opposite extreme, which may be worse than what you started with. Once again, a helpful solution is to apply compost. Compost moderates soil pH and is one of the best ways to maintain the 6.5 ideal.

How to Choose a Fertilizer

In most cases, an all-purpose, 5-5-5 fertilizer will provide the nutrients all plants need for healthy growth. If a soil test reveals certain nutrient deficiencies, or if you want to tailor your fertilizer to the needs of particular plants (tomatoes vs. flowers), you can select a special formulation. What you choose will depend on your soil and what you are growing.

The three numbers that you see on a fertilizer label, such as 5-5-5, tell you what proportion of each macronutrient the fertilizer contains. The first number is always nitrogen (N), the second is phosphorus (P) and the third is potassium (K). This "N-P-K" ratio reflects the available nutrients by weight contained in that fertilizer. For example, if a 100-pound bag of fertilizer has an N-P-K ratio of 5-7-4, it contains 5 pounds of nitrate, 7 pounds of phosphate (which contains phosphorus), 4 pounds of potash (which contains potassium) and 84 pounds of filler.

Note that the N-P-K ratio of organic fertilizers is typically lower than that of a synthetic fertilizer. This is because by law, the ratio can only express nutrients that are immediately available. Most organic fertilizers contain slow-release nutrients that will become available over time. They also contain many trace elements that might not be supplied by synthetic fertilizers.

Fertilizers offered by Gardener's Supply are either all-organic, or contain primarily organic materials. To build the long-term health and fertility of your soil, we recommend using granular organic fertilizers. Supplementing with a water-soluble fertilizer ensures that your plants have the nutrients they need when they're in active growth

Our slow-release, granular GSC Organic Tomato Fertilizer (5-6-5) gives your tomatoes all the nutrients they need, including plenty of phosphorus for big, abundant fruit. For a healthy start, mix a handful into the soil at transplant time and then side dress when tomatoes begin to set fruit. Also ideal for peppers and eggplant.