Standing in the fertilizer aisle of a garden or farm store, you are faced with a dizzying array of fertilizer options, many with a series of three numbers like 10-10-10, 20-20-20, 10-8-10 or many other combinations of numbers.

You may be asking yourself, “What do the numbers on fertilizer mean?”. These are NPK values, which leads to the next question of, “What is NPK?”. Keep reading to learn more about fertilizer numbers and NPK.

What Do the Numbers on Fertilizer Mean?

The three numbers on fertilizer represents the value of the three macro-nutrients used by plants. These macro-nutrients are nitrogen (N), phosphorus (P), and potassium (K), or NPK for short.

The higher the number, the more concentrated the nutrient is in the fertilizer. For example, numbers on fertilizer listed as 20-5-5 has four times more nitrogen in it than phosphorus and potassium.

A 20-20-20 fertilizer has twice as much concentration of all three nutrients than 10-10-10. The fertilizer numbers can be used to calculate how much of a fertilizer needs to be applied to equal 1 pound (453.5 g.) of the nutrient you are trying to add to the soil. So if the numbers on the fertilizer are 10-10-10, you can divide 100 by 10 and this will tell you that you need 10 pounds (4.5 kg.) of the fertilizer to add 1 pound (453.5 g.) of the nutrient to the soil. If the fertilizer numbers were 20-20-20, you divide 100 by 20 and you know that it will take 5 pounds (2 kg.) of the fertilizer to add 1 pound (453.5 g.) of the nutrient to the soil. A fertilizer that contains only one macro-nutrient will have “0” in the other values. For example, if a fertilizer is 10-0-0, then it only contains nitrogen.

What is NPK and Why is it Important?

So now that you know what the numbers on fertilizer mean, you need to know why NPK is important to your plants. All plants need nitrogen, phosphorus, and potassium to grow. Without enough of any one of these nutrients, a plant will fail.

Nitrogen (N) – Nitrogen is largely responsible for the growth of leaves on the plant.

Phosphorus (P) – Phosphorus is largely responsible for root growth and flower and fruit development.

Potassium (K) – Potassium is a nutrient that helps the overall functions of the plant perform correctly. Knowing the NPK values of a fertilizer can help you select one that is appropriate for the type of plant you are growing.

For example, if you are growing leafy vegetables, you may want to apply a fertilizer that has a higher nitrogen number to encourage leafy growth. If you are growing flowers, you may want to apply a fertilizer that has a higher phosphorus number to encourage more blooms.

Before you apply fertilizer to your garden beds, you should have your soil tested. This will also help you determine what balance of fertilizer numbers will be appropriate for your garden’s soil needs and deficiencies.

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

Plants need to be fertilized because most soil does not provide the essential nutrients required for optimum growth. Even if you are lucky enough to start with great garden soil, as your plants grow, they absorb nutrients and leave the soil less fertile. 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.

Six Primary Nutrients

There are six primary nutrients that plants require in fairly large quantities.

  1. carbon from CO2 in the air
  2. hydrogen from water
  3. oxygen from water and air
  4. Nitrogen helps plants make the proteins they need to produce new tissues. 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. Growth may actually be stunted because the plant isn’t absorbing enough of the other elements it needs.
  5. Phosphorus stimulates root growth, helps the plant set buds and flowers, improves vitality and increases seed size. It does this by helping transfer energy from one part of the plant to another. To absorb phosphorus, most plants require a soil pH of 6.5 to 6.8. Organic matter and the activity of soil organisms also increase the availability of phosphorus.
  6. Potassium improves overall vigor of the plant. It helps the plants make carbohydrates and provides disease resistance. It also helps regulate metabolic activities.

Three Additional Nutrients That Matter

Plants also need these three nutrients, but in much smaller amounts:

  1. 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.
  2. Magnesium is the only metallic component of chlorophyll. Without it, plants can’t process sunlight.
  3. Sulfur is a component of many proteins.

When crops are harvested, important nutrients are removed from the soil, because they follow the crop and end up at the dinner table. If the soil is not replenished with nutrients through fertilizing, crop yields will deteriorate over time.

Careful analyzing and fertilizing of crops enables a chain that provides humans with nutritional food:

  • The nutrients feed the soil
  • The soil feeds the plants
  • Plants feed animals and people

The three most common mineral fertilizers are those based on nitrogen, phosphorus and potassium.

The International Fertilizer Association (IFA) estimate that 85% of the soils globally are deficient in nitrogen(1). 73% of the soils are deficient in phosphorus, whereas 55% lack potassium.

What is fertilizer used for?

Often, the plants have few possibilities to avoid nutrient deficiencies without the help of fertilizers.

Take nitrogen for example: Since plants are not capable of absorbing it from the air directly, the soil is their only means of acquiring this important nutrient. If the soil is low on nitrogen, fertilizers are needed to boost nutritional levels.

Large concentrations of potassium sources occur deep below the soil surface (often around one kilometer) and are far beyond the reach of plant roots. Mining of potassium brings this naturally occurring nutrient to the soil surface and within the grasp of plant roots.

Phosphorus exists in certain rocks, but for plants to access this nutrient, it needs to be water soluble. The correct use of phosphorus fertilizers helps plants absorb it through the soil and ensures a high production and rapid growth.

What is the difference between mineral fertilizers and organic fertilizers?

In nature there are 17 nutrients necessary for plants to thrive. What kind of fertilizer you need, depends on what crop you grow and the nutrient deficits in each specific soil. Different crops remove different amounts of nutrients from the soil.

Many farmers use NPK compound fertilizers that provide a combination of several nutrients at the same time.

Organic fertilizers such as animal waste and compost have been used for centuries and are a valuable source of nutrients and organic matter, which enhances soil structure.

But since the 20th century, mineral fertilizers have been required to meet the increasing food requirements of a growing world population. The amounts of nutrients in organic fertilizers vary and are much less concentrated than those in mineral fertilizers.

Mineral fertilizers reduce the amount needed and the number of vehicles to transport the fertilizing products.

By 2050, the global population is estimated to reach 9.8 billion, according to the United Nations(2). Increasing crop yields is essential if we are going to be able to produce enough food for everyone.

This increase is not possible without carefully planned fertilizing.

Each year soybean cyst nematodes rob millions of dollars of income from soybean farmers due to lost yield. To add insult to injury, the impact to the crop cannot typically be seen until harvest. It is likely that above ground SNC stress symptoms may never be noticed. The best way to minimize SCN impact is by planting resistant varieties, or varieties with multiple resistances.

The SCN lifecycle is very interesting.

“The nematode overwinters as eggs,” said Horacio Lopez Nicora, assistant professor in plant pathology at The Ohio State University. “It will remain in the soil enclosed in the SCN female nematode body — which we call a cyst — until it hatches into a second stage juvenile. That is the only infective stage of the entire life cycle of the nematode. It is the only stage that the nematode can penetrate into a root of a soybean plant.”

Nematodes can penetrate all soybean plants.

“Nematodes can penetrate the roots of both susceptible and resistant varieties,” Lopez-Nicora said. “The resistance does not prevent the nematode from getting inside the root. The nematode will penetrate the root and migrate to the root’s vascular system. Once the nematode penetrates and gets to the vascular system, it will initiate a feeding site if the plant is susceptible. That feeding site will allow the nematode to continue to grow until the female nematode is ready to be fertilized by the male and then lay eggs every 24 to 40 days depending on the time of year. In the summer roughly every 25 days a generation is produced.”

This process is not completed on resistant varieties.

“In a soybean plant with SCN resistance, the nematode will penetrate the root and migrate to the vascular system and attempt to initiate a feeding site. When the nematode initiates a feeding site it becomes sedentary. The nematode relies on the cells surrounding its head of the nematode to feed it,” Lopez-Nicora said. “In the SCN resistant soybean varieties the plant cells at the feeding site around the head of the nematode will die once the site is initiated and not feed the nematode to complete its life cycle.

“I find this amazing. Ever since I was young, I have always been fascinated by microscopic organisms and how they can impact our crops. It is like the resistant soybean plant is tricking the nematode and getting it inside to stop moving, and then it stops feeding it and it dies.”

There are different levels of resistance.

“There is moderate resistance or highly resistant varieties. Resistance to SCN is quantitative,” Lopez-Nicora said. “We measure the level of SCN resistance of the variety by the ability of the nematode to reproduce on those roots.”

Plant appearance is not a sufficient measure to determine if SCN is a problem in a soybean field.

“In our research, it is very consistent with research across the United States,” Lopez-Nicora said. “We are able to detect significant yield reduction between resistant versus susceptible lines without detecting any above ground visible symptoms. We have found yield reduction of 15% to 30%, all the way up to 50% yield reduction in SCN susceptible plants without any visible symptoms, including plant height and color. This makes it difficult to raise awareness. When we do find areas with visibly stressed plants and find areas on yield maps that show a reduction and go back and check for the presence of SCN in soil tests, we find SCN levels that are very high, and difficult to reduce.”

Farmers are re-evaluating their fertilizer application due to high fertilizer prices. Farmers who regularly applied fertilizer may not need as much or any additional fertilizer if their soil tests are optimal; especially for lime, phosphorus (P), and potassium (K). Most agricultural clay-based soils have around a ton of P and perhaps 40 tons of soil K. Applying high priced fertilizer when it may not be needed is not a good investment.

Soil testing is a good investment for both rented and owned cropland.

Soil tests are accurate for 3 years, but knowing what fields need fertilizer is the most important. Fields that are nutrient low should be prioritized over high or optimal fields, especially if fertilizer dollars are limited. Without recent soil tests, farmers are just guessing their crop’s nutrient needs. Soil samples are usually taken in the fall or early spring. Soil tests are often taken at the same time of year and then compared over several years to see soil fertility trends. Weather, crop rotation, and the soil environment may change your soil fertility and nutrient availability.

Start by looking at field’s soil pH which should be between 6.0 -6.8 for most field crops. If the soil pH is in the right range, then most other soil nutrients should be available at optimal levels. A low or high soil pH generally ties up critical soil nutrients. Next look at soil P and K levels. Most current soil tests are now using Melich-3 instead of Bray P1. Mehlich-3 P returns approximately 35% higher soil test phosphorous (STP) values than Bray P1 and approximately 14% higher soil test potassium (STK) values.

Due to improved plant breeding, most crops are now more efficient at obtaining soil nutrients, leading to lower fertilizer needs. While P has stayed relatively stable, K levels in crops harvested has dropped 26% in corn, 19% in soybeans, and 35% in wheat. Currently, our crops are yielding more bushels per acre with less nutrients removed. These new numbers can save farmers money on additional fertilizer inputs, especially when fertilizer prices are high.

New crop removal rates for corn are now estimated to be .74# of N, .35# P2O5, and .2# K2O for each bushel harvested. These removal rates are based on P2O5 and K2O and NOT actual fertilizer rates. For example, for potassium chloride (0-0-60), it takes 100# of 0-0-60 to equal 60# K2O. For soybeans, .79# P2O5, and 1.14# K2O are removed for each soybean bushel harvested. For wheat, estimates are .96# of N, .49# P2O5, and .24# K2O for each wheat bushel harvested. These numbers are based on recently updated Tri-State Fertilizer guidelines.

The optimal range for P is around 20-40# P2O5 based on Mehlich-3 which is very close to the old standard of 30-60# P2O5 Bray P1. For loamy and clay soils, the optimal range for K2O is 120-170 while for sandy soils the range is 100-130.

When fertilizer prices are high, fields that are in the optimal range have enough nutrients for at least one or several years to grow crops without any nutrient deficiency signs. When fertilizer prices come down, farmers can start applying fertilizer again to keep yields and fertility levels up. In general, soils with good soil health can maintain and even raise crop fertility levels. Beneficial soil fungi (mycorrhizae) supply the plant with 6X more P than roots can alone. Good soil structure and maintaining high soil organic matter (SOM) levels create good soil fertility.

When fertilizer soil tests are less than optimum, start by correcting your pH with lime first then address P and K fertilizer needs. Soil pH along with SOM determines when most nutrients are optimally plant available. Soil P levels remain steady longer than soil K, especially if forages are harvested. In high fertilizer cost years, avoid trying to “build up” or over applying fertilizer, because it does not pay. Just use up fertilizer in your “soil bank”.

For phosphorus (P) fertilizer, avoid broadcasting by applying P starter fertilizer in a band to increase P fertilizer efficiency. Potassium (K) fertilizer can be broadcast because it moves into the soil easier than P. For nitrogen, consider lowering your application rates. Based on the cost of N fertilizer ($.90 per # of N) and the price of corn ($5.85/ bushel), lowering your N fertilizer rate 15-20# per acre may only cost you 1 bushel of corn, so it’s more economical to cut back N rates slightly. Always follow the 4 R’s when applying fertilizer. Apply the right rate, use the right source, apply it at the right time, and in the right place.


The application of too much fertilizer not only affects the environment we live in, it also damages the plants we are trying to maintain. Heavy fertilizer applications may lead to nitrogen leaching into ground water and phosphorus washing into surface water.

Is Too Much Fertilizer a Problem

Over fertilization often leads to excessive plant growth that can cause a variety of problems. Some vegetable plants, like tomatoes, with too much growth will not produce blooms so production is delayed.

Excess fertilizers, when washed into lakes and ponds, may result in algae blooms. These algae blooms may deplete oxygen in the pond and can cause fish kills.

One common overuse is fertilizers in the lawn. In order to get a nice green lawn, many homeowners believe they need to put out high fertilizer rates.

However, over-fertilized lawns are more prone to diseases like brown patch, pithium, and helminthsporium. Placing too much fertilizer around plants can lead to fertilizer burn.

The high amount of salts associated with over fertilization can disrupt water uptake by the roots. Plants with fertilizer burn will often have leaf scorch type symptoms.

Over fertilization often leads to excessive plant growth that can cause a variety of problems. Some vegetable plants, like tomatoes, with too much growth will not produce blooms so production is delayed.

Bushes and shrubs with dense canopies are more prone to disease problems because they do not dry out as quickly as plants with open canopies.

In order to maintain healthy plants, just remember that a little bit of fertilizers can go a long way.

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