The letters "NPK" on a fertilizer label stand for nitrogen, phosphorus, and potassium, the three primary nutrients plants need to grow. The numbers on the label indicate the ratio (by percentage) of nitrogen, phosphorus, and potassium in the fertilizer container.
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Even if you do not see the letters N-P-K, but you see a set of three numbers, for example, 5-10-5, you can correctly assume it stands for 5% nitrogen, 10% phosphorus, and 5% potassium, always in that order. This fertilizer contains 20% nutrients; the remaining 80% is minor nutrients or fillers. Plants need about 16 nutrients; some they get from the air and water, and others are nutrients like iron, calcium, and chlorine from soil.
A smaller quantity of fertilizer with higher ratios can be the same as a larger quantity with lower numbers. For example, a five-pound bag of 10-20-10 fertilizer has the same nutritional value as 10 pounds of 5-10-5 fertilizer.
What Do the Ingredients in NPK Do?
Here's a brief look at what each nutrient contributes to plant growth.
Nitrogen
Nitrogen, the first number referenced in an NPK sequence, plays a key role in a plant's coloring and chlorophyll production, making it an important factor in leaf development. Fertilizers high in nitrogen are often used for grass or other plants where green foliage growth is more important than flowering. At the opposite end of the spectrum, gardeners sometimes encounter the problem of nitrogen depletion—the yellowing of typically green plants often indicates a nitrogen deficiency.
Phosphorus
The middle number in an NPK series refers to the percentage of phosphorous in the fertilizer product. Phosphorus plays a key role in the growth of roots, blooming, and fruiting, which is why it is an essential nutrient for your plants in spring. Phosphorus contributes to many fundamental plant processes, such as rooting and seed formation.
Potassium
The final number in the major ingredients listing gives the percentage of potassium in the product. Potassium contributes to the overall health and vigor of plants. It is known to help strengthen plants' ability to resist disease, assist in the movement of water and nutrients in the plant, and can be especially important in areas that experience cold or dry weather.
Other Ingredients
In addition to the major nutrients that are usually noted on the front label, most fertilizers also include additional ingredients that are listed on a side or back label. This may include other nutrients like calcium, magnesium, iron, micronutrients, and even the percentage of organic matter and fillers. Although the minerals and micronutrients are less critical than the major nutrients, a good fertilizer product will include small amounts of other ingredients as well.
Types of NPK Fertilizers
Balanced Fertilizers
A fertilizer listed as "10-10-10" is considered a balanced or "complete" fertilizer because its nitrogen, phosphorus, and potassium proportions are equal. Meanwhile, a fertilizer labeled "10-0-10" is an "incomplete fertilizer."
Incomplete Fertilizers
An incomplete fertilizer is not necessarily inferior to a complete fertilizer. Identifying the right fertilizer for your needs depends on a variety of circumstances. If your soil already has an excess of one of the three nutrients in NPK, you could be harming some of your plants by adding more of it to the soil—in this case, an incomplete or unbalanced fertilizer may be the right choice.
For this reason, it is important to test your soil before applying fertilizer. Otherwise, whenever you add anything to your soil, the effect (whether positive or negative) is left to chance.
Organic Fertilizers
Organic fertilizer ingredients are soil nutrients from plant, animal, or mineral sources. Products labeled as "organic fertilizers" must specify which nutrients are organic and must be identified as synthetic or natural by percentage. For example, the label might read, "20 percent of nitrogen (6 percent synthetic, 14 percent organic)."
Organic fertilizer sources might include manure, alfalfa meal, kelp, blood and bone meal, fish emulsion, cottonseed, sewage sludge, soft rock phosphate, and green sand.
Technically speaking, "organic" material is anything that contains carbon atoms. The modern definition of "organic" means it doesn't contain anything synthetic; however, "organic fertilizers" may have natural or synthetic materials, but the percentages of each must be marked. So, if what you want is "non-synthetic fertilizer," read the label carefully for all ingredients before purchasing since the term "organic" does not cover it.
Worthy of note: Organic fertilizers made wholly from natural ingredients often have lower concentrations of the three major nutrients, so you will often need to use larger amounts. The upside is they usually contain many additional nutrients that feed the plant and the soil. It is strongly recommended that if you are only using commercial synthetic fertilizer, supplement it with some type of organic matter, such as compost or manure, for more holistic soil health.
How to Choose the Right Fertilizer
To properly give your plants the nutrients they need, you need to evaluate what they currently have (or will have if doing a new planting). Not all types of plants have the same nutrient requirements, and you can cause ill-wanted effects when applying amendments haphazardly.
Amendments are supplements like fertilizer that you add to the soil to improve its condition; they can fix a number of factors, including nutrition, water permeability, or pH level. Agricultural lime is a type of amendment you use to reduce acidity in the soil, just like sand or grit is an amendment used to increase the porosity of the soil, aiding in drainage.
If you do not choose the right fertilizer, and if your soil pH is already too high or too low, your plants cannot access some nutrients, even if you give them fertilizer. Knowing what's already in your soil is crucial because you can do more harm than good when applying a fertilizer high in nitrogen to an already nitrogen-rich soil. For example, nitrogen feeds foliage at the expense of flowers, which can affect flower production.
Here are the steps for choosing the right fertilizer:
Get your soil tested. Having a soil test done before you start adding amendments will tell you what you actually need. You can do a home test, send a soil sample for testing, or contact your local cooperative extension office for guidance.
Determine what you are planting. Some plants might need more or less of a certain type of nutrient. For example, leafy greens need more nitrogen, while more phosphorus is required for fruiting and flowering. Before you start any gardening or planting project; have a firm understanding of the plant you're growing and its fertilizer needs
Check for fertilizers specially formulated for the plant you're growing. You can find preformulated fertilizers for lawns, trees, bulbs, cactus, houseplants, flowers, fruits and vegetables, acid-loving plants, and much more. Most will match or come close to the nutrient requirements you need, considering your soil is neutral, so adjust accordingly based on your soil analysis.
Read the fertilizer analysis label before buying fertilizer. If you cannot find a fertilizer with the exact N-P-K ratio you need based on your soil test, come as close as possible. Of the three nutrients, it's more important to get the nitrogen correct and not overdo the phosphorus since it can affect water intake.
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Determine the form of fertilizer you need. If you are establishing a new lawn, you might consider using a quick-release fertilizer that is sprayed in liquid form. Or, you might try a granulated fertilizer for a slow-release form of food for the plant and its roots over time. Most plant growth and care guides will list nutrient ratios, liquid vs. dry forms, and quick vs. slow-release recommendations.
If not sure, use compost. If you have not had your soil tested and do not understand how well it meets a plant's nutritional needs but still feel the need to feed it, use compost instead of commercial fertilizer or consider using a slow-release fertilizer. Slow release is less likely to harm plants to any great degree.
How to Use Fertilizer
Since every plant has different fertilizer needs, we know that one plant may require more fertilizer while another might require less or none at all. In most cases, a regular fertilizer schedule will keep your plants vigorous. However, too much fertilizer can kill your plant, often burning the foliage and roots if given too much too quickly or applied directly on the foliage.
How do you know how much to use and how often? Follow the guidelines on the fertilizer label combined with the plant's specific care instructions. Also, check your local extension office for recommendations for fertilizer needs in your region; some regions have more acidic soils than others.
For a simple example, we know in the spring a lawn requires nitrogen-rich fertilizer. One pound of nitrogen might be recommended per 1,000 square feet of lawn. If your lawn is 1,000 square feet and your fertilizer contains 20% nitrogen, you should evenly apply 10 pounds of the fertilizer over the entirety of your lawn. In most cases, lawns benefit from fertilizer once a month from April to November, with a pause during the hottest months of July and August; however, this is variable based on how rich the soil already is. That's why soil analysis is important. You never want to over-fertilize, if you can avoid it.
Variables that affect how much and how often you should fertilize:
The type of plant
Slow-release vs. quick-release fertilizer
Mowing practices (lawns)
Watering
Weather and temperatures
Soil texture
Soil pH
Avoid over-fertilizing your plants, especially lawns. Nutrients that aren't taken up by plants may run off into sewer systems and rivers, leading to serious pollution problems.
How to Apply Fertilizer
It is generally recommended that the best time to apply fertilizer is when the sun is not beating down on the plants. However, applying it before or after rainfall or plant watering depends on the form of fertilizer you use. Different forms of fertilizer are granules, liquid, powder, or spikes.
Granular fertilizer: This dry fertilizer comes in the shape of pellets, typically used for lawns. Most granular fertilizers are slow-release fertilizers, giving off nutrients to grass over a prolonged period. This type of fertilizer uses special fertilizer spreaders as tools—broadcast, drop, or hand-held spreaders. It's best to apply this fertilizer before the rain comes. This slow-release fertilizer uses rainwater as its method of dispersal. On wet grass, the granules might stick to foliage, increasing the risk of burning it.
Liquid, powder, or crystal fertilizer: This form is often sold as a liquid concentrate, dry powder, or crystals that require further dilution with water. Most liquid fertilizers are quick-release solutions, which means nutrients pass to the plant more immediately. Sprayers and hose attachments make spreading this fertilizer an easy task. It's best to spray this fertilizer on already-wet grass to limit "burn" from the chemicals.
Spikes: For a set-and-forget-it approach to fertilizing, spikes are your go-to. Spikes are usually the most expensive type of fertilizer but are easy to use. Spikes are slow-release, leeching nutrients from the spike to the soil over time. Spikes are formulated for trees, shrubs, outdoor plants, and houseplants. Generally, plant the spike in soft, damp soil at the drip line (the point where water falls from the tree or plant down toward the roots).
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The three major nutrients that plants need are nitrogen, phosphorus, and potassium. These are referred to by the initials of their chemical symbols: N (Nitrogen), P (Phosphorus), and K (Potassium).
Fertilisers are food for plants that can be bought in concentrated and compact form such as pellets, granules, powder, or liquid. Fertilisers that contain the three major nutrients, NPK, are known by a variety of terms. These include All purpose, All round, Complete, Compound, General Purpose and Universal. There are many different brands and types of all-round fertiliser. As these ‘general’ fertilisers, there are also fertilisers for specific plants such as tomatoes and roses.
Back of a fertiliser bottle, showing its NPK ratio
Product labels carry a detailed breakdown of the three major nutrients N, P and K, always in the same order and shown as a percentage. If the numbers are the same or similar, this is a ‘balanced’ fertiliser. Where one number is higher than others (as pictured above), this means the proportion differs and the fertiliser is best used at a particular time of year. Plants need different amounts of nutrients at certain times of year, so it helps to understand what each one is used for.
Nitrogen (N) boosts leafy growth and is most needed in spring and early summer. Avoid applying a high-nitrogen fertiliser late in the season as soft leafy growth is easily damaged by frost. High nitrogen feeders include lawn food.
Phosphorus (P) promotes root development, so higher levels of phosphorus are required when planting, particularly in autumn. Bonemeal contains high levels of phosphorus.
Potassium (K) helps flowers and fruit to form and should be applied from late spring to the end of summer. A good example of a high potassium feed is a tomato feed (above) which is used to increase flower and fruit production in fruiting plants, particularly tomatoes.
Applying lawn feed
When choosing fertilisers think about what you need it for. If you're looking for a lawn feed you'll need a high nitrogen (N) feed, if you want a bumper crop of chillies, tomatoes or aubergines, go for a high potassium (K) feed, like a tomato feed. If you're planting and want to aid root growth then add a generous helping of bonemeal to the planting hole, which has high levels of phosphorus (P). Think also of ease of application – granular or powdered fertiliser is quick and simple to apply and usually needs only one or two applications in a growing season. Liquid or concentrated fertilisers need diluting and watering on and tend to need more frequent application.
Think also about the desired effect and the speed with which you want the fertiliser to feed your plants – liquid feeds tend to act immediately whereas pelleted fertilisers take longer to release their nutrients. Consider also whether you want organic or inorganic feeds. Fertilisers can be artificially produced (inorganic or synthetic) or based on plant or animal products (organic). Inorganic fertilisers tend to feed the plant in one application but offer no long-term benefits to the soil. Organic fertilisers not only feed the plant but they feed the soil, too, contributing to its long-term health, helping to support long-term plant growth and wildlife.
Gardener using a liquid plant food
A healthy soil, particularly when regularly improved with organic matter such as compost or manure, may provide everything that plants need, although hard-working plants like vegetables and some flowering roses benefit from an additional boost in summer, particularly those growth in pots and sandy soils.
Getting the balance right is important as too much fertiliser can be as damaging as too little. Also bear in mind the potential of harm to the wider environment too, as surplus synthetic nutrients can pollute water and harm soil life.
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Be aware that plants can take up nutrients only when the soil is moist, so dry conditions often result in plants showing signs of deficiency when all they really need is moisture.
Contact us to discuss your requirements of wholesale blue NPK fertilizer. Our experienced sales team can help you identify the options that best suit your needs.