Understand NPK fertilizers and use them correctly

Understand NPK fertilizers and use them correctly

the essentials in brief

  • NPK fertilizer consists of mineral salts. Nitrogen, phosphorus and potassium are the main components. Plants need these nutrients for healthy growth.
  • When used properly, NPK fertilizer is not dangerous. Its toxic effect on humans, animals and the environment occurs when used improperly.
  • NPK fertilizer can be made by yourself, although organic alternatives make more sense. Because of their rapid availability, NPK fertilizers must be precisely dosed so that the advantages outweigh the disadvantages.

What is NPK fertilizer?

The core nutrients of a complete fertilizer are nitrogen, phosphate and potassium. Such compositions are also called NPK fertilizers, which is derived from the symbols of the chemical elements. How high the concentration of the individual nutrients is is given in percentages.

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Content information on the NPK fertilizer: Composition

Information on the packaging of the NPK fertilizer such as 15-15-15 or 10 52 10 state that the fertilizer contains either 15 percent nitrogen, phosphate and potassium or ten percent nitrogen, 52 percent phosphate and ten percent potassium. If another number is listed, it indicates the magnesium content.

Plants need:

  • Nitrogen : promotes the growth of the green parts of the plant
  • Phosphorus : supports the development of flowers and fruits
  • Potassium : regulates the water balance and strengthens the plant tissue

What an NPK fertilizer brings: application


The composition provides information about the possible uses of the fertilizer. They give the plant everything it needs to grow. The complete fertilizers can be used both for basic supply and for subsequent fertilization. The mixtures are sprinkled by hand or added to the irrigation water. An oversupply of individual components can negatively affect growth.

NPK fertilizer: fertilizer contains important nutrients for plants


Plants need this element for healthy growth. Nitrogen is part of the plant pigment chlorophyll and proteins. In its gaseous state it makes up 78 percent of the air, whereby the plants can only absorb nitrogen in the form of nitrate or ammonium ions. This happens through the roots, which is why special fertilizers are applied to the soil. NPK fertilizers for lawns are nitrogen-based and are mainly applied in spring. A nitrogen deficiency is noticeable through light green leaves and an oversupply through blue-green leaf mass.


To prevent permanent irreversible damage to plants, you should give your plants phosphorus. Phosphorous fertilizers are used where the plants bloom poorly or where the underside of the leaves is red. They are mainly intended for beds and hedges with flowering ornamental plants and balcony flowers. However, the soil has an impact on how much phosphorus a plant can absorb. Plants can absorb the element more poorly if the soil is too moist, compacted or the pH value is too low. An overdose of phosphorus leads to poor growth.


If the leaves of your ornamental plants droop despite a good water supply, this may be due to a lack of potassium. Use a complete fertilizer with a high concentration of potassium. Herbs and green plants need more potassium and nitrogen. Their need for phosphorus is lower because they do not depend on the development of flowers. Highly consuming vegetables produce higher yields if they are adequately supplied with potassium. Over-supplied plants often develop brown leaf margins.

Can I use NPK fertilizer in the aquarium?

Aquarium plants also need these nutrients for healthy growth. When choosing, you should make sure that the complete fertilizer is suitable for the aquarium. Which fertilizer you choose also depends on your water parameters. Often there are nutrients in the outlet water that get into the water through feeding or stocking. Existing nutrients should no longer be administered.

How to fertilize the aquarium:

  1. Measure water values ​​such as iron, nitrate and phosphate
  2. Select complete fertilizer (NPK), mixed forms (PK, NP, NK) or individual components (N, P or K) according to the values
  3. Adjust the nutrient concentration to the pool
  4. Bring other water values ​​such as CO2 and light into the optimum range

Is NPK Fertilizer Toxic?

Heavy metals play a role in human health; they are absorbed by plants from the soil and thus enter the food chain. These elements are poisonous in high concentrations, but at the same time the plant organism and humans need essential trace elements for important metabolic processes. Some NPK fertilizers can contain additional trace elements in addition to the main nutrients, which is why you should pay attention to the exact composition.

NPK fertilizer is toxic if used improperly.

Blue grain


These blue colored balls also belong to the NPK fertilizers. In addition to the main nutrients, blue grain also contains magnesium and sulfur, whereby the concentrations of the individual substances vary depending on the recipe. This complete mineral fertilizer is produced chemically and, like comparable NPK fertilizers, can cause various problems.

Toxic - to animals

If pets accidentally eat the blue-colored grains from the bed, this can lead to health problems. Consumption leads to bloody diarrhea, salivation or vomiting as well as shortness of breath and tremors. The cycle can collapse, which in the worst case scenario can be fatal.

If you suspect your pet has eaten bluegrain, watch them and see a veterinarian at the first symptoms. Make sure that the animal's airways remain clear and do not try to induce vomiting. One possible treatment method is the administration of charcoal tablets to bind the toxins.

Toxic - to humans

The nitrate contained in NPK fertilizers is converted to nitrite in the plant or in the human body. In large quantities this substance can lead to blue rash. Small children in particular should not come into contact with the fertilizer. In blue rash, there is an increased level of methemoglobin, which disrupts the oxygen supply. Drowsiness, confusion, and headache develop. In severe cases, coma-like conditions occur that can be fatal. The blue rash is treated with a blood transfusion.

Toxic - for the environment

Most garden soils are oversupplied with certain nutrients and do not need complete fertilizers. Unused elements are washed out or accumulate in the soil. This affects the micro-organisms in soil and water. Nitrogen is difficult to store in the soil. If the element gets into the groundwater as nitrate, the way into the drinking water is not far.

Consequences for agriculture:

  • Arable farming focuses on a few crops that are favored by mineral fertilizers
  • nitrogen-fixing legumes or undemanding crops are meaningless
  • further measures required to maintain the humus layer
  • Weakly competitive plants and species from poor locations are being pushed back
  • Biodiversity is being lost

Mix NPK fertilizer yourself

Compost fertilizer

If you have a liquid fertilizer at home that does not offer the optimum nutrient concentration, you can mix it yourself with purchased individual components. However, it takes a bit of tact to bring the individual substances into the desired ratio. Hence, you should think about alternative organic fertilizers that are similar to an NPK fertilizer.


Liquid fertilizers with a pH that is too high can be neutralized with a dash of salad vinegar. Get a test strip to check the value.

Making organic fertilizers

Such mixtures contain natural components that are broken down by microorganisms in the soil. The nutrients are not immediately available to the plant, which reduces the risk of overfertilization. Compost is an ideal fertilizer, the NPK concentration of which differs depending on the composted material. Pay attention to what you dispose of on the compost and add animal substances if necessary.

Banana peels--xhigh magnesium content
Nettlesx--Liquid manures have a high pH
Animal dungxxxin different concentrations depending on the species
Coffee groundsxxxcontains minerals
ash-xxhas a strong alkaline effect

Correctly dose NPK fertilizer

How many nutrients a plant needs depends on the stage of growth and soil conditions. For plants that are grown in the greenhouse under controlled conditions, the optimal nutrient concentration can be achieved more easily than outdoors under fluctuating conditions. The following example shows which concentrations of NPK fertilizer the hemp needs in the individual growth phases.


Hemp cultivation in Germany

Cannabis can be legally purchased in Germany because it is a prescription drug for patients with chronic pain. However, growing hemp is illegal. The intoxicating herb may only be grown and harvested for a limited period of time with a special permit. Such special permits are issued for research purposes. Most of the medicinally used hemp products are imported. In the coming years, the need for cannabis is to be covered by German cultivation.

Vegetative phase


Seedlings with one or two pairs of leaves require few nutrients. A concentration of 2-1-2 NPK is sufficient to stimulate the growth of the plants. As the number of leaves increases, so does the nutritional requirement. As soon as the young plants have developed five pairs of leaves, they are optimally supplied with a 4-2-3 fertilizer.

Then the need for nitrogen increases rapidly. To encourage vigorous growth, the plants need an NPK concentration of 10-5-7. Only in the late vegetative phase does the nitrogen requirement decrease by around 25 percent. Just before flowering, values ​​of 7-7-7 are sufficient.

Generative phase

During the early flowering phase, hemp needs a high level of phosphorus, so that NPK values ​​of 5-10-7 are optimal. Subsequently, continuously increased concentrations ensure balanced growth, with the entire flowering phase being accompanied by a predominant proportion of phosphate. During the middle phase, concentrations of 6-15-10 are optimal. When the flowering period is coming to an end, the nutritional requirements are slowly reduced. NPK fertilizers with values ​​4-10-7 provide sufficient elements.

advantages and disadvantages


NPK fertilizers are extremely convenient. They ensure improved growth in the short term if deficiency symptoms occur. Therefore, such products are used not only in agriculture, but also in the private garden when growing vegetables. The plants are completely supplied and can absorb the nutrients directly from the soil without them first having to be made available by microorganisms. Compared to organic alternatives, NPK fertilizers are cheap. Blue grain costs between one and four euros per kilogram.

Disadvantages of improper use:

  • Environmental pollution : unused nutrients get into the groundwater through leaching and pollute the soil
  • Short-term : in the long run the fertility of the soil and the crop yields decrease
  • Growth disorders: unbalanced nutrient concentration leads to shooting and prevents flower development


In order to benefit from the advantages of NPK fertilizers, you should first carry out a soil sample. If necessary, administer individual components and avoid using complete fertilizers.

frequently asked Questions

What types of NPK fertilizers are there?

A mineral NPK fertilizer consists of inorganic substances. This is mainly made from water-soluble salts that are of synthetic origin. Complete fertilizers contain additional trace elements. The salts are available in liquid or solid form. Organic NPK fertilizers are not actually NPK fertilizers. Their nutrients come from vegetable or animal raw materials. There is also an intermediate form called organic-mineral NPK fertilizer. These consist of synthetic materials to which organic substances such as horn meal have been added.

How much NPK fertilizer on 1 ha of grassland?

How high the nutrient requirement of grassland is depends on the cutting intensity and the types of grass present. Intensively used areas on which high-quality grass mixtures thrive have a nitrogen requirement of 240 to 300 kilograms per hectare. However, there are regulations as to how much NPK fertilizer can be applied. A maximum of 170 kilograms of nitrogen may be applied to grassland per hectare and year. A soil analysis provides information about how much NPK fertilizer is required in which concentration.

What are the prices of NPK fertilizers?

Prices vary depending on the manufacturer and composition. A basic price per kilogram of between 80 cents and two euros is common, although this price drops for larger purchases. This means that mineral NPK fertilizers are significantly cheaper than organic alternatives. Here the price per kilogram fluctuates between three and five euros. A self-made fertilizer from biological waste is free of charge.

Examples of prices:

  • 25 kg NPK fertilizer (15-15-15): approx. 22 euros
  • 50 kg NPK fertilizer (10-6-18): approx. 42 euros
  • 100 kg NPK fertilizer (15-10-10): approx. 95 euros

Are NPK fertilizers harmful to health?

Mineral fertilizers available in Germany must not have any harmful effects on health or the environment. Correct use is therefore very important. If fertilizers are not misused, they are considered harmless.

Since NPK fertilizers consist of soluble salts, the dust on the skin or in the eyes in contact with water can cause irritation or dehydration. Improper use such as consumption can damage your health. In order to protect children and pets, the fertilizer should always be worked into the soil and never be stored freely accessible.