PK Fertilizer: Explanation Of Terms, Effect And Application

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PK fertilizers primarily supply the plant with phosphorus and potassium. We explain why you need PK fertilizer, how it works, and how you use it.
If plants have to be supplied with phosphorus and potassium, but nitrogen would not be beneficial, a PK fertilizer is used instead of complete fertilizer or NPK fertilizer. You can find out here when the use of PK fertilizers makes sense and how it is done.

PK fertilizer: what exactly is it?

PK fertilizers are complex fertilizers that primarily consist of the salts of two macronutrients: phosphates and potassium salts. It does not contain nitrogen, but – depending on the mixture – further macro or micronutrient elements. PK fertilizers are usually mineral fertilizers. They are mainly used by professionals and, more rarely, by amateur gardeners.

Which PK fertilizers are there?

The following table shows various PK fertilizers and their composition. Because the content of nutrients in fertilizers is always related to a common chemical form, P2O5 stands for phosphorus, K2O for potassium, MgO for magnesium, S for sulfur, and CaO for calcium.

fertilizer P2O5 K20 MgO S. Cao
Thomaskali / Thomas flour 8th % 15% 6% 4%
PK plus 12% 24% 2% 7% 14%
Patent-PK 12% 15% 5%

In addition to the solid fertilizer mixtures shown above, there are several other mixtures with phosphate contents of up to 25% and potassium oxide contents of up to 40%, especially for professional use. In addition, you can mix your own PK fertilizers from various single nutrient fertilizers:
The 40s, 50s, and 60s potash, potassium magnesia, and potassium sulfate can be mixed with superphosphate, Novaphos, Thomas phosphate, or Hyperphos without any problems.

How do PK fertilizers work?

Phosphorus and potassium are essential nutrients that plants have to consume in relatively large quantities in order for normal and healthy growth to be possible. Potassium is essential as a mediator in building up sugar, starch, and entire cell walls. It is also indispensable in maintaining the flow of water that supplies the plant and making it frost hardy. Phosphorus forms a basic building block of the energy carrier ATP and is needed to build up the DNA. In addition, it is crucially involved in photosynthesis and promotes root development, blossoms, and fruit formation.

If they are applied, mineral PK fertilizers quickly dissolve in the soil solution. However, to a large extent, phosphorus also precipitates again very quickly as calcium phosphate, aluminum phosphate, or iron phosphate. So it turns back into an undissolved salt that cannot be absorbed by plants. Plants and other soil organisms actively acidify the soil in order to obtain


The situation is different with potassium: it remains in the soil solution as a positively charged cation until it is retained in the soil by a negative surface charge – for example on clay minerals or humus. However, a certain amount of potassium is always dissolved in the water and is available to the plants.

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Advantages and disadvantages of PK fertilizers

Unfortunately, PK fertilizers are almost always mineral. Since the breakdown of fossil rock phosphates is to be viewed critically, we do not recommend the use of phosphate-based PK fertilizers. Potassium, on the other hand, is still available in large quantities in fossil form for many centuries and is often in short supply of inorganic fertilizers. In this respect, the use of potassium-based PK fertilizers makes perfect sense. However, both P and K in mineral form have a problem: P changes chemically in the soil, accumulates, and is no longer available to plants. Only about 50% of the fertilized phosphorus can be used. In the case of potassium, it is exactly the other way round: it can easily be washed out of sandy and acidic soils, which means that it can no longer be used.

The clear advantage of PK fertilizers, however, is the following: Since it does not contain nitrogen and P and K are hardly washed out on soils rich in humus or clay, it can be used for stock fertilization without stimulating plant growth with nitrogen. The use is also more flexible than if only rigid NPK ratios can be


Tip: Anyone who promotes the build-up of humus in the garden can use it to optimize the availability of phosphate and potassium. Precipitated phosphate is then better dissolved and potassium is better stored in the soil.

Application of PK fertilizer

If nitrogen does not have to or may not be fertilized – for example in order not to disturb the frost hardiness and fruit formation – you can fertilize with a PK fertilizer. This is useful in late summer or autumn, for example. But even if nitrogen is fertilized in another form – for example as liquid manure, urea, or mineral nitrogen fertilizer – it can be supplemented with a PK fertilizer as required. The use of individual nutrients makes the user more flexibility. Professional users benefit greatly from this flexibility: Since they regularly check the nutrient reserves of their soil and know the individual nutrient withdrawals of the cultivated crops, they can make targeted additional fertilizers. Laypeople who usually lack this complex information, recommend the use of PK fertilizers only after a soil analysis. This shows you how much phosphorus and potassium are in your soil.

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Because PK fertilization by feeling harbors the risk of over-fertilization or a nutrient imbalance in the soil. Both would have negative effects on the plants that grow in it. With mainly organic complex fertilizers such as our Plantura organic fertilizers, your soil can neither be overfertilized nor become impoverished. The reason for this is that mainly organic fertilizers release the phosphate and potassium they contain in a slowly flowing manner and excesses are bound to organic particles with the help of the organisms living in the soil. These surpluses are released again by the hardworking organisms when required.

Tip: Of course, research is being carried out into organic alternatives to mineral PK fertilizers. In addition to phosphorus and potassium, fertilization with wood ash also contains magnesium and a lot of calcium – they have a strong alkaline effect and are therefore not suitable for every soil. They can also be contaminated with heavy metals. However, after further research, they may still become a worthwhile alternative. Fertilizing with banana peel also provides phosphorus and potassium. Banana peels can be composted or fermented in the Bokashi for fertilization.


Using nitrogen-free PK fertilizers is for real professionals! But there is also a more convenient way of doing things: natural organic fertilizers made primarily from organic raw materials supply the soil and plants just as it does in nature. Here microorganisms, fungi, and plants work together to ensure fertile soil and keep the available nutrients in balance.

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