PK fertilizers mainly provide the plant with phosphorus and potassium. We explain why you need PK fertilizer, how it works and how to use it.
If plants need to be supplied with phosphorus and potassium, but nitrogen would not be beneficial, a PK fertilizer is used instead of a compound fertilizer or NPK fertilizer. You can find out here when it makes sense to use PK fertilizers and how.
PK fertilizer: what is it exactly?
PK fertilizers are compound fertilizers that consist primarily of the s alts of two macronutrient elements: phosphates and potassium s alts. Nitrogen is not included, but - depending on the mixture - other macro or micronutrient elements. PK fertilizers are usually mineral fertilizers. They are mainly used by professionals and less often by hobby gardeners.
What PK fertilizers are there?
The table below lists 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, K 2O for potassium, MgO for magnesium, S for sulfur and CaO for calcium.
| Fertiliser | P2O5 | K20 | MgO | S | CaO |
|---|---|---|---|---|---|
| Thomaskali/Thomas Flour | 8 % | 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 - especially for professional use - a number of other mixtures with a phosphate content of up to 25% and potassium oxide content of up to 40%. In addition, you can mix PK fertilizer yourself from various straight fertilizers:
40, 50 and 60 potash, potash magnesia and potassium sulphate can be mixed with superphosphate, Novaphos, Thomas phosphate or Hyperphos without any problems.
How do PK fertilizers work?
Phosphorus and Potassium are essential nutrient elements that plants need in relatively large amountsmust be ingested so that normal and he althy growth is possible. Potassium is essential as a mediator in the construction of sugar, starch and entire cell walls. In addition, it is irreplaceable in maintaining the flow of water that feeds the plant and provides frost hardiness. Phosphorus forms a basic building block of the energy carrier ATP and is required for the construction of DNA. In addition, it plays a key role in photosynthesis and promotes root development, flowering and fruit formation.
When applied, PK mineral fertilizers quickly dissolve in the soil solution. However, large amounts of phosphorus also quickly reprecipitate as calcium phosphate, aluminum phosphate or iron phosphate. So it turns back into an undissolved s alt that cannot be absorbed by plants. Plants and other soil organisms actively acidify the soil to obtain phosphorus.
Potassium is different: It remains in the soil solution as a positively charged cation until it is held 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 available to plants.
Pros and cons of PK fertilizers
Unfortunately, PK fertilizers are almost always mineral. Since the degradation of fossil rock phosphates is to be viewed critically, we do not recommend the use of phosphate-rich PK fertilizers. Potassium, on the other hand, will still be available in large quantities from fossil sources for many centuries to come and is often in short supply in organic fertilizers. In this respect, the use of potassium-rich PK fertilizers makes 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. Potassium is the other way around: it can easily leach out of sandy and acidic soils, making it unusable as well.
However, the clear advantage of PK fertilizers is as follows: Since it contains no nitrogen and P and K are hardly washed out on soil rich in humus or clay, it can be used for storage fertilization without stimulating plant growth with nitrogen. Also the usage is more flexible than when only rigid N-P-K ratios can be used.
Tip: Anyone who promotes the build-up of humus in the garden can use it to increase the phosphate andOptimize potassium availability. The 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, for example, in late summer or autumn. 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 flexible. Professional users benefit greatly from this flexibility: since they regularly check the nutrient reserves in their soil and know the individual nutrient withdrawals from the cultivated crops, they can add fertilizer in a targeted manner. For laypersons, who usually lack this complex information, we recommend the use of PK fertilizers only after a soil analysis. This shows you how much phosphorus and potassium are in your soil.
Because PK fertilization by feel carries the risk of over-fertilization or nutrient imbalance in the soil. Both would have negative effects on the plants growing in it. With primarily organic compound fertilizers such as our Plantura organic fertilizers, your soil can neither be over-fertilized nor impoverished. The reason for this is that primarily organic fertilizers slowly release the phosphate and potassium they contain and surpluses are bound to organic particles with the help of the organisms living in the soil. These surpluses are released again by the industrious organisms when needed.
Tip: Of course, research is being carried out on organic alternatives to mineral PK fertilizer. In addition to phosphorus and potassium, fertilizing with wood ash also contains magnesium and a lot of calcium - so 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 become a worthwhile alternative. Fertilizing with banana peels also provides phosphorus and potassium. Banana peels can be composted or fermented in the Bokashi for fertilization.
The use of nitrogen-free PK fertilizers is something for real professionals! But there is also a more convenient way: Natural organic fertilizers made from primarily organic raw materials nourish the soil and plants just as they do in nature. Here, microorganismsFungi and plants work together to keep soil fertile and balance available nutrients. In our Plantura Shop you will find organic fertilizers for all requirements.