Potassium fertilisers: types, their role in plant nutrition and when to apply them
· Valerii Korolko · 5 min read
What potassium fertilisers are: basic information
Potassium is one of the three main nutrients that plants consume in large quantities. It does not build the structure of cells, as nitrogen or phosphorus do, but it affects almost all physiological processes in the plant:
- Regulation of water balance: it affects moisture evaporation, which allows the plant to use water more efficiently and reduces losses in drought conditions.
- Resistance to diseases and stress: it strengthens plant immunity. Plants that receive enough potassium better tolerate cold spells, drought, fungal diseases and pests.
- Formation and ripening of fruit: it activates the enzymes responsible for the accumulation of sugars and starch. As a result, fruits become sweeter, more fleshy and keep better after harvest.
- Stimulation of photosynthesis and metabolism: it promotes the formation of chlorophyll and activates enzymatic reactions. This means that energy is converted more efficiently and plant growth accelerates.
- Improvement of crop quality: the weight of fruit and seeds increases, the content of vitamins and dry matter rises, and the number of deformed or damaged fruits decreases.
Without potassium plants weaken, leaves turn yellow at the edges (marginal necrosis), shoots become thinner, and fruits fail to set or ripen.
Types of potassium fertilisers and their features
There are various types of potassium fertilisers, which differ both in form and in their effect on the soil:
- Potassium chloride (KCl)
- Potassium sulphate (K₂SO₄)
- Potassium salt
- Potassium magnesia (K₂O + MgO)
- Natural potassium fertilisers
It is important to choose correctly which potassium fertilisers to use for a particular crop. For crops sensitive to chlorine, it is better to use potassium sulphate or natural potassium fertilisers. For less demanding ones, potassium chloride.
When to apply potassium fertilisers for maximum effect
When to apply potassium fertilisers is a question that directly affects the effectiveness of plant nutrition:
- Autumn application – especially relevant for chloride fertilisers, since over the winter the chlorine has time to be washed out of the soil without harming the plants. It is suitable for the main fertilising of fields and orchards.
- Early spring – the time to apply chlorine-free forms of potassium (potassium sulphate), when plants need starter feeding.
- Growing season – for foliar or root feeding if there are signs of potassium deficiency (yellow leaves, slowed growth, weak flowering).
For perennial crops, such as grapes, apple trees and apricots, fertilisers are applied in advance – in autumn or early spring. For vegetables – depending on the growth stage, best of all when digging over or into planting holes at planting.
Potassium fertilisers should not be applied "at random". An excess of potassium can upset the balance of other elements (for example, calcium or magnesium), which will adversely affect fruit quality. Always take the soil type into account – light sandy soils need more frequent application in small doses, clay soils less often but in larger portions.
How to apply potassium fertilisers correctly
To get the maximum benefit, you need to know how to apply potassium fertilisers correctly:
- When digging over – the classic method for main application (in autumn or spring).
- Into the planting hole – for vegetable crops or tree saplings.
- Foliar spraying – quick help in case of potassium deficiency or during periods of stress.
Natural potassium fertilisers: an eco-friendly alternative
For those who practise organic farming or simply want to reduce the chemical load on the soil, there are natural potassium fertilisers. They act gently, do not accumulate toxic residues and improve soil structure.
The most common natural sources of potassium:
1. Wood ash: contains up to 10–12% potassium in an easily absorbed form and is rich in calcium, phosphorus and magnesium; it is used for feeding vegetables, berries, trees and even houseplants.
2. Compost from potassium-rich plants: cut nettle, horsetail, beet tops, banana peels. After decomposition in the compost, the potassium becomes available to plants.
3. Green manures (siderates): some plants (mustard, phacelia), after growing and being ploughed into the soil, decompose and release useful substances, including potassium.
4. Bird droppings and manure: in small doses, especially well-rotted, they contain available potassium. Use with caution – an excess of nitrogen or salts is possible.
Advantages of natural potassium fertilisers: they do not contaminate the soil with chlorine or heavy metals, preserve soil fertility for longer, promote the multiplication of beneficial microflora, and are ideal for ecological and home farming.
MKP and potassium nitrate (saltpetre) from Prayon – what are these fertilisers and how are they useful?
These fertilisers are not purely potassium ones, but contain potassium and phosphorus or potassium and nitrogen – that is, they are combined fertilisers that act in a complex way.
Monopotassium phosphate (MKP, KH₂PO₄)
🔹 Composition: potassium (K₂O) ~34%, phosphorus (P₂O₅) ~52%.
🔹 Chlorine-free, highly soluble.
Role and benefits:
- A source of pure potassium and phosphorus – ideal for critical stages (flowering, fruit formation).
- Promotes growth of the root system, flowering and fruit formation.
- Increases resistance to diseases and unfavourable conditions.
The best time for application is the period of budding, flowering and fruit filling. It is often used for foliar spraying – a quick effect without soil salinisation.
Suitable for: grapes, tomatoes, cucumbers, fruit trees, berry crops.
Potassium nitrate (saltpetre, KNO₃)
🔹 Composition: potassium (K₂O) ~46%, nitrogen (N) ~13% in nitrate form.
🔹 Chlorine-free.
Role and benefits:
The combination of potassium and readily available nitrate nitrogen gives a powerful boost to growth and improves fruit quality, colouring and dry matter content. Ideal for fruit and berry crops, especially during the period of active growth and fruit filling.
Application period – spring and summer, especially at the fruit formation stage.
- For drip irrigation or spraying (in open and protected ground).
Suitable for: berries, potatoes, tomatoes, peppers, salad crops, grapes.
Tips for use in a nutrition system
| Fertiliser | Best time of application | Main effect | What to combine with |
|---|---|---|---|
| Monopotassium phosphate (MKP) | flowering period, fruit filling | potassium + phosphorus, improved crop quality | alternate with calcium fertilisers |
| Potassium nitrate | fruit filling, summer | potassium + nitrate nitrogen, growth and sugars | with phosphorus fertilisers and microelements |
If you need high-quality Belgian mineral fertilisers – contact our managers! We are always in touch and ready to help.
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