This guide explains what potassium fertilizer does and how to make organic versions at home.  

What is in fertilizers?

All plant fertilizers contain nitrogen, phosphorus, and potassium. These are the macronutrients, the elements found in the largest quantities in plants. 

Nitrogen (N) is critical in chlorophyll and amino acid building. It aids leaf production. A nitrogen deficient plant will send the nutrient to young shoots first and then to mature leaves. Dying mature leaves can be a sign that your plant needs nitrogen. 

Phosphorus (P) is involved in energy transfers for cell metabolism. It comprises the structure of cell membranes and nucleic acids. Stunting, purple leaves, and lesions are signs of deficiency.

Potassium (K) regulates stomata (small pores on the leaves that allow gases to move in and out of plant cells), maintains turgor (water pressure in the plant that keeps it upright), and osmotic equilibrium (the transfer of nutrients in the plant from areas of high to low density). Leaf lesions are a sign of potassium deficiency, with mature leaves dying first. Potassium is particularly crucial in fruit growth.

While plants also require thirteen other elements in smaller quantities, these micronutrients are usually found in sufficient quantities within the soil. Plants need all three macronutrients and thirteen micronutrients to carry out essential physiological maintenance and achieve growth.

How do plants eat?

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All nutrients are taken up from the soil by the root and transported through the plant.

Nutrients are moved through the plant through transpiration, root pressure that pushes water and nutrient ions upwards, and the source-sink phenomenon, which draws water and ions from less active parts of the plant to places that are actively growing, such as shoots or fruit.

What are potassium-rich fertilizers?

Potassium-rich fertilizers contain a higher ratio of potassium compared to the two other macronutrients (nitrogen and phosphorus). Read more on what potassium does for your plant here.

You can identify commercial brands of potassium-rich fertiliser by looking for nutrient percentages like 10:12:22, which means that the macronutrient content is 10 % nitrogen, 12 % phosphorus, and 22 % potassium. 

The ratio figures you find on commercial fertilizer brands are called ‘NPK’ ratios. Each number indicates the percentage of nitrogen, phosphorus, and potassium respectively in the fertilizer.

A 5-5-5, 10-10-10, or 12-12-12 fertilizer contains nutrients in a 1:1:1 ratio to one another. All three of these NPK ratios indicate a balanced fertilizer, containing equal amounts of each macronutrient. The only difference between a 12-12-12 fertilizer and a 5-5-5 fertilizer would be that you need to use less of a 12-12-12 product than a 5-5-5 product since the first contains 7 percent more nitrogen, phosphorus, and potassium. The rest of the product is filler material that does not impact the plant. 

Fertilizers with lower macronutrient percentage figures should be used on smaller plants and those with higher macronutrient percentage figures should be used on larger ones.

Homemade, organic fertilizer recipes tend to contain less concentrated amounts of the target nutrients than commercial, chemically synthesized ones. This lessens the chance of killing plants with over-fertilizing. It is also more environmentally sound, as there is less chance of releasing excess fertilizer into the wild.

When to use potassium fertilizer

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Potassium fertilizers are used for shrubs and plants that produce flowers and fruit. Potassium improves the size, appearance, color, acidity, taste, and vitamin content of fruit. This is because it activates enzymes that produce proteins and sugars. Potassium-rich fertilizers are often on rose plants and orchids. Tomatoes also require high levels of potassium to produce lycopene.

Flowering houseplants like the lipstick plant, hibiscus, or African Violent will require potassium-rich fertilizers during the blooming season. During the reproductive flowering phase, plants will need fertilizers that have a nitrogen to potassium ratio of 1:2. 

Potassium can also be used to increase the alkaline content of the soil. Acidity and alkalinity are measured in pH, which runs on a scale of 1 to 10. pH numbers over 7 indicate alkaline soil, and anything below that indicates acidic soil. Some plants have strong preferences for alkaline soils, such as winter jasmine and Boston ivy.

Too much potassium however will inhibit magnesium uptake. As with all fertilizers, feed less than you think is required if you are unsure of the correct amounts. 

Homemade potassium fertilizer recipes

Recipe for homemade potassium fertilizer #1: Banana

Dry four banana peels and 3 eggshells. Combine them and add 4 tablespoons of Epsom salt. Grind the mixture into a powder in a food blender. Pour 75 ml of water onto the powder, shake to combine, and water your plants with the liquid. 

Alternatively, you can create potassium-infused compost by adding banana peels to your compost bin. 

Recipe for homemade potassium fertilizer #2: Kelp

If you are lucky enough to live near the coast, foraging for potassium-rich kelp may be an option. Make sure you collect kelp rather than seaweed - the latter contains much less potassium. 

To make concentrated liquid kelp fertilizer, fill a bucket with foraged kelp, cover it with rainwater, and leave to soak for a month, stirring every few days. For added nitrogen, add a few stinging nettles stems into the mixture. 

If you would like to experiment with your own organic fertilizer recipes, Oregon State University has compiled a useful table showing average NPK ratios in various organic substances. You can use it to figure out which ingredients you need to create a perfect NPK ratio mixture for your plant.   

If you want to buy Kelp or other Potassium rich fertilizers click on the link below.
(as an Amazon associate we earn from qualifying purchases).

Learn More

• What happens when a plant gets too much Nitrogen?
• The N in NPK - Nitrogen: What does it do for Plants?
• The P in NPK - Phosphorus: What does it do for Plants?
• The K in NPK - Potassium: What does it do for Plants?
• Make a homemade balanced fertilizer recipe
• How to make Organic Phosphorus fertilizer
• DIY - Homemade Nitrogen Fertilizer - Natural sources of Nitrogen for Plants