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The K in NPK - Potassium: What Does It Do for Plants?

August 09, 2021 3 min read 1 Comment

Potassium the K in NPK

Part of our series on NPK Plant Nutrients

Intro to Potassium

Potassium is one of the three plant macronutrients, the other two being nitrogen and phosphorus. Although plants need thirteen other elements to grow and develop, they need these three in the greatest amounts. 

Plant fertilizers contain these three macronutrients in varying proportions since different species and developmental stages require different levels of nutrition. The NPK ratio you see on bottles of fertilizer indicates the percentage of nitrogen, potassium, and phosphorus it contains.NPK stands for nitrogen, phosphorus, and potassium respectively. A. 5:5:15 ratio would indicate a potassium fertilizer. 

Here are two recipesfor your own home-made, organic potassium fertilizers. 

Tomato potassium
Potassium-rich fertilizers are essential to healthy fruit harvests 
Josephine Baran

Humphry Davy (1778-1827) discovered the chemical element potassium. Martin Heinrich Klaproth (1743-1817) was the first to identify the element in plant sap. Long before potassium was identified as a plant nutrient, people had applied manure and wood ash to soils, noticing that they improved fertility. These substances contained high concentrations of potassium. Nowadays, potassium-rich plant fertilizers are sourced from natural deposits of potassium chloride.

What does potassium do for plants? 

Heavy-flowering plants like orchids need a lot of potassium just before and during their flowering period The LEAF Project

From algae and moss to roses and monstera, all plants need potassium to grow and develop. It is the mineral nutrient required in the largest amount by plants after nitrogen. Fruits and tubers need particularly high doses of potassium. Any deficiency will affect harvest quality. Plants with large flowers like hibiscus, rose, and orchids will also need lots of potassium.

Potassium is quickly taken up by roots and transported around the plant through the xylem and phloem vessels to where they are needed in the plant. All plants need potassium for cell metabolism. They are also an essential ingredient in making protein for the plant. For this reason, it is often present in young developing tissue. Potassium is linked to the absorption of carbon dioxide as potassium concentration increases track increases in carbon dioxide in plants. 

Potassium is integral to the mechanism that pumps other essential elements and sugars around the plant. Potassium ions (positively charged potassium atoms) are especially important in the uptake and transport of sucrose and glutamine, which are directly involved in plant growth and metabolism. Potassium also activates enzymes, molecules that speed up chemical reactions, by altering the shape of the enzyme protein. 

At the level of the whole plant, potassium regulates the opening and closing of stomata. Stomata are small pores in the leaves which allow for transpiration - where water and oxygen in the plant are released into the air and carbon dioxide is taken in. Potassium also maintains turgor pressure - the water pressure that keeps cells taut and the plant overall upright.

Plant organs that need a heavy supply of potassium will be young growing leaves, the reproductive organs, storage cells in roots, and fleshy fruits. Fruits high in water need potassium because potassium is necessary for maintaining osmotic balance  - the optimal pressure needed to balance the entry and exit of minerals and water from the fruit.

What are the signs of potassium deficiency? 

Potassium deficiencyA potassium deficient soybean leaf paling at the edges. Alandmanson 

Potassium deficiency symptoms are very similar to symptoms of nitrogen and phosphorus deficiency. Plant growth will slow when they are low on potassium.  Leaves will discolor around the edges and tips at an advanced stage of potassium deficiency. Leaves will also droop. Because potassium is responsible for the formation of leaf cuticles that keep in moisture, plants will lose water through the leaves at a higher rate. This is why plants that are suffering from potassium deficiency have a high water demand. 

Because a plant deficient in potassium will be less able to maintain high water content in its tissues, the plant will be more susceptible to frost damage and stress from drought. Without potassium, plants are at greater risk from fungal diseases. 

Without potassium, the plant will eventually die.

Where to buy Potassium?

Below are some of our favorite sources of Potassium. (as an Amazon associate we earn from qualifying purchases)


Learn More

1 Response


December 10, 2021

Looking for iron pumpkin fertilizer program. Can you assist me. Or contact nr please.
Southside Plants replied:
We don’t sell iron rich fertilizer. Check with your local nursery!

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