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What happens when you overfertilize your plants with Potassium

August 27, 2021 3 min read


Without potassium, plants quickly die but an excess of the mineral can be equally fatal. 

Along with phosphorus and nitrogen, potassium is one of the three plant macronutrients. These are the three elements that plants need in the greatest amounts. Plants cannot take up all forms of potassium: only the potassium ion K+ can be absorbed by plants. Ions with a positive charge like K+ are known as ‘cations’. 

Read on for what happens to your plant when it receives too much potassium.

Potash mineral

Fertilizer potassium comes from mineral potash (above). Potash is mined in Canada, Russia, Mexico, and Utah

What does potassium do?

Bahia potassium experiment
Bahia Grass growing in different levels of phosphorus and potassium.
State Library and Archives of Florida

Potassium is responsible for many critical plant functions: potassium ions activate enzymes in the plant that drive chemical reactions and they contribute to protein synthesis, especially the protein polypeptide. Polypeptide is a type of plant hormone that allows for cell-to-cell communication. 

Potassium also allows for Co2 assimilation and is essential in maintaining a high pH in the chloroplast, something required to turn light energy into chemical energy for plant functions. 

Cabbage potassium deficiency

Potassium is crucial for plant growth. The cabbages on the left are deficient in potassium. Alandmanson

How much potassium is too much? 

Potassium toxicity in plants is rare because the uptake of potassium ions from the is regulated well by plants. Further, plants tend to have a high tolerance to potassium. When there is too much potassium in its tissues, plants will use anions (negatively charged particles) to counterbalance the potassium’s cations (positively charged ions) and move any excess into the cytoplasm. 

This means that plants won’t immediately die if they take up more potassium than they expend on performing life functions. At normal levels, potassium concentration in plants varies between 0.2 and 2.5 % but can reach as high as between 3-8%. 

Only give potassium-rich fertilizers to plants that need high levels of the nutrient and always follow manufacturer instructions on the amounts you should use.

What happens when a plant takes up too much potassium? 

Although extra potassium may not damage plants directly to begin with, it will eventually have serious indirect effects on overall plant nutrition. 

Excess potassium affects overall plant nutrition by preventing the plant from taking up other mineral nutrients, in particular magnesium, iron, zinc, and calcium. This phenomenon is called ion antagonism or cation competition (cation refers to positively charged ions), where the presence of one element limits the absorption of others. A potassium deficiency encourages plants to absorb substitute minerals at a high rate (especially magnesium) while a potassiumexcess will stop other minerals from being taken up. Excess potassium in grazing pasture can be problematic for sheep and cattle farmers for this reason: their animals may become magnesium deficient by eating plants too high in potassium.

The overall nutrient imbalances caused by excess potassium will limit plant growth (particularly in the stems and roots) and fruit yield. Excess potassium will affect the most mature tissues first because this is where the mineral has had the most time to accumulate. 

Another damaging effect of excess potassium is lipid peroxidation. This is a chemical reaction that produces free radicals. These free radicals react with, injure, and eventually kill cells. Excess potassium also increases electrolyte leakage, the loss of minerals from cells through their membranes. This is a typical stress response by plant cells. Electrolyte leakage is often used by scientists as a measure of plant industry, for example, freezing temperatures and other stresses. 

What causes potassium toxicity? 

Potassium is already quite abundant in soils so overzealous gardeners can easily add too much in the form of liquid fertilizer.  If in doubt about whether or not to add potassium fertilizer to your soil, get a soil testing kit to know precisely what nutrients are in your soil.

When adding potassium fertilizer to your soil, remember that clay soils generally have greater potassium content while sandy types have less. The structure of clay soil particles mean that they can retain the potassium ions much more effectively meaning you won't have to fertilize as frequently. 

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