3/17/2022 - By Lynn Griffith

Potassium is one of the three major, or macro, nutrients for plants. It’s represented by the third number on the fertilizer bag. Some will include calcium, magnesium, and sulfur in the macronutrient category, as well. But clearly, nitrogen, phosphorus, and potassium are the mineral nutrients present in the largest quantities in plants.

Why Potassium Matters

Potassium is known to have more than 200 functions in plants, including enzyme activation, sugar transport, water absorption, flower and fruit quality, and many others. One of the most important potassium functions in ornamentals and other plants is that it has an important role in guard cell functions. Guard cells are the cells surrounding the stomates in leaves. These are the pores in leaves that regulate moisture and gas exchange.

When potassium is deficient in plants, the stomates don’t open and close properly. Deficient plants exposed to heat, drought, or low humidity stress can’t “close the window.” This often results in numerous necrotic spots in the older leaves, as well as leaf tip burn in many varieties, or diminished fruit and flower quality . Potassium is very mobile in plants. When deficient, plants will steal potassium from the older leaves and move it into the new leaves. Therefore, a common symptom of potassium-deficient plants is numerous spots or tip burn in the older leaves, while the newer leaves look relatively normal. Drought or heat stress aggravates potassium deficiency.

You hear the terms “potassium” and "potash" used interchangeably, but they are really not the same thing. “Potassium” refers to the element, signified by the chemical symbol “K,” atomic number 19. The term “potash” generally refers to alkaline potassium compounds such as potassium carbonate or hydroxide. The number on the fertilizer bag technically refers to potash, or K2O, not elemental K.

The major sources of potassium in fertilizer are potassium nitrate, potassium sulfate, potassium chloride (muriate of potash), and sul-po-mag or K-mag. Potassium is readily leached from both field soil and container media via both irrigation and rainfall. It doesn’t stick around long in wet conditions. Therefore, coating potassium fertilizer sources in controlled-release forms such as POLYON® is a great way to maintain potassium levels long-term in ornamentals.

Correct Doses and Products

Plants that create flowers and fruits generally have higher potassium requirements than trees, shrubs, or houseplants grown primarily for their foliage. Good ranges for potassium in potting media are 60 to 80 ppm or higher. Ideal potassium levels in the leaf tissue are often 1.5% to 4%, sometimes even as high as 6%.

Since 1% is 10,000 ppm, a 4% potassium level in leaf tissue is 40,000 ppm. That is 500 times higher than a desirable 80 ppm in potting media. Plants are great concentrators of potassium.

Great sources of potassium from the Harrell’s line include coated and uncoated k-mag, potassium nitrate, potassium sulfate, Harrell’s 4-0-8, and Stress Relefe , among others. Just keep in mind that potassium is readily absorbed by most plants via foliar sprays of soluble potassium sources. For more information on potassium, potassium products, and how they can better improve your plants, reach out to your local Harrell's Representative.