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What You Need to Know About Potassium

November 9, 2010
By: Darrell Smith, Farm Journal Conservation and Machinery Editor
1 Slide 20   potassium deficiency in corn leaves
Potassium deficiency begins at the tip of a leaf and runs down the outside, resembling a burning process.  

The more you know about each nutrient necessary for crops to grow, the better you can manage them to produce higher yields. At last summer’s Farm Journal Corn College, Ken Ferrie, Farm Journal Field Agronomist, focused an entire presentation on potassium (K), one of the three major nutrients you need to grow a top crop.

What K does. K plays a major role in regulating the water pressure and water movement in plants, Ferrie explains. “In field crops, potassium is important for managing water and standability,” he says. “When K deficiencies occur, lodging can happen. Photosynthesis and respiration are somewhat controlled by potassium. Because potassium has a role in water movement, it affects the movement of carbohydrates throughout the plant.

“Potassium is the activator in more than 80 essential enzyme reactions within the plant. It’s a big part of how a plant manages environmental stress. Fields that are low in potassium tend not to be able to handle stresses, such as drought tolerance, winterhardiness in perennial crops and resistance or tolerance to disease and insects.”

In the plant, K is found mainly in the cell sap. Unlike some nutrients, such as nitrogen, K does not form other compounds, but remains as a K ion in the cell solution.

“Potassium is required for cells to maintain internal pressure, so plants don’t wilt,” Ferrie says.

“A study in a Hawaiian sugarcane field showed 18 lb. of K per acre was more effective at preventing plants from wilting than changing the irrigation cycle from 15 days to seven days.

Potassium plays a role in taking water into plant cells. The positive charges on potassium ions draw in the negative charges on water molecules. If potassium moves out of the cells, it draws water out.

“Potassium also regulates the leaf stomata, the openings which allow water vapor to escape. Potassium moves in or out of the ‘guard cells,’ drawing water along with it. When the guard cells fill with water, the stomata open, allowing oxygen to enter the plant and water vapor to leave,” Ferrie says.

“When water leaves the guard cells, it causes the openings to close. If water leaves the plant faster than it comes in, the stomata close to protect the plant from wilting and overheating.

“The closing of stomata on the bottom of the leaves makes the bottom surface area larger than the top. That’s what causes corn leaves to roll and soybean leaves to tilt up during dry-weather periods,” Ferrie explains.

K in the plant. Early in the season, plants take in more K than they need at the time and store it for later use. Plants can store K in much greater concentrations than is available to them in the soil solution. (That’s a good thing, as we’ll explain later.)

If a plant needs more K later in the season, it moves the nutrient from the oldest part of the plant to the newest. The element moves easily because it is in the cell sap. If too much K is pulled from the lower leaves, plant cells get weak, letting disease organisms move in.

“The highest level of potassium should be in the newest growth at the top of the plant,” Ferrie says. “You can’t scout for potassium deficiency from the road, because the symptoms are down lower on the plant. The deficiency starts at the tip of a leaf and goes down the outside. It’s like a burning process. The plant is cannibalizing its lower leaves to supply the newer tissue that is higher on the plant.”

When K is removed from storage in the stalk, it leaves empty cells that look like Styrofoam or cotton pith, which is visible when you split the plant. This appearance starts at the ear node and runs up and down the stalk in both directions. Too much of this pithy appearance as corn is finishing pollination and coming into the blister stage indicates a problem.

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FEATURED IN: Farm Journal - October 2009
RELATED TOPICS: Agronomy, Corn College, Production

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