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Potassium Insight

March 9, 2013
By: Darrell Smith, Farm Journal Conservation and Machinery Editor

Drought emphasizes the value of this vital nutrient

If there was an upshot to the drought of 2012 it was that many farmers had a prime opportunity to observe the effects of low potassium (K) levels. "Potassium helps plants manage stress, especially drought, so the effects of low K levels show up faster in dry weather," says Farm Journal Field Agronomist Ken Ferrie.

Because of the dry conditions this past year, plants could not take up the K they needed. When corn plants pulled K out of the lower leaves to transfer it where needs were more urgent higher in the plant, producers saw classic deficiency symptoms.

"As plant cells die, you see yellow color starting on the tips of corn leaves and working down the outer edges," Ferrie explains. "After a while, that color turns from yellow to brown. It looks like someone burned the leaves.

"In soybeans, the outer leaves start to discolor. Then you see a speckling of dead brown tissue through the leaf. In many fields in 2012, corn and soybeans began showing these symptoms when plants were only ankle high."

Plants wilt faster when K levels are low, and wilting sets the stage for lower yield. "The wilting stage is a downhill run," Ferrie says. "A plant can’t be wilted long without sustaining damage.

"With plentiful moisture, you can get by with lower K levels in the soil," Ferrie summarizes. "But you may pay the price if you get a drought."

K serves various functions inside a plant, many of which involve water.

FJ Potassium Insight p28

In corn, potassium deficiency shows up as yellow coloring on the tips of lower leaves and runs down the outside of the leaves. Finally, the tissue turns brown, appearing as if the leaf had been burned.

"Potassium’s biggest role is to maintain turgor pressure by regulating osmotic pressure in the plant, by moving water around," Ferrie says. "This keeps the plant somewhat inflated. It’s sort of like the blow-up snowmen you see in yards at Christmas. The water in a plant functions like the air in the snowman, and the potassium functions like the pump that supplied the air."

K regulates water loss by opening and closing stomata in the leaves. The stomata allow water to exit the plant through a process called transpiration. Water moves from the soil through the roots and upward through the plants. The water carries nutrients upward through the plant to be used in photosynthesis. As water moves upward, it also cools the plant.

With high temperature and low humidity, plants might lose water faster than the roots can replenish it. This triggers the stomata to close and the leaves to roll up, reducing the rate of transpiration. "If the potassium level is inadequate, the stomata do not close," Ferrie says. "Rather than rolling, the leaves wilt and the plant overheats."

Regulate water loss. "In a corn plant, there are 10,000 stomata openings in a leaf area the size of a dime," Ferrie says. "Most of the openings are on the bottom of the leaf. The plant opens and closes stomata by moving water in and out of guard cells. This process is driven by potassium. You need a high concentration of potassium in the leaf to open and close the stomata according to the plant’s needs."

Closing of stomata results in the corn leaves rolling, the visible sign that plants are conserving moisture and fighting the effect of drought.

"When the stomata cells close, they are longer than when they are open," Ferrie explains. "Because most of the stomata are on the bottom side of the leaf, the leaf rolls. In the right conditions, you can see this happen in 15 minutes to an hour. The plant is protecting itself. But if there is not enough potassium, the stomata can’t close, so the plant leaks water through transpiration. The top of the leaf remains exposed to the sun, it gets hotter and wilts, and you get cell damage."

K deficiency in corn usually leads to standability problems and lodging. The nutrient plays a role in building vascular bundles, which hold xylem and phloem tubes—the stringy part of corn stalks through which water and nutrients move up and down.

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FEATURED IN: Farm Journal - March 2013

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