What You Need to Know About Potassium

01:45PM Nov 09, 2010
1 Slide 20 potassium deficiency in corn leaves
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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.

“We like the pithy appearance to stop before it gets more than three nodes from the bottom of the plant,” Ferrie says. “If the plant has pith all the way to the crown before the black layer is formed, you have a potassium deficiency. Once disease gets in empty cells, it’s like a fire taking off. Stalk rots can move in and take the crop down.”

K in the soil. K that is taken up by plants comes from the soil solution. “There’s only about 6 lb. of exchangeable potassium—the type of potassium available to plants—in the top 6" to 12" of soil at any one time,” Ferrie explains. “That’s only about a day’s supply—a minute amount, considering the crop’s total needs.”

The exchangeable K taken up by plants is replenished from the non-exchangeable portion—40,000 lb. to 50,000 lb. per acre of soil. The non-exchangeable K gradually “leaks” into the soil solution as it goes through the weathering process.

“In order to keep up with plant requirements, we need to apply potassium, to add to the soil-reased K, to replenish the K in the soil solution,” Ferrie says.

How plants take up K. Because most K is stored in stalks and leaves, only about 65 lb. of K per acre is removed in a 180-bu. corn crop. However, to produce that crop, the plants must take up about 250 lb. of K per acre.

Roots come in contact with K in three ways: interception, mass flow and diffusion. In interception, roots grow into new soil, which has not yet been depleted of nutrients—“like a blind sow finding an acorn,” Ferrie says. In mass flow, nutrients are drawn to the roots as they take up
water—“like a leaf floating in the gutter.” In diffusion, nutrients move from an area of higher concentration to an area of lower concentration—“like a drop of food coloring dispersing in a glass of water.”

All three processes happen at the same time. “Because the amount of potassium in the soil solution is very low, we need diffusion and mass flow to get the job done,” Ferrie says.

Once roots encounter K in the soil, the nutrient has to make its way inside. To greatly simplify what actually happens, plant cells expel positively charged hydrogen ions, leaving a surplus of negative ions in the root lining. That attracts positively charged K ions to enter the cells.

The same hydrogen ions that need to be expelled to bring in K are carrier molecules, which bring nitrogen into the plant. “If there’s too much nitrate in the soil, you bring in an excessive amount of nitrate at the expense of potassium,” Ferrie explains.

“It may not be low potassium levels that cause corn to lodge, but excessive nitrogen rates. When you push corn populations and are tempted to increase your nitrogen rate, remember that nitrogen and potassium are interrelated.”

K uptake. If you apply too much K fertilizer, plants will take up more than they need, a situation agronomists call “luxury feeding.” That can create an imbalance with magnesium and calcium, leading to a deficiency of the latter nutrients.

Plants will also luxury-feed on magnesium. If you push soil magnesium levels too high, it can result in a K deficiency in the plants.

“The most important thing about fertilizer is that uptake by the plant is a metabolic process which takes energy,” Ferrie says. “Nutrients don’t just flow into plants; it takes energy to get them inside. So a plant must have a living, breathing, healthy root system.

“The plant takes in carbon dioxide through its leaves. It builds sugars and sends them to the root system. To get the energy needed for nutrient uptake, the root system burns the sugars during cell respiration, and it needs oxygen to do that. It is taking in carbo-hydrates and letting out carbon dioxide, and it needs oxygen to drive that process.”

That’s why corn dies or grows unevenly after being submerged in water, Ferrie explains.
A lack of oxygen is shutting down the metabolic processes required to take up nutrients.

Managing K leaching. K can be leached out of soil by water—up to 30 lb. or 40 lb. per acre per year—especially if the soil has a low cation exchange capacity (CEC), a relative measurement of the soil’s ability to hold nutrients.

In an acid soil environment, there are lots of positively charged ions, such as aluminum and hydrogen, attached to soil particles, leaving no opportunity for K ions to attach, Ferrie explains. So as K ions change from non-exchangeable to exchangeable through the weathering process and enter the soil solution, they are unable to attach to soil particles. These unattached K ions can be leached away by water. “Acid soils leach potassium, so you need to lime soils and keep the pH up,” Ferrie says.

As a crop manager, recognize that your light soils with low water-holding capacity are also more subject to leaching, Ferrie advises. “If you’re in a highly leachable situation, consider applying your K in the spring. Split your K application by applying some with your starter fertilizer and when you sidedress nitrogen.”

Making a K application ahead of every crop, rather than applying enough K for two years in one application, will also reduce leaching and luxury feeding.

Fight K fixation. K can be “fixed” to certain type of clay particles and made unavailable to plants. Soils with high CECs may have most of the K fixed to the exchange sites, leaving very little in the soil solution.

In another type of tie-up, K ions may get trapped between clay lattices when soil dries out and the lattices collapse. “Water is needed for mass flow of nutrients and to prevent soil from collapsing,” Ferrie says. “Both dry weather and soil compaction can cause potassium deficiencies.”

On heavy soils that fix large amounts of K, Ferrie recommends farmers apply K for every crop, just as on leachable soils. “Don’t apply a surplus, which can get tied up in the clay colloids,” he advises.

“Band applications when possible. Banding puts the potassium in contact with fewer clay particles, so it can’t be tied up as easily. You can band potash fertilizer in the spring, or in the fall if you strip-till. You also can add potassium to your starter and sidedress fertilizer.”

K advice for all soils. In any soil, manage your pH. Don’t overlime or underlime. Either situation creates opportunities for K to be leached away by water.

Be aware of and manage any soil compaction in your fields. “Compacted soils tend to have poor potassium uptake, no matter what the level of K in the soil test,” Ferrie says. “You must preserve the ability of potassium to move through the soil solution.”

How plants lose K. “Because potassium is in the cell sap, it can be leached out of the plant when leaves are damaged,” Ferrie says. “This often is overlooked when we assess hail damage. Hail followed by rain can cause heavy losses of potassium out of the plant, from the torn leaves. If you rip a leaf and run water across it, it will actually bleed K right out on the ground. So, on tissue tests, potassium levels can drop drastically after a minor hail situation.

“This may not be a problem if it’s early in the season and the plants didn’t have a lot of leaf destruction. But if the plants have a lot of leaves torn up midway through the season, they won’t be very drought-tolerant after losing potassium in the sap.”

K recommendations. There are various ways to make a K fertilizer recommendation. Some create the recommendation solely on parts per million (ppm) or pounds of K in the soil, as shown on a soil test.

Others adjust that recommendation based on the CEC of the soil. (To do this, the CEC must be included on your soil test.) Others key their recommendation from base saturation. And still others use all three—ppm, base saturation and CEC—to formulate their recommendation.

Depending on only one factor for your recommendation is risky. “If you go strictly by ppm or pounds of K2O, your goal is just to get the soil potassium level to the optimum range,” Ferrie says. “Most laboratories would say you want somewhere around 350 lb. to 400 lb. of potassium per acre. But what is low and what is high depends on a soil’s CEC. And if you base recommendations only on the ppm that is recorded on a soil test, you can’t account for variability of soil types within the field.

“The higher the CEC, the higher the K level must be to ensure enough potassium in the soil solution,” Ferrie says. “For example, 190 ppm of K could be excessive in a soil that has only a 6 CEC. On that soil, you could experience luxury feeding, leading to uptake problems with magnesium and other nutrients. But 190 ppm could be low on soil with a 25 CEC. There, you could encounter potassium shortages in the plants.

“Crop advisers who use base saturation levels for their recommendation like to be in the 3% to 5% range for potassium,” Ferrie says. “Basing K recommendations only on CEC can create problems in low- and high-CEC soils. You could have 85 ppm potassium on the soil test, and that would give you the desired base saturation on a very light soil with 4 CEC. But the potassium level wouldn’t be high enough to meet the demands of the crop.

“And if you have a soil with a CEC of 30, 200 ppm of K may not be enough to reach 3% to 5% saturation. On that soil type, it may not be financially feasible to put on enough K to reach the saturation level you need. There, you may have to band your K application, apply it in strips or with your starter or sidedress fertilizer to keep it available.

“You need to consider the CEC or soil type, and how much of the potassium you apply is going to be fixed versus soluble,” Ferrie says.

Soil test accuracy. Laboratories use different procedures to test for K, but the results are fairly similar across all procedures. However, they also use different procedures for calculating CEC, which can vary fairly dramatically depending on the procedure used.

“So, if you are using CEC to calculate potassium needs, make sure you use the same laboratory, or the same CEC extraction procedure, to determine changes in your soil,” Ferrie advises.

“Most laboratories will also have their own standards for high, medium and low ratings, based on which extraction procedure they use,” he adds.

Facts about Potassium

  • Unlike nitrogen, potassium (K) does not convert to other compounds inside plants or animals. It always remains as potassium ions.
  • 90% of the potassium fertilizer used on farms is muriate of potash (0-0-60).
  • Potassium sulfate (0-0-50-18S), also called K-Mag or Sul-Po-Mag, is the second most popular potassium fertilizer.
  • Whether muriate of potash is white, red or pink in color doesn’t matter to crops. But the white form—which contains the fewest impurities—must be used for fertigation, to prevent the fertilizer from plugging irrigation equipment.
  • Symptoms of potassium deficiency are similar to those for nitrogen deficiency. But with potassium deficiency, yellowing runs down the outside of the leaves. With nitrogen deficiency, yellowing starts at the tip and goes down the leaf’s midrib.
  • Potassium was one of the first products American colonists could sell for cash. They cleared trees, burned the stumps and scraps, boiled the ashes down and shipped the remaining product to England. It was called “pot ash” or, later, potash.
  • The first patent ever issued in the U.S., signed by George Washington, was for an improved method of making potash.
  • Potash mining began in the 1860s, when deposits were discovered in Germany.
  • Most potash comes from deposits that were laid down by ancient seas.
  • The discovery of potash deposits in New Mexico in 1925 led to what became a thriving North American potash industry.
  • Today, most potash supplies come from mineral deposits in Canada.
  • At present levels of production, there are enough potassium sources to last the world for 600 to 1,000 years. The only issue is meeting demand on a yearly basis.
  • Potassium is represented on soil tests with a “K.” It comes from the Latin word for potassium, kalium. Phosphorus already had dibs on the letter P.