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. 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. 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.
Potassium serves various functions inside a plant, many of which involve water.
Potassium’s biggest role is to maintain turgor pressure by regulating osmotic pressure in the plant, by moving water around. 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. “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. 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. 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.
In soybeans, when stomata close, it looks as if the three trifoliate leaves are folding inward on each other. A field may look normal, and then two hours later you can see the back side of the leaves. That’s equivalent to corn leaves rolling.
In plants, K helps facilitate the movement of nutrients and water.
“Potassium plays a big role in photosynthesis and respiration,” Ferrie says. “If a plant is deficient in potassium, both processes slow down.”
In plants, potassium aids in:
- Cell turgidity and osmotic pressure
- Photosynthesis
- Respiration
- Drought tolerance
- Winter hardiness
- Disease resistance
- Insect tolerance
Potassium to the rescue
K also activates more than 80 enzymes that trigger important plant functions. Sufficient K levels also help plants ward off disease.
The vital nutrient aids flowering and kernel and pod set. Corn plants that are grown in soil that is low in K have a higher risk of missing their pollination nick in drought years.
Plants take up K throughout the vegetative stage. During the early vegetative stage they take up more K than it can use and stores the excess. It’s an unusual nutrient in several ways.
Unlike other major nutrients, potassium is not a component of proteins or carbohydrates in the plant. It does not convert to other compounds. It enters and exits the plant as a potassium ion.
Most of those K ions are found in the plant’s saps and juices. Because of that, K can literally leak out of plants if leaves are shredded by hail.
Inside the plant, potassium is mobile. If the plant’s daily uptake doesn’t meet its needs, it will move potassium from the oldest part of the plant to the newest. That’s why deficiency symptoms show up first in the lower leaves.
Only about 40 lb. of K per acre leave the field in a 200-bu. corn crop. But to grow that crop, plants remove 240 lb. or 250 lb. per acre of K from the soil. Most of it is returned to the field in the crop residue.
Potassium in the soil
Hence, in dry conditions, it’s harder for plants to take up the K they need. Clay lattices, or layers in the soil, can collapse during drought periods trapping K ions in the soil between the layers—another reason for K deficiencies in 2012.
In the soil, K differs from nutrients such as nitrogen and phosphorus. Those nutrients cycle through the soil in organic (unavailable) and inorganic (available) forms, converted from one form to the other by microbes. K is in the mineral form. It must go through a weathering process to become available to plants.
In many soils, there’s typically 40,000 lb. of K per acre in a slice of soil 6" deep. But most of it is bound up in a non-exchangeable form that plants can’t get to. “This potassium must be processed, or weathered, into the exchangeable form by acids given off by microbes or plant roots,” Ferrie says.
“Typically, 500 lb. to 600 lb. of the total amount of potassium will be in the exchangeable form,” Ferrie continues. “This exchangeable potassium is like a cloud of K ions, which, because of their positive electrical charges, are held close to the organic matter in clay particles, which carry a negative charge.
“The K ions farthest from the clay particle diffuse into the soil solution. They are held with water in the macropores, or large air spaces, between soil crumbs. This is the soluble potassium that plant roots and soil microbes are able to access.
“At any given time, there will be about 6 lb. of soluble potassium available for plants to use. As plants remove this potassium from the soil, more exchangeable K ions diffuse into the soil solution.
“The 6 lb. of potassium is less than one day’s requirement for an acre of 200-bu. corn. If we don’t have good growing conditions to constantly replenish the supply of soluble potassium, deficiencies show up in the crop. That’s what happened in 2012,” he says.
How to prevent K deficiency
One way to reduce the chances of K deficiency is to maintain soil test levels. At low or moderate levels, we are almost guaranteed to have a deficiency if we have prolonged dry weather.
Another way is to prevent and remove soil compaction. It’s similar to drought in that soil particles collapse, so potassium is held tightly and cannot get into the soil solution. With comparable levels of soil fertility, plants growing in compacted soil will show potassium deficiency sooner than plants growing in un-compacted soil.
During the growing season, tissue analysis will reveal deficiencies. You also can do a visual inspection to check how well your fertility program is working. First, look for symptoms on the lower leaves.
Later in the season, as plants move into the reproductive stages, split a stalk. Starting at the ear, look inside the stalk for a Styrofoam appearance. A deficiency shows up as ‘cotton pith’—what you’re seeing is cells from which K (and also nitrogen) have been drawn out. This is a natural process in which, at the reproductive stage, the plant translocates potassium to areas where it’s needed.
The farther down the stalk you see cotton pith, the more K has been pulled. At pollination, you want to see three solid nodes at the bottom of the plant with no cotton pith.
How Potassium Finds Its Way to Plants
In soil, roots find some potassium by intercepting it as they grow, says Farm Journal Field Agronomist Ken Ferrie.
Nutrients in the soil solution are carried to the plant as water is taken up. This is called mass flow. The potassium ions are carried along like a leaf flowing with water.
As plants take up potassium ions from the soil solution, other exchangeable potassium ions move from their positions around soil particles into the soil solution. This process is called diffusion—from an area of higher concentration into an area of lower concentration. It’s like a drop of food coloring spreading out in a glass of water.
The three processes occur simultaneously in the soil.
Types of Potassium Fertilizer
About 90% of the potassium fertilizer applied by farmers is muriate of potash (potassium chloride—KCL). Because it contains 60% potassium, farmers know it as 0-0-60.
Another form is potassium sulfate, which contains sulfur (0-0-50-18S). A third form is potassium magnesium sulfate (K2SO4-2MgSO4), which contains potassium, sulfur and magnesium. Farmers know it as K-MAG. Its analysis is 0-0-22-22S-11Mg. Finally, there is potassium nitrate (KNO3), with an analysis of 13-0-44.
Potassium comes in three colors, notes Farm Journal Field Agronomist Ken Ferrie. Along with white potassium, there is red potassium, which results from iron impurities not removed during process, and pink, which is a mixture of the two. Plants don’t care which color you apply, Ferrie says; but if you apply K through a center pivot irrigation system or foliar feed through a sprayer, you must use white because the impurities in red potassium might plug the nozzles.
