Written by Mike Hamilton, CCA & President of Turf Dietitian
“I’m the Diva of Plant Nutrients”
Potassium is a vital component for the healthy development of plants. Dissimilar to nitrogen and phosphorus, Potassium doesn’t get utilized in any plant’s structural formation. Potassium is found within the plant cell solution and is responsible for many life-sustaining metabolic functions.
Here’s a good analogy for Potassium: Wood, concrete, shingles, windows, doors, and hardware are all materials used to build a house, but the paint, carpet, drapes, decorations, and furniture give the home its magnificence. Potassium is the nutrient that provides a plant with its magnificence.
We all learned about Liebig’s Law of Minimums in school, so I can’t say Potassium is any more essential than cobalt. But, Potassium is involved in more plant processes than any other nutrient besides nitrogen.
The more I learn about plant Physiology, the more I’m amazed at how similar plants and animals are in their cellular development, structure, and interaction.
One of Potassium’s primary responsibilities in a plant cell is to execute the respiration and transpiration processes throughout the plant. If a plant is deficient in Potassium, it is also inadequate in the amount of food it digests, the amount of water it drinks, and it can’t pee or poop properly. If you’re as old as me, you’ll understand how vital that last part is.
Potassium is vital to many plant processes. A review of its role involves understanding plants’ basic biochemical and physiological systems. While K doesn’t become a part of the chemical structure of plants, it plays many important regulatory roles in development.
THE PRIMARY ROLES OF POTASSIUM WITHIN THE PLANT:
- Stomatal Activity (Water Use
- Assist in the production of ATP
- Enzyme Activation
- Transport of Sugars
- Water and Nutrient Transport
- Protein Synthesis
- Starch Synthesis
- Turf Quality
- Increases root growth and improve drought resistance
- Maintains turgor; reduces water loss and wilting
- Builds cellulose
Photosynthesis and Food Production
The role of Potassium in photosynthesis is both critical and complex. Potassium initiates the necessary enzymes that produce adenosine triphosphate (ATP) during photosynthesis. ATP is the energy source for many chemical reactions that occur in plants. The electrical charge maintains balance at the site of ATP production with potassium ions.
Potassium also triggers enzymes during photosynthesis to combine carbon dioxide and water to form sugars that feed plant cells. When the plant is deficient in Potassium, the production rate of ATP and sugar is reduced proportionately to the deficiency. Conversely, plant respiration increases which also contributes to slower growth and development.
How Important and what is ATP
All life begins and depends on a miraculous enzyme called adenosine triphosphate (ATP). ATP is the world’s first and smallest biological rotary motor. Every living cell performs the ATP synthesis reaction about one million times per hour.
ATP is the energy source that sustains life and powers all metabolic functions of a plant cell, including manufacturing DNA, RNA, proteins, clean-up of debris, and transporting chemicals into, out of, and within cells.
ATP synthesis executes the relocation of electrons from the internal membrane, through the membrane tissue, and back into the inner membrane. The transfer of electrons in and out of the plant tissue leads to a substantial pH difference between the two sides of the membrane. The outer side of the membrane is typically equal to the optimum uptake pH of each plant species. The ATP reaction pushes positive charged ions and acids out in exchange for negative charged ions and alkalis.
Although several steps are involved in creating an energized cell membrane, there’s a simple hypothesis. When the water molecule (H2O) splits inside the cytoplasm of the cell, the positive protons (H+) get pushed out of the cell membrane, and the negative hydroxide ions (OH–) get pulled inside the cell membrane.
An ion’s charge makes it impossible for them to cross the membrane independently. Trapping the ions on either side of the membrane creates two things, a pH and charge difference, that make the proton motive force. A difference in charge on the inside and the outside of a cell is called an electrochemical potential and is a huge energy source.
Stomatal Activity (Water Use)
Potassium synchronizes the opening and closing of stomates. The stomatal opening allows the leaves to exchange carbon dioxide, water vapor, and oxygen with the atmosphere. Proper functioning of stomates is essential for photosynthesis, ATP synthesis, water and nutrient transport, and plant cooling.
When Potassium travels into the guard cells around the stomates, the cells accumulate water causing the pores to open. The stomate opening properly allows gases to move freely in and out of the plant. When the water supply is short, Potassium moves out of the plant through osmosis. When potassium levels in the plant are adequate, the pores close tightly to prevent water loss and minimize drought stress. When potassium levels are low in the plant, the stomates open slowly, causing water vapor to escape into the atmosphere. Closure may take hours rather than minutes and is incomplete. As a result, plants with an insufficient supply of Potassium are much more susceptible to water stress.
Accumulating K in plant roots produces a gradient of osmotic pressure that draws water into the roots. Plants deficient in K are less able to absorb water and are more subject to stress when water is in short supply.
Potassium “activates” at least 60 different enzymes involved in plant growth. Enzymes serve as catalysts for thousands of chemical reactions in the plant. Once activated, the plant utilizes enzymes to achieve chemical transformation but won’t get consumed. Enzymes bond molecules together in such a way that a chemical reaction can take place.
Potassium also deactivates several organic anions and other complexes within the plant, helping to buffer pH between 7 and 8 (which is optimum for most enzyme reactions). The volume of Potassium in the cell influences the number of enzymes activated, which controls the rate at which chemical reactions occur.
Transport of Sugars
The carbohydrates and starches produced in photosynthesis must be transported through the phloem tubes to all plant cells; for utilization and storage.
The plants’ transference system uses energy created by ATP, which Potassium initiates. If potassium levels in the plant are insufficient, there is less ATP. Therefore, the transference system breaks down. The slowing of carbohydrate movement through the plant causes sugars to build up in the leaves, slowing the rate of photosynthesis. An ample supply of Potassium in the plant helps keep all these processes functioning normally.
Water and Nutrient Transport
For all the reasons mentioned above, Potassium plays a significant role in transporting water and nutrients throughout the plant in the xylem tubes when potassium levels are low in the plant, nitrates, phosphates, calcium, magnesium, and amino acids translocation decreases.
Plants require Potassium for every major step of protein synthesis. When plants are deficient in Potassium, proteins won’t synthesize regardless of the nitrogen content. Without an adequate supply of potassium amino acids, amides and nitrate accumulate.
Potassium activates the enzyme responsible for the synthesis of starch. If Potassium is in short supply, the level of starch declines, while soluble carbohydrates and nitrogen compounds accumulate, making the leaf succulent.
As you can see by all that Potassium is responsible for, it plays a significant role in enhancing turf quality. Adequate levels of available Potassium improve physical quality, disease resistance, and plant vigor. Even short periods of deficiency, especially during critical developmental stages, can cause a severe decline in turf quality.
Improves Root Growth and Drought Resistance
During drought stress, root growth and the rates of potassium diffusion in the soil significantly reduce. Maintaining adequate plant potassium is, therefore, critical for plant drought resistance. However, because soil solution is minimal in drought-affected soils, Potassium should be applied as a foliar feed.
Improving crop tolerance in low-moisture soils is worthwhile by inducing deeper rooting, larger absorption surfaces, and greater water retention in plant tissues. Deeper rooting is achievable by the deep placement of potassium fertilizer.
POTASSIUM IN THE SOIL
The behavior of Potassium in the soil is determined more by physical and environmental factors than by chemical or biological processes.
Because Potassium is a small ion (1/2 of the size of calcium) and has only one positive charge attached (unlike Ca++ and Mg++, and Fe++), it leaches quickly through the soil. Conversely, for the same reasons, Potassium can get trapped in tiny pore spaces in clay and heavy organic matter soils. Trapped Potassium can act like a slow-release fertilizer by leaking small amounts into the soil when the soil reaches saturation field capacity.
Soluble and exchangeable and non-exchangeable Potassium make up the pool of available Potassium. Unfortunately, commonly available soil tests do not evaluate the non-exchangeable component. Plants grown in clay or with heavy organic accumulations may receive Potassium even when soil tests indicate a deficiency.
The only way to determine if the plant is receiving enough Potassium is to tissue test frequently.
POTASSIUM FERTILIZER: THERE ARE MANY SOURCES AND PHILOSOPHIES
If you’ve gotten this far, you probably realize by now that sufficient levels of Potassium are critical for turf health and quality. But, excess Potassium can be equally detrimental to plant health and quality; because it limits the uptake of other essential elements. Therefore, supplemental Potassium must be precisely managed.
Precise nutrient management requires constant monitoring of nutrient reserve, nutrient availability, and nutrient uptake.
I’ve read articles that say, “a grower can supply enough potassium through weekly foliar application.”. With all the data we’ve collected from years of tissue testing, I’ve yet to see where the spoon-feeding method can supply enough Potassium to maintain sufficient levels of Potassium through all environmental conditions.
I’ve read articles that say, “as long as there is a minimally sufficient level of potassium in soil reserve, the plant will take care of itself and pull up what it needs.” This theory believes all that a grower requires to maintain healthy turf is to maintain sufficient levels of each nutrient in the soil. Since this is a newer philosophy, I don’t have enough data to support whether it’s a good or bad hypothesis.
I have seen over-fertilized properties maintain quality turf for years with minimal inputs. Yet most of those properties struggle eventually – once the nutrient bank has been leached away or consumed.
I do agree that we don’t give plants enough credit for pulling up what they need. Nevertheless, I don’t believe this method considers physical factors, environmental conditions, or maintenance levels. Plus, I don’t believe that even the best scientists know the ideal ranges of nutrients in soil reserve, soil solution, or plant tissue. Those ideal ranges frequently change due to environmental conditions and plant growth stages.
I know one consultant that recommends supplemental applications of 50 pounds of Potassium per year. The philosophy is that Potassium leaches so quickly that the only way to ensure an adequate supply of Potassium to the plant is to apply it in excess.
WOW, where do I start with that one?
I believe if you’re managing a property with high sodium, you will need to apply more Potassium than a property with low sodium levels. Sodium replaces Potassium in the plant if it’s deficient. I also know that excess Potassium is not toxic to a plant and will not destroy soil structure. However, what’s attached to that potassium molecule can and will.
You may remember from above I mentioned that excess Potassium in the plant would initiate deficiencies of other nutrients. By applying one pound of Potassium each week, you are taking a greater chance of potassium luxury consumption and likely causing other deficiencies.
As bad as luxury consumption can be to a plant, the real challenge is the negative effect on soil structure caused by the anion attached to Potassium.
Here’s just one example: If you apply sulfate of potassium magnesium (sol po mag) to achieve the goal of fifty pounds of Potassium, you would also be applying 25 lbs of magnesium and 20 lbs of sulfur. Excess accumulation of either magnesium or sulfur causes tremendous obstructions to the soil’s structure.
Applying just the right amount of Potassium will frequently change with plant growth and environmental changes. And using the correct sources is essential for uptake efficiency and consistency of potassium levels.
If you wish to take most of the guesswork out of potassium management, you need to test plant tissue before every application, and you should test soil solution a minimum of 4 times per year. It’s important to understand that anions leach quickly through the soil because they can not attach to soil particles. The potassium source that’s most efficient during one season will likely be completely inefficient a few months later when temperatures and precipitation change.
When Potassium is moderately deficient, the effects first appear in the older tissues and progress towards the growing points. Acute deficiency severely affects growing points, and die-back commonly occurs.
- Chlorosis: May cause yellowing of leaves. The margin of the leaves may fall off or lead to shedding and defoliation of the leaves.
- Stunted growth: Potassium is an important growth catalyst. Its deficiency might lead to slow growth or poorly developed roots and stems.
- Poor resistance to ecological changes: Reduced Potassium availability will directly result in less fluid circulation and translocation of nutrients in plants which causes them to become more susceptible to temperature changes.
There is no level at which Potassium becomes toxic to plants. That said, most Potassium consumed is taken into the plant through osmosis. Therefore, when potassium levels are high in soil solution, plants consume more Potassium than needed for healthy growth. Excess potassium accumulation within the plant will cause nitrogen and other cations to be deficient.
POTASSIUM AND SALT STRESS
If potassium levels in the plant are low, the plant will take sodium up in its place. The accumulation of high salt concentrations in the soil makes it harder for plant roots to take up Potassium. Thereby disturbs a plant’s water balance, while high concentrations of sodium in plant tissue may be toxic.
When sodium replaces Potassium, it inhibits all the processes that Potassium is responsible for in the plant. Therefore, if you are in an area with high concentrations of sodium in the soil, more frequent potassium applications will be required to compensate for the high sodium levels.