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Potassium: The Elusive Regulator

Matt Brill |Director of Marketing, Ferticell USA
Steve Trotter | Agronomist
Sponsored by Ferticell®

Potassium (K+) is one of the three main pillars of essential macronutrients for plant growth. Irregularities in uptake or availability can negatively affect yield size and quality when not closely monitored and planned for. Understanding the relationship potassium plays in soil profiles is essential to delivering optimal efficiency, uptake and yield results.  

Often described as the “weakest” member of the cation family, potassium may share space with sodium in many environments. Plant uptake of potassium, or Potassium Use Efficiency (KUE), is highly selective and synchronized to the rate of metabolism in the plant. As a result, potassium is highly mobile across all levels of plants and travels well in long distance transport via the xylem and phloem.

The Roles of Potassium

In the relationship between plants and water, potassium plays a crucial role, forming weak complexes that readily provide exchangeable potassium. While potassium may not strongly compete with calcium and magnesium, in alkaline conditions, sodium may actually replace potassium.

When plant-available, potassium directly promotes carbohydrate production for both plant health and Brix levels. To assist that role, we know now that the frequency of application of potassium will directly affect and increase KUE. All crops and soils throughout the country do not have the same relationships with potassium and it is important to remember crop types like grapes and citrus have high potassium removal demands while stone fruits have a lesser removal demand and should be calculated accordingly. There are several components that affect potassium uptake by plants differently by region, including soil moisture, aeration, oxygen level, soil temperature and tillage systems.

Potassium is an enzyme activator for many plant functions and is required in higher concentrations for protein synthesis than for enzyme activation. Proteins facilitate the movement of potassium through plants. Concentrations of potassium can occur during the accumulation of soluble N compounds like nitrates, amino acids and amides.

Designing a Program for Potassium

When designing a program for potassium, it can be challenging due to the soil source or solubility. The three locations potassium can be found in the soil are structural soil minerals, the soil colloid and exchangeable elements within the soil solution.

It is valuable when managing potassium levels to have the knowledge of soil limiting factors such as iron and aluminum. High levels of Fe (iron) in soils, or high application rates, should be considered as a risk. And as pH drops, aluminum levels should also be monitored in relation to potassium.

When soils have low OM (organic matter), carbon and/or an active biomass, boron – which is beneficial for potassium – can also become a limiting factor. A light rate of supplemental amino acids would be a wise strategy when this occurs.

Potassium-deficient tomato plant leaves.
Potassium-deficient tomato plant leaves.

Selecting Your Potassium Source

When selecting your source for potassium, it is important to take into consideration the adulteration of each source and what limiting factors can be avoided to provide optimum uptake potential of potassium. All traditional sources of potassium, KCL, K2SO4, KNO3 and KOH all carry with a limiting factor which should be calculated during program design. One example is a possible negative effect on chlorophobic plants like grapes, fruit trees, tomatoes, strawberries and cotton by being exposed to potassium chloride. When analyzing nitrate sources, due to competition with potassium and nitrates, a potassium nitrate may be less efficient. When reviewing soil tests where additional sulfur is not required, it would be difficult to use K2SO4.

Factors to Consider with Potassium Availability

  • Low K+ will reduce photosynthesis in leaves.
  • Deficiencies will increase respiration rates.
  • Deficiencies will reduce cell turgor and cell size.
  • Deficiencies will display low energy or energy transfer.
  • Necessary for synthesis of ATP (energy).
  • Will increase osmotic potential as well as support stabilizing cell pH.
  • K+ increases will stimulate CO2 fixation by 3x.
  • K+ and reducing sugars assists turgor potential for cell extension.
  • Increases cell elongation rates with gibberellic acid.
  • A dominant counter ion for nitrate transportation.
  • Elevated levels of potassium will increase resistance to biotic and abiotic stress.
  • Drought resistance requires higher K+ concentrations.
  • Deficiencies may increase risks for frost damage.
  • Severe deficiencies will create yellowing or death of mature leaves and stems depending upon light intensity.

The development of potassium programs can be made simpler with the knowledge of potassium removal by crop type as well as the source. The use of a non-altered potassium will yield efficiencies with lower rates and Potassium Use Efficiency (KUE).

Finally, improving soil health through biofortification is a major factor in allowing plants and crops to maximize their genetic potential by maintaining a balance in metabolic energy. Fertility programs should include several known growth regulators vital for the performance and efficiency of potassium crops. After limits and corrections have been identified, a proper balance can be executed to sustain soil vibrancy and health.

Sponsor Message

When comparing sources of potassium, consider Pro K™ 0-0-20, a liquid potassium plant derived fertilizer, approved for organic use. With little to no salt content, Pro K™ makes the design of your potassium programs predictable and reliable without uptake concerns as either a foliar, or soil application. Learn more at https://ferticellusa.com/pro-k-0-0-20/.