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Potassium for crop production

Potassium (K) is an essential nutrient for plant growth. It’s classified as a macronutrient because plants take up large quantities of K during their life cycle.

Minnesota soils can supply some K for crop production, but when the supply from the soil isn’t adequate, a fertilizer program must supply the K.

Here, we’ll give you a basic understanding of K, including plants’ K nutrition, how it reacts in soils, its function in plants and its role in efficient crop production. In addition, you’ll find information about soil tests, K sources, predicting potash needs and effectively applying K to your fields.

Role in plant growth

Potassium is associated with the movement of water, nutrients and carbohydrates in plant tissue. It’s involved with enzyme activation within the plant, which affects protein, starch and adenosine triphosphate (ATP) production. The production of ATP can regulate the rate of photosynthesis.

Potassium also helps regulate the opening and closing of the stomata, which regulates the exchange of water vapor, oxygen and carbon dioxide. If K is deficient or not supplied in adequate amounts, it stunts plant growth and reduces yield.

For perennial crops such as alfalfa, potassium plays a role in stand persistence through the winter. Other roles of K include:

  • Increases root growth and improves drought resistance.

  • Maintains turgor; reduces water loss and wilting.

  • Aids in photosynthesis and food formation.

  • Reduces respiration, preventing energy losses.

  • Enhances translocation of sugars and starch.

  • Produces grain rich in starch.

  • Increases plants’ protein content.

  • Builds cellulose and reduces lodging.

  • Helps retard crop diseases.

Potassium in soils

The total K content of soils frequently exceeds 20,000 ppm (parts per million). While the supply of total K in soils is quite large, relatively small amounts are available for plant growth at any one time. That’s because nearly all of this K is in the structural component of soil minerals and isn’t available for plant growth.

The amount of K supplied by soils varies due to large differences in soil parent materials and the effect weathering has on these materials. Therefore, the need for K in a fertilizer program varies across the United States.

Three forms of K – unavailable, slowly available or fixed and readily available or exchangeable – exist in an equilibrium in the soil system. Below, we describe these forms and their relationship to one another. Figure 1 also illustrates the general relationship among these forms.

illustration of the potassium cycle
Figure 1: Generalized soil cycle of K in the soil and where K may be annually applied or removed.

 

Forms of potassium

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Plant uptake: Key factors

Several factors affect potassium uptake by plants, including soil moisture, soil aeration and oxygen level, soil temperature and tillage system.

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Symptoms of potassium deficiency

Some crops exhibit characteristic deficiency symptoms when adequate amounts of K aren’t available for growth and development. Potassium is mobile in plants, and will move from lower to upper leaves.

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Predicting potash needs

You can monitor K status of soils with plant analysis and routine soil testing.

Soil testing is the most reliable predictor of a fertilizer program’s need for potash. Plant analysis can confirm a suspected deficiency indicated by visual symptoms or routinely monitor the effects of a chosen fertilizer program.

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Soil tests: Soil sample drying’s effect on results

Soil testing labs commonly air dry soil samples prior to analysis. Drying soils high in clay can affect the amount of K extracted.

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Sources of potassium

There are a limited number of fertilizer materials that can supply K when needed. Table 3 lists these materials.

Table 3: Common potassium fertilizer sources

Note that mentioning brand name materials does not mean the University of Minnesota endorses these products over similar products that might be available.
Material Chemical formula Approximate K2O
Potassium chloride KCl 60 to 62%
Potassium sulfate K2SO4 50%
Potassium-magnesium sulfate (K-mag or Sul-Po-Mag) K2SO4�2MgSO4 20%
Potassium thiosulfate K2S2O3 17%
Potassium nitrate KNO3 44%
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Applying and managing potassium

Suggested management practices for K vary with each crop.

You can apply potassium fertilizer either in the fall or spring for most soils in Minnesota. Sandy soils with a low cation exchange capacity have a low ability to hold K. Consider potassium to be partially mobile on sandy soils and apply it closer to the time of planting.

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Liquid forms of potassium

Liquid forms of potassium are available for in-furrow application as a starter fertilizer. These forms of fertilizer are manufactured with either potassium chloride (KCl) or potassium hydroxide (KOH).

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Daniel E. Kaiser, Extension nutrient management specialist and Carl J. Rosen, Extension nutrient management specialist

Reviewed in 2018

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