Potassium in Mississippi Soils

Potassium (K) is absorbed in large amounts by plants. Potassium uptake by crops under good growing conditions is high, which is often equal to nitrogen (N) uptake. Potassium is used by plants in photosynthesis, sugar transport, water and nutrient movement, protein synthesis, and starch formation. Adequate potassium in plants increases disease resistance, water stress tolerance, winter hardiness, tolerance to plant pests, and uptake efficiency of other nutrients.

Potassium mobility is often related to soil texture; movement is greatest in soils with more sand content. The buildup of potassium in soils is related to soil texture, with the greatest accumulation generally in clay soils, followed by loam and coarse-textured sands.

Although soils provide a great deal of potassium through natural processes, fertilization with potassium may be necessary to maximize plant growth. The total amount of potassium in soil ranges from 5 to 25 tons per acre. While this seems like a lot of potassium, only a small amount is plant-available at a particular point in time. Most potassium is in the structural component of the soil. Sandy-textured soils have much less potassium than fine-textured, clay soils. Where levels of soluble potassium in the soil are high, plants may take up more potassium than needed in a “luxury consumption” that does not increase yields.

Potassium in Soils

There are three forms of potassium “pools” in the soil: unavailable, slowly available or “fixed,” and readily available.

Unavailable potassium is within soil minerals such as feldspar and mica. Over extremely long periods, these minerals will break down and potassium will be released. However, this process is much too slow to provide the full potassium needs of growing plants.

Slowly available or “fixed” potassium is trapped between layers of clay minerals. This type of soil potassium is not measured by soil-testing procedures, but it will become plant-available over time. However, it is possible for potassium measured in the soil-testing process to become “fixed” in the slowly unavailable pool to plants during the growing season.

How much potassium is “fixed” varies with the particular types of clay present in a soil. Many Mississippi soils are dominated by clay with large shrink-swell capacities (i.e., cracks form in dry seasons). These soils can fix large amounts of potassium upon drying but release substantial amounts when rewetted.

Sandy, low organic-matter soils have little capacity to hold or retain potassium. These soils should be managed to minimize potassium loss. Readily available potassium is the pool measured by soil-testing procedures. This is potassium present in the soil solution and potassium easily removed from soil clay edges. Potassium in the soil readily and regularly interchanges between the water in the soil and clay solids. Note that this potassium is not the same pool within the clay structure.

When plants use the potassium present in the soil solution, more potassium is released from the clay particles to the solution in response to the decrease in concentration. This interchange of potassium in the soil is extremely important to plant nutrition.

Plant Uptake of Potassium

Potassium used by growing crops must be absorbed from the soil. Potassium constitutes 1 to 4 percent of plant dry-matter weight. The quantity removed by plants varies among crops. Cotton contains about 20 pounds of potassium oxide (K2O) equivalent potassium per bale harvested, and hybrid bermudagrass hay contains about 50 pounds per ton. Extension Publication 2647, Nutrient Management Guidelines for Agronomic Crops Grown in Mississippi, contains detailed information about potassium uptake by various crops.

Plant root systems cannot intercept sufficient potassium in the soil or soil water to maintain plant function. About 90 percent of the potassium needed by plants must move to the root surfaces by diffusion, which is movement from an area of high concentration to one of low concentration within moisture films surrounding soil particles.

Factors affecting potassium diffusion include soil moisture conditions, soil aeration, and soil temperature. Higher soil moisture usually increases potassium movement in the soil. However, when soils are saturated with water, the resulting decrease in root function will decrease potassium uptake. Soil temperature affects all plant functions. Potassium uptake is optimum at temperatures of 60 to 80°F, but uptake is reduced at lower temperatures.

Managing Soil Potassium

Soil-fertility management should follow the four Rs: using the right fertilizer at the right rate at the right time and at the right place. Determining the right rate begins with a sound soil-testing program. Information on soil testing in Mississippi is available for farmers and homeowners. Common potassium soil-test procedures use a chemical procedure that results in assessing the potassium in soil solution and the potassium on soil solids that exchanges with it. The Mississippi State University Extension Service Soil Testing Laboratory uses an extraction solution and procedure developed for the diverse soils of the state.

Research develops the relationship between potassium soil-test levels and potassium fertilizer requirements. Plant potassium uptake and yield are related to measured quantities of potassium in the soil. These results are integrated into “soil-test K indices”; these are commonly seen on soil-test reports as very high, high, medium, low, or very low. Each category reflects the probability of plant response to potassium application. Crops grown on soils testing with a very high potassium index normally will not respond to potash fertilizer application, but crops on very low-potassium soils should usually respond.

The potassium indices used by the MSU Extension Soil Testing Laboratory are listed in Tables 1, 2, and 3. These indices are categorized by crop and by cation exchange capacity (CEC). The CEC of a soil is a measure of its ability to store the positively charged nutrients that are determined during the soil-testing procedure.

Potassium Fertilization

If the soil-test recommendations associated with that sample’s index call for potassium fertilizer to optimize crop production or plant growth, several materials are available. Determining the right fertilizer is relatively simple for potassium. Most commercial potassium fertilizer used in the state is potassium chloride, or muriate of potash. Potassium chloride may be pink, red, or white. The color difference is due to the iron content of the materials; there is no difference in the amount of plant-available potassium in the material. Potassium-magnesium sulfate is a good potassium source if magnesium is also needed for the crop.

The MSU Extension Service does not recommend potassium fertilization for soils that have high or very high indices, except for cotton in the high index. In these cases, the right rate is zero. In most crop production systems, the right time to apply potassium is at or before planting. While potassium may be fall-applied for row crop production in Mississippi on soils with low loss potential (CEC > 8), potassium is vulnerable to overwinter loss on sandy, low CEC soils.

The right place to apply potassium is where it will be useful to the growing plants, which can be a site-specific decision. Detailed information on commercial fertilizer recommendations, options, and management is available for field crops. See Extension Publication 2647, Nutrient Management Guidelines for Agronomic Crops Grown in Mississippi, and Publication 2500, Inorganic Fertilizers for Crop Production.

Potassium Deficiency Symptoms

Plants lacking potassium will have shortened internodes, weak stalks, excessive lodging, and more leaf and stalk disease; they will also be a lighter green when viewed from a distance. Severe deficiency will cause leaf drying along the outer margins. Because potassium is mobile in plants, symptoms begin at the tips of lower leaves and move up the plant as the deficiency persists.

Inner portions of the leaves may have a striped appearance. This often can be confused with deficiency symptoms for sulfur, magnesium, and zinc. Potassium deficiency has been an issue in cotton production during times of high “demand” growth. Guidance is available elsewhere for this situation.

The information given here is for educational purposes only. References to commercial products, trade names, or suppliers are made with the understanding that no endorsement is implied and that no discrimination against other products or suppliers is intended.

Publication 3962 (POD-09-23)

Reviewed by Corey Bryant, PhD, Assistant Professor, Delta Research and Extension Center, from an earlier edition by Larry Oldham, PhD, Extension Professor (retired), Plant and Soil Sciences, and Keri Jones, PhD, Laboratory Coordinator, Plant and Soil Sciences.