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Ohio State University Extension


Potassium Uptake and Ohio Crop Response

Agriculture and Natural Resources
Manbir Rakkar, Soil Fertility Specialist/Assistant Professor; Food, Agricultural, and Environmental Sciences; Ohio State University Extension
Greg LaBarge, Field Specialist/Professor; Food, Agricultural, and Environmental Sciences; Ohio State University Extension

Potassium (K) is a macro nutrient needed for crop production. Annual K removal rates for grain crops at state average yields are 45–100 pounds per acre annually, while forage crops are 200–300 pounds per acre annually. The primary function of K is to maintain a charge balance between positively charged ions (cations) and negatively charged ions (anions). Maintaining this balance supports plant functions. While most Ohio soils have an abundance of total K, only a small portion of total K is plant-available. Plant uptake of K and the movement of K between different soil K forms occurs by diffusion.

A recent summary of 458 K trials provides information on yield response at various soil-test potassium (STK) levels for grain crops. The frequency of response to different STK values was evaluated to identify a measure of potential yield loss.

Plant Use

Plant use of potassium differs from how plants use nitrogen, phosphorus, and most micronutrients. Potassium is not incorporated into plant products like enzymes, cell structure, and proteins. Instead, it exists in a solution within the plant sap or plant cell surfaces. Its primary function is to provide a charge balance that supports plant function. The role of K is critical to plant biological processes, water movement between soil and roots and within the plant, plant stem strength, and reduced disease susceptibility.

Plant Uptake and Soil K Forms

Plant potassium uptake is a diffusion-driven process. Diffusion is a nutrient uptake process that occurs close to the plant root’s surface. Diffusion is concentration-driven—nutrients move from areas of higher concentration to areas of lower concentration. An example of diffusion is when a tea bag is placed into hot water. When the tea bag (high concentration) is placed in hot water (low concentration), tea diffuses out of the bag into the surrounding water. Similarly, a plant’s root surface (low concentration) attracts K from the surrounding soil (higher concentration) during active nutrient uptake. Soil conditions that impact root growth will influence K uptake and can lead to temporary or season-long nutrient deficiencies in plants.

The principles of K diffusion occurring at the root surface also occur with soil K forms. The total K content of Ohio soils is between 12,500 ppm and 30,500 ppm (Ames & Gaither, 1913). Even though an abundance of K exists, only a small portion of the total K in the soil is plant-available at any given time. Soil K has three primary forms: mineral, nonexchangeable, and exchangeable. The readily available K form consists of exchangeable and solution forms. Potassium exchanges between these three forms, but the exchange rate depends on the soil’s texture and composition. Other soil factors, such as clay mineralogy, cation exchange capacity (CEC), soil moisture, pH, and soil wetting-drying cycles, also impact the exchange rate and thereby K-availability to plants.

Figure 1 shows a simplified K cycle. Mineral K is the largest quantity, consisting of 95%–98% of the total soil K. Mineral K becomes very slowly available with weathering. Nonexchangeable K is 1%–3% of the total K. Nonexchangeable K gradually becomes available to exchangeable K. Exchangeable K is readily available K, plus soil-solution K. These two forms have a fast exchange rate.Graphic showing how potassium (K) is exchanged from mineral and nonexchangeable sources into readily available forms (exchangeable K and soil solution K), which crops can then use.

Actively growing plants reduce the soil-solution K concentration. Exchangeable K form then replenishes the soil-solution K. Potassium from fertilizer or manure may be needed if the readily available forms are insufficient to meet plant needs.

Applying K in manure or fertilizer increases soil-solution K concentration. When the soil-solution concentration exceeds the demand from plant uptake, some soil-solution K concentrations will move into exchangeable or nonexchangable K forms.

Corn, Soybean, and Wheat Response to K Fertilization

A total of 458 potassium rate trials have been conducted on corn, soybean, and wheat in Ohio. The trials occurred across 40 counties between 1976 and 2021. The trial types include multi-rate, multi-year, single-year, and large strip trials. Culman et al. (2023) provides an analysis of these trials. This fact sheet presents the highlights of that study.

Table 1 (click to download PDF). Soil-test values in the Tri-state reference Bulletin 974 maintenance range. The lower STK value represents the critical value. The higher number is the maintenance limit (Culman et al., 2020).
Table showing soil-test values that are available in the Tri-state reference bulletin 974.

The study sites had a range of soil-test potassium (STK) values from 39 to 365 ppm, with a median of 98 ppm. The Tri-State Fertilizer Recommendations for Corn, Soybean, Wheat, and Alfalfa (Bulletin 974), uses a critical STK value of 120 ppm for loam and clay soils with cation exchange capacity (CEC) greater than 5 milliequivalents per 100 grams. For sandy soils with a CEC of less than 5 milliequivalents per 100 grams, the critical level is 100 ppm (Table 1; Culman et al., 2020). Bulletin 974 defines the critical level as "deficient: thus, a yield response to fertilizer is more likely." The maintenance limit is the point where additional fertilizer would not result in increased yield. For loams and clays, this would be 170 ppm; for sandy soils, it is 130 ppm. The range of soil test values in the given study was below the critical level and above the maintenance limit, providing sites where a yield response to fertilizer is expected and sites where a yield response to fertilizer is not expected.Two triangle-shaped graphics representing soil texture triangle. Left side is a blank chart showing texture class names. The right chart shows the site data  that is represented in the trials indicate clay, sand, loam, and silt.

Soil texture was determined for 118 studies conducted between 2014 and 2018. The trials in this period were primarily on-farm trials conducted in 37 counties. Figure 2 shows the classes of soil texture by site. The range of soil types studied is representative of Ohio farmland soils, where 77% are classified as loam and silt loams, 15% are silty clay loam and clay loam, and 4% are silty clay and clay textures. The range of soil-test K values, nutrient management approaches, and soil textures provides a robust dataset to discuss K management in Ohio.

The study used relative yield to evaluate the yield response of various crops in different field conditions.

The following formula calculated relative yields for each trial:

Maximum relative yield = (unfertilized control yield1 /maximum yield of all treatments2) × 100
1The mean of the unfertilized control.
2The numerical maximum among all treatment means, including the unfertilized control for each trial.
Note: In this calculation, the maximum possible value for relative yield is 100%.

Three separate graphs for corn, soybean, and wheat showing the relationship between the relative yield of each crop and if it was responsive to fertilizing with potassium in relation to various levels of potassium in the soil based on soil testing. The data shows a general relationship between low potassium levels in the soil and increased crop yields after fertilizing with potassium.Potassium fertilizer application increased grain yield in 25% of the trials across all crops. Yield increases were more common in corn, where about approximately 30% responded, while only 20% of soybean trials had a positive response. Figure 3 shows the responsiveness of corn (246 trials), soybean (195 trials), and wheat (17 trials) to added fertilizer at various soil-test K (STK) levels.

While a lower STK increases the probability of crop response to fertilizer, it is not certain that added fertilizer will always maximize yield. Figure 3 shows that a crop response to fertilizing with K is not guaranteed, even at very low STK. In Figure 3, regardless of the crop, a 75 ppm STK can result in a 95%–100% relative yield with no response to fertilizer. Remember that the critical value used in Tri-state reference bulletin 974 is 120 ppm (or 100 ppm for CEC < 5 milliequivalents/100 grams) for all crops (Culman et al., 2020). This study and similar studies show a general relationship between declining STK and the probability of crop response to fertilizer. The classification of soil test values based on crop response may be helpful for farmers and advisers looking to refine their fertilizer recommendations.Two box graphics, with the box on the left displaying red, blue, green, purple, and orange dots on a graph representing relative crop yields for STK values less than 70 (red), from 70 to 100 (blue), from 100 to 130 (green), from 130 to 160 (purple), and greater than 160 (orange). The box on the right displays boxes in red, blue, green, purple, and orange, depicting relative yields ranging from 75% to 100% for STK values less than 70 (red), from 70 to 100 (blue), from 100 to 130 (green), from 130 to 160 (purple), and greater than 160 (orange).

Figure 4 shows all 440 K trials grouped by STK. Figure 4, panel A, displays the relative yield of the individual trials by STK grouping. For example, the individual trials' relative yield is shown in green for STK between 100 and 130 ppm. Figure 4, panel B, provides a statistical summary of relative yields by STK grouping. For example, with STK between 100 and 130 (green), the combined line and box values show relative yields ranging from 75% to 100%. The line in the green box defines the 50th percentile (median) with a relative yield of 96%.

Knowing how relative yield responds at different STK levels is important, but the next question is whether fertilizer is needed to maximize yield. Table 2 summarizes all trials shown in Figure 4 based on the response to fertilizer. This information provides a measure of the risk of yield loss at a given soil-test K level without fertilizer applied during the growing season. With STK greater than 160 ppm, only 2% of 92 trials had a K fertilizer response with a median relative yield of 98%—an extremely low risk of yield loss without fertilizer. The probability of a response to fertilizer increases when STK ranges from 70 to 100 ppm, with 38% of the trials in this range responding to added fertilizer and the relative yield decreasing to 89%. The lower relative yield and the higher response rate indicate adding fertilizer should be considered when STK is in this range.

Table 2 (click to download PDF).
Summary of trial showing responsive and nonresponsive crop yields to fertilizer by STK classification (adapted from Culman et al., 2023).
Table displaying a summary of trials showing the percentage of crops that responded positively to fertilizer and increased their yields.

The study shows that 3% of the trial resulted in yield reductions when K fertilizer was added. A total of 14 trials (five corn, nine soybean) resulted in a significant yield reduction. Reduced soybean yields could have resulted from salt injury, chloride toxicity, lodging, and days to maturity. Reports of small but significant soybean yield decreases with K fertilizer have recently been reported in north central Ohio (Culman et al., 2023).


Results from 458 potassium response trials were conducted on a wide variety of STK soils and soil types common to Ohio crop production regions. The trials show low response to K fertilization when STK was greater than 130 ppm. When STK was between 100 and 130 ppm, fertilizer responses increased but the median relative yield was near or at 100%. In contrast, when STK was 100 ppm or less, the fertilizer response had a positive impact that approached 50%. Maximum yields of corn, soybean, and wheat can be produced over a wide range of STK values. The greatest value of fertilizer in the year of application occurs at STK values of less than 100 ppm.


Ames, J. W., & Gaither, E. W. (1913). Composition of calcareous and non-calcareous soils (with special reference to phosphorus supply) (Number 261). Bulletin of Ohio Agricultural Experiment Station.

Culman, S. W., Fulford, A., LaBarge, G., Watters, H., Lindsey, L. E., Dorrance, A., & Deiss, L. (2023). Probability of crop response to phosphorus and potassium fertilizer: Lessons from 45 years of Ohio trials. Soil Science Society of America Journal, 87(5), 1207–1220.

Culman, S., Fulford, A., Camberato, J., Steinke, K., Lindsey, L., LaBarge, G., Watters, H., Lentz, E., Haden, R., Richer, E., Herman, B., Hoekstra, N. Thomison, P., Minyo, R., Dorrance, A., Rutan, J., & Warncke, D. (2020). Tri-state fertilizer recommendations (Bulletin 974). College of Food, Agricultural, and Environmental Sciences. The Ohio State University.

Originally posted Jan 19, 2024.