Potassium (K) is an essential plant nutrient. We measure available soil K with a soil test and add fertilizers when the soil available K is insufficient to meet crop needs. There are economic benefits to making informed decisions about K fertilizer use. The under-application of K fertilizer can result in reduced yields, while over-application adds to input costs, with economic losses resulting from both scenarios.
A few frequent questions about K fertilizer use are: Does K fertilizer always result in a positive yield response? How much will yields increase with applied K? What is the likelihood of yield penalty if K fertilizer is not applied?
A recently published factsheet, Potassium Uptake and Ohio Crop Response https://ohioline.osu.edu/factsheet/anr-0147 (Rakkar and LaBarge, 2024), provides a general overview of soil K and highlights the findings of Culman et al. (2023) to answer these practical questions. The study summarized 458 replicated field K trials conducted over the last 45 years across 40 counties in Ohio. The robust dataset evaluated corn, soybean, and wheat response to added K fertilizer in trials conducted on farms and at research stations.
Below are some key takeaways:
Does K fertilizer always result in a positive yield response?
No. A significant crop yield increase due to K application occurred in 25% of the 458 K field trials. The yield response to added K varied by crop type. Corn responded to K application in 30% while soybean showed a response in 20% of trials with an average yield increase of 11% (Fig. 1).
Fig. 1. Relation of relative yield and soil Mehlich-3 K for corn, soybean, and wheat across 457 field trials (Culman et al., 2023).
How much will yield increase with applied K?
Yield increase will depend on the Mehlich-3 soil test K level. The Mehlich-3 soil measures the soil K that is readily available for crop uptake. Culman et al. (2023) classified Mehlich-3 soil K levels into <70, 70-100, 100-130, 130-160, and >160 ppm to evaluate the yield increase for each soil K category.
The crop yields were presented as Relative Yield, which refers to the yield with no K application divided by the maximum yield obtained across all K treatments. In other words, 100% relative yield means no yield increase with added K. The lower the relative yield, the higher the yield increase from K fertilizer.
Generally, as the soil test K levels decreased, the yield increment from fertilizer K increased (Table 1). When the soil test K was less than 70 ppm, the median relative yield was 86%. As the soil test K level increased above the critical level of 100-120 ppm (Cation Exchange Capacity (CEC) dependent), the median relative yield ranged from 96% to 100%, signifying minimal or no yield increase with added K.
Table 1. Summary of crop response to K fertilizer by soil K classification. (adapted from Culman et al., 2023)
What is the likelihood of a yield penalty if K fertilizer is not applied?
We can also determine the likelihood of yield penalty based on Mehlich-3 soil K with the information in Table 1. When the soil K level was less than 70 ppm, 47% of trials showed increased crop yields with applied K. When the K levels were above the critical level of 100-120 ppm (CEC dependent), only 22% of trials showed increased crop yields. In other words, the likelihood of yield penalty with no K application decreases as soil K levels go above 100 ppm. If the soil test K level is less than 100 ppm, there is an increased risk of yield penalty with no K application.
For more soil fertility resources, information, and tools, use the link go.osu.edu/fertilityresources.
Reference:
Culman, S., Fulford, A., LaBarge, G., Watters, H., Lindsey, L. E., Dorrance, A., & Deiss, L. (2023). Probability of crop response to potassium and potassium fertilizer: Lessons from 45 years of Ohio trials. Soil Science Society of America Journal, 87, 1207-1220. https://doi.org/10.1002/saj2.20564
Rakkar, M. & LaBarge, G. 2024. Soil Potassium and Crop Response to Potassium Fertilizer in Ohio. Ohioline. (https://ohioline.osu.edu/factsheet/anr-0147)