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Crop Response to Potassium Fertilizer in Ohio

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).  

A graph showing different types of soybeans

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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)  

 

Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.