Yield responses to potassium fertilizer

By Greg LaBarge, Ohio State University Extension

An analysis of potassium fertilizer response research is giving Ohio farmers clearer guidance on when potassium (K) fertilizer boosts crop yields—and when it does not. The study, conducted by Ohio State University researchers and summarized in a recently released Extension factsheet, draws on 458 field trials across 40 counties to pinpoint crop responses at varying soil potassium levels.

Potassium is essential for plant health, influencing water movement, stem strength, and disease tolerance. Yet most Ohio soils already contain large amounts of total K—only a small fraction of which is plant‑available.

Results showed potassium fertilizer increased grain yields in 25% of all trials. Corn was the most responsive crop, showing yield increases in about 30% of trials, while soybeans responded only 20% of the time. Wheat had limited data but followed similar trends. The likelihood of a yield bump was strongly tied to soil‑test potassium (STK) levels.

The Tri-State Fertilizer Recommendations designate 120 ppm as the critical K level for most soils, and 100 ppm for soils with a CEC less than 5. The Critical soil test K value answers the question Do I need to apply K fertilizer this year. Fields below the critical soil test K value have a higher risk of yield loss without fertilizer during the current growing season.

The risk of yield loss by STK class and the magnitude of yield increase from fertilizer additions provide farmers with more information on fertilizer response than simply looking at the relationship between their test and the critical STK value.

The OSU dataset identifies response frequencies and the yield change that can be expected.

• STK < 100 ppm: Up to 50% of trials showed a positive fertilizer response, and median relative yields were reduced 11 to 14% in unfertilized plots.
• STK 100–130 ppm: Response frequency increased to 20–30% of the trials.
• STK > 130 ppm: 7% of sites responded to added potassium, but median yields of unfertilized crops still approached 99%, signaling very low risk of yield loss without fertilizer.

The report also clarifies how potassium moves from soil to plant roots. Because uptake occurs primarily through diffusion, soil moisture, compaction, texture, and clay mineralogy all affect how quickly K travels to plants. These results highlight that many Ohio fields already have enough plant‑available potassium to reach maximum yield.

Although uncommon, 3% of trials showed yield reductions, mostly in soybean fields. Researchers cited possible causes, including salt injury, chloride toxicity, lodging, or delayed maturity—factors growers may want to monitor when applying K near planting.

The comprehensive analysis suggests farmers can make more confident, data‑driven decisions:

• Prioritize K fertilizer on fields with STK below 100 ppm.
• Evaluate economics carefully for fields between 100–130 ppm, where response is possible but not guaranteed.
• Avoid unnecessary applications above 160 ppm, where yield benefits are highly unlikely.

With fertilizer prices volatile and margins tight, the findings provide farmers with more details on when and where potassium is needed. The factsheet can be found at https://go.osu.edu/kresponse.