Dry conditions can restrict potassium movement from the soil into plants during a drought.
Potassium management remains an important consideration for crop production, particularly as farmers pursue higher yields and face increasingly variable weather conditions.
Agronomist Greg Stewart discussed potassium deficiency, crop nutrient removal, and fertilizer recommendations during a presentation focused on improving nutrient management decisions.
Stewart, who previously worked with the Ontario Ministry of Agriculture, Food and Agribusiness (OMAFA), Mazex and Syngenta, reviewed the fundamentals of potassium nutrition before examining challenges associated with current fertilizer recommendations and high-yield cropping systems.
Potassium deficiency can become visible when crops cannot access sufficient nutrients, even when soil tests indicate adequate levels. Stewart explained that dry conditions can restrict potassium movement from the soil into plants, causing deficiency symptoms to appear during periods of limited moisture availability.
Stewart noted that some fields experienced these symptoms during a dry spell in early July, despite having reasonably good soil potassium levels. The issue can be particularly noticeable in lighter soils, where moisture availability may decline quickly.
Stewart also highlighted the relationship between nitrogen application and potassium deficiency symptoms. When farmers apply nitrogen at relatively high rates and crops encounter a sudden dry period, potassium-related symptoms may become more apparent than nitrogen deficiency. This reinforces the importance of considering soil moisture and nutrient interactions when diagnosing crop problems.
Another major topic was the relationship between potassium removal and fertilizer recommendations. Stewart reviewed Ontario Ministry of Agriculture recommendations alongside potassium removal figures for different crops and yield levels.
High-yield corn production presents a particular challenge because greater grain production removes more potassium from the soil. Stewart cited removal levels of approximately 60 to 70 pounds of potassium oxide (K2O) in high-yield corn systems producing around 240 to 280 bushels per acre.
He questioned whether existing recommendations adequately account for these removal rates. Under the guidelines he discussed, fertilizer applications designed to replace crop removal may not be recommended until soil-test potassium declines to relatively low levels, around 75 parts per million.
Stewart described this as a longstanding issue in nutrient management, raising concerns about allowing soil potassium levels to decline before recommendations call for replacing the nutrients removed by harvested crops.
However, Stewart cautioned if applying fertilizer at the same rate as crop removal is always the most appropriate strategy. Fertilizer decisions must consider soil-test results, crop yield potential, nutrient availability, and economic returns.
At the other end of the soil-test scale, the recommendations he reviewed indicated that additional potassium fertilizer may not be required when soil-test levels exceed 120 parts per million.
Stewart noted that while this approach may support short-term profitability, questions remain about how well it accounts for long-term soil fertility under consistently high yields.
Photo Credit: Farms.com
Watch the full Greg Stewart demo presentation from the 2026 Great Ontario Yield Tour below.