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Over recent growing seasons, we have observed and seen reports of potassium deficiency across our territories. While some blame the drought and dry soil conditions, we also know that many of our soils are continuing to show lower K supply levels. In this article, we will discuss how K deficiency can be forced by two interacting soil conditions: K soil supply and drought stress (e.g., low soil moisture) and what to do about it.
The Last Few Years
Drought stress was felt and seen across many acres in 2023 and 2025 (see the comparison by toggling between the 2023 and 2025 maps in Figure 1).
Dry soil can drive K deficiency because the K+ ions cannot move through the soil solution (the liquid phase) to be taken up via mass flow, nor can the ions move properly via diffusion to the root tips. Also, K-containing fertilizers may not properly dissolve and deliver nutrients to the crop when the soil is dry. A few of our agronomy peers advised that “K deficiency would resolve itself once the rains came in” during the 2023 drought; however, we also know that farm soils are becoming increasingly deficient in K. Some farms may have experienced soil conditions that promote K deficiency when the soil supply of K is low and drier than usual.
Growing Soil K Deficiency and Critical Values
We know that soil K deficiency is trending upward based on the analysis of more than 2 million soil samples across the U.S. and Canada (TFI 2020). This is due to several factors, including K soil removal rates in the harvested part of the crop that are outpacing the input rates of K back to the soil (e.g., from fertilizer). The “K budget” has been out of balance (K removals > K inputs) for many years, and, as a result, the percentage of soil tests that are prone to deficiency continues to increase year over year.
A critical level is tied to a specific nutrient metric in the soil on the X-axis (e.g., K ppm), and the corresponding yield is shown on the Y-axis. Many years of testing will reveal that there is a certain level of a soil nutrient that supplies optimum yield (100%). Below this point, crop yield may decrease quickly, and a fertilizer response is highly likely. Above this point, the crop does not respond, and the fertilizer cost may not justify the expected yield return.
So, what is driving the increased observation in K crop deficiency symptoms over recent years? Is it dry soil due to drought or low soil K levels? Read below for an explanation on how two factors (drought intensity × soil K levels) interact with each other to influence K uptake and crop yield.
Some Explanation on Drought Intensity × Soil K Levels
In a recent social media post, Dr. John Jones from the University of Illinois, Urbana-Champaign showed how drought and K supply can interact to influence crop performance (Fig. 2). In the post, Dr. Jones showed several photos of corn plots growing under different soil K levels and their average yield (poor K supply, left; okay K supply, middle; optimal K supply, right). These plots are in the same area and subjected to the same D3 extreme drought stress during the active growing season (see green star on map). The photos clearly show an interaction between drought intensity and soil K levels and their effect on yield.
Notice the differences in corn growth across the low K/drought stress and higher K/drought stress spectrum. Dr. Jones concluded crops grown in soils with optimal K levels or higher should perform better under lower soil moisture conditions than their K-deficient counterparts, assuming nothing else restricted crop growth. This is a great example of how drought conditions (e.g., soil moisture) can interact with soil K supply to produce an impact on crop growth.
The combination of drought and low soil K values (76 ppm Bray and 159 bushels per acre yield) led to a 95-bushels-per-acre yield loss relative to plots that had similar drought conditions but higher soil K values (127 ppm and 254 bushels per acre yield).
Uptake and Removal Data
Dr. Jones, in a separate webinar, discussed the interaction of K supply on K uptake (Fig. 3, right) and K removal from the harvested portion of the crop (Fig. 3, left). The dark circles represent years when growing season precipitation did not deviate much from the 30-year average, and the open circles represent data from drier years. The corn crop shows higher total K uptake when soil moisture conditions are conducive to nutrient movement and uptake (dark marks, Fig. 3, right). On the other hand, nutrient movement and crop uptake are limited in dry conditions, and this is reflected in the graph (open circle, right graph). Since removal of K from the field moves in tandem with uptake, it is not surprising that a crop grown under dry conditions removes much less K relative to corn grown under more optimal conditions (Fig. 3, left). This becomes important for fertilizer budgets for the crop planted after the drought as some nutrients will still be available to drive crop growth in the next season.
Yield Data
So, what does the yield data tell us about how soil K supply and drought-stressed, dry soils interact? We can illustrate the interaction below with corn (Fig. 4, left) and soybean (Fig. 4, right). Wetter, more optimal years are marked by solid symbols, while crops grown under dry conditions are marked with open circles.
Key Takeaways
• At low soil K levels (red bar), the crops grown under optimal conditions can produce much higher yields than their drought-stricken counterparts, indicating a moisture limitation. This is not surprising. However, on the line for the dry years, yield responses to K fertilization differ, with corn requiring higher soil test K and soybean lower. This tells us that, under low soil moisture conditions, diminished K supply begins to affect yield in a major way and that K fertilizer applications should maintain priority in a crop nutrient management plan.
• On the other hand, keeping high levels of K in the soil beyond the optimum range (green bar) is not going to provide much “insurance” for yield (dry or wet year) relative to optimum K conditions. This region of soil test ranges does not support high probabilities of agronomic or economic returns to K fertilization; however, fields in this range may require K to prevent crop removal of K from pushing soil test levels too low over the long term.
• When considering how yields will respond near the critical concentrations of soil test K (yellow bar), notice how annual moisture fluctuations might be of more concern. In this range, large previous K removals or dry conditions may lead to observed deficiencies. The probability of yield responses to K fertilization is commonly double that of the green range, and goals should be to supply enough K to optimize yield and replenish removal if necessary. Notice the yellow bar is narrower than the others, requiring both accurate removal estimates and up-to-date soil test level numbers to watch closely.
Organizing Our Thoughts
Because soil K supply interacts with the dry conditions forced by drought, we can think about our K supply to the crop in three different ways (Fig. 5).
• Under optimal soil K and moisture conditions, excellent K uptake and yields can be achieved (green box).
• Under conditions with poor K supply and dry soil, a decline in crop performance and yield is expected due to the one-two punch of simultaneous K nutrient deficiency and unavailable soil moisture (red box).
• Not surprisingly, it is the subtle areas of these two endpoints where we need to pay the most attention (yellow boxes). Under the two interacting conditions of dry soil and soil K supply, the system can tilt to a yield-limiting direction very quickly (Fig. 2). This is where proactive planning and management are important. When these conditions are observed, irrigation can be turned on (Fig. 2, lower right box) or prescriptive fertilizer applications can be made (Fig. 2, upper left box).
Next Steps
Drought intensity and soil K levels interact with each other to influence K uptake and crop yield. However, there is some nuance to the relationship that is worth some consideration. As we move into soil sampling season, it is important to run a “systems check” to help explain any observed K deficiency this year and to also calibrate future K applications. Dr. Jones makes the following notes:
• Optimum soil test K levels should supply sufficient nutrients in dry conditions (water stress is restricting other physiological processes.)
• Adding supplemental K may only supply a response in low-testing soils.
• Variable in-field K deficiencies may indicate low soil test K or “troublesome” soils (great opportunity to zone soil sample and apply K where it is needed most).
• Crop removal values will be affected by drought and generally leave K “behind” that can be used by the next crop. Consider this for future fertilizer plans and adjust application rates accordingly.
• Consider soil conditions (particularly abnormally dry) when interpreting soil pH and K soil test results; they may deviate from when moisture is sufficient.
For more information on agronomy topics, including drought and K-related topics, please visit nutrien-ekonomics.com.
References
Jones, J.D., Laboski, C.A., Arriaga, F.J. (2023). Soil Fertility Challenges in a Dry 2023. 2023 Badger Crop Connect Meeting. Univ. Wisconsin-Madison.
The Fertilizer Institute. 2020. Soil Test Levels in North
America: Summary Update.
Resources
Potassium and Drought: A Two-fold Water Uptake Problem (Agvise Laboratories)
Nutrient Uptake Considerations Under Drought (nutrien-ekonomics.com)
Dr. Karl Wyant | Director of Agronomy, Nutrien
Lyle Cowell | Senior Agronomist, Nutrien
