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In Pursuit of Potassium: Learning about Placement, Timing, and Rate Implications

On the top of many corn producers to-do lists each fall and winter is making fertilizer decisions. One of the main nutrients they commonly consider is potassium (K). These management decisions can be guided by the considerable past, as well as more recent, research on this nutrient. Producers are able to take soil sampling data to examine levels of phosphorus, potassium and micro nutrients, determine which will be limiting to crop growth and yield goals, and ultimately make an informed decision for application rates. However, significant challenges still exist for producers in making their final decision. In 2015, over 560,000 soil samples were collected across Indiana, and of those, approximately 40% were below 120 ppm exchangeable K. Additionally, exchangeable K across the state is often stratified in layers near the surface, so while present in the soil, it is not always available for plant uptake. Recent research has indicated the need for more K availability due to higher plant populations, as well as indications that higher yields are possible when ear leaf K is well above the previous researched threshold value of 1.9% as suggested in the Tri-State Fertility Recommendations. These issues suggest there is a need for better understanding of potassium fertilizer placement, timing, and rate.

The Purdue Cropping Systems Lab group, under the direction of Dr. Tony Vyn, set out to look at some of these issues their multi-year Potassium Study. This article will cover the background of the project, as well as some results seen in the last 3 years.

The Tri-State fertility guidelines for critical levels of potassium as well as application rates for build, maintenance, or draw down scenarios were developed from research in the 1970’s and 1980’s.  The critical level for ppm K is equal to 75 +(2.5 x CEC). So, for a soil with a Cation Exchange Capacity of 10 meq/100g, the critical potassium level would be 100 ppm. For a soil with 20 meq/100g CEC, the critical potassium levels would be 125 ppm. This critical level is used to indicate the minimum K concentration in the soil necessary to supply enough K as to not limit plant growth and development. While this equation has served us well in the past, changes in genetics and management practices in the last 40-50 years are not reflected in this equation. A general trend across the state is that about 60% of the soils currently meet the critical level of potassium. According to the old standard, no applications of potassium would be needed. So why the need for more potassium research? Anecdotally, producers, crop consultants, and researchers alike had been noticing an interesting phenomenon with potassium applications. Even in soils meeting the critical levels of potassium, there still seemed to be a yield increase for additional K applied. Was this impact coming from a change in genetics, more precise placement of potassium products, the distribution of potassium in the soil associated with changing tillage systems, the timing of the potassium applications, or maybe something else entirely?

Graduate Research Assistant, Lauren Schwarck, began to look into this phenomenon in her research at Purdue University. “As minimum tillage practices become increasingly common for producers in Indiana, and deposition of plant material on the surface leads to higher concentrations of potassium building up in the surface inches of soil, our objective was to understand the impact of potassium fertilizer placement on corn uptake utilizing an alternative fertilizer source.” Additionally, research also focused on timing of potassium fertilizers. Within the same study, questions regarding timing of application and placement with tillage influence could both be investigated. The first part of the study looked at three rates of potassium fertilizer and two application timings (fall versus spring) within a strip tillage system. The second component of the study investigated common conservation tillage systems (no till, strip tillage, and fall chisel plow) with combinations of placement and timing of potassium fertilizer.

Strip Tillage Fertilizer Efficiency

The first part of this study compared rates, 0, 100, and 200 lbs of Aspire® under spring and fall strip tillage. Aspire (0-0-58-0.5(B)) is a relatively new granular fertilizer product produced by the Mosaic Company, with potassium and two forms of boron incorporated into fertilizer granules. The goal of analyzing this subset of treatments was to understand if the incorporation of fertilizer in the rooting zone would allow for the reduction in fertilizer application rate without sacrificing plant health and yield.  Additionally, this study aimed to discover if this product and placement, even in soils at or above the critical K level, could lead to a yield advantage.

Across the multiple locations and years of conducting this experiment, soil tests ranged from 150-269 ppm plant available K. Utilizing the Tri-State Fertilizer equation for calculating the critical K level discussed previously, the average critical K level across locations was 137 ppm (well below the range measured across experiment locations, indicating low probability of a response to fertilizer application). However, results for this study showed that even where 100-200 lbs Aspire® was applied, an average grain yield increase of 9 bushels per acre was documented over 5 site-years of trials in Indiana. So why are we seeing this dramatic response for supposedly adequate soil potassium levels? Dr. Tony Vyn surmises “Corn yield increases to potash applications (utilizing Aspire®) appeared to come about because corn plants accumulated more K and B during vegetative growth stages and then maintained higher leaf K levels in leaves at flowering. Those higher plant K concentrations in early grain fill allowed heavier grain weights to be achieved.”

The analysis of this subset of treatments documented a 9 bushel per acre yield response in corn yields (comparing 0 to the 100 and 200 lbs per acre for fall strip tillage treatment) in soils that met the critical K level recommended by the Tri-State Fertilizer Recommendations.  Banding potassium effectively placed nutrients near the root system for easy access and eliminated some of the issues arising with soil stratified K from many years of broadcast applications with reduced tillage systems.

Conservation Tillage/Placement and Timing Impacts

Research has shown that placement of fertilizer can change efficiency of K uptake and has the potential to change the rate needed for application.  The second component to this study was to investigate impact of the addition of Aspire under common conservation tillage systems in Indiana, with a timing component within the strip tillage placement/tillage. Three placement/tillage combinations were used for this analysis, (fall and spring strip tillage with coulter driven incorporation), broadcast unincorporated (no-till), and broadcast incorporated (utilizing a chisel plow in the fall, followed by secondary tillage in the spring). Two rates of Aspire® (0 and 200 pound per acre) were assessed for each placement/tillage method. The timing component for this analysis is related to the fall and spring strip tillage timings. Table 1 shows the combination of placement/tillage and timing for this study as well as the five year yield average.

Yield adjusted to 15.5% moisture. Values with the same letter are not significantly different at alpha=0.05.

Overall, the 200 pound per acre Aspire applications in the fall strip tillage, spring strip tillage and fall chisel tillage systems, all had average yields exceeding 250 bushels per acre. Essentially, the higher rates of Aspire resulted in higher yields. Practices that incorporated Aspire in the soil, by strip tillage or fall chisel resulted in significantly higher yields. It is important to acknowledge the likely impact tillage had on creating seedbed conditions conducive to improved plant emergence in the spring (drier and warmer soil conditions). The lowest yields of the study were in the 0 pound per acre Aspire rate in no-till and spring strip-tillage systems.

The plants with Aspire™ applied to them had higher grain moistures at harvest time as well. This is likely due to improved plant health conferring some level of delayed maturity on these plants. In this case, delayed maturity = more time for grain fill = higher yields.

This study also showed the dramatic stratification of plant available K ppm in the soil with depth. The concentration of K was much higher in the 0-2 inches zone than 2-4 inches, or 4-8 inches. Potassium is a slow-moving nutrient in the soil, leading to the development of stratification if continually deposited on the soil surface. Consequently, anytime potassium fertilizer is applied, the majority will stay close to that zone of application for some time, as dictated by the unique soil texture and soil moisture conditions as well as root uptake. For broadcast applications, most of the applied potassium will remain near the soil surface and spread across the crop row. Strip tillage operations allow for a targeted approach for fertilizer application, enriching the intended crop row typically within the top 6 inches of the soil. Mixing of stratified soil and the addition of Aspire improves the distribution of potassium throughout the rooting zone. Utilizing strip tillage operations may provide an advantage to uptake of K and also to the efficient recovery of Aspire fertilizer allowing for reduced application rates. Dr. Tony Vyn suggests using strip till as a way to combat potential limitation in potassium from potassium stratification. “Soil K stratification is mostly the result of natural plant K uptake and return of crop residues on or near the soil surface. However, that stratification can be exacerbated by continual broadcast application of potash. Strip-till systems offer an opportunity to incorporate potash fertilizers in bands centered on the intended rows.” Strip till has the potential to place the supply of nutrients right at the root zone for optimum uptake.

Over three years of this study, yield results showed that the 200 pound rate of Fall Chisel, Spring Strip Till and Fall Strip Till had the highest yields. Again, this is despite having soils meeting threshold levels of K at most sites. So, what does this mean for potassium recommendations for the state? “The critical K soil level were commonly close to optimum (slightly above or below) for experimental locations, yet there was commonly a yield increase seen following application of Aspire; however, it is not clear if this increase in yield was due to K or B” states Schwarck. Further research drilling down on those specific nutrients will provide clarification.

Another component of this study looked into the soil sampling methods and K analysis. Samples were pulled at 0-2 inches, 2-4 inches, and 4-8 inches both in the strip till row and in-between the strip till rows. This was to test the effect of potassium movement and placement on accessibility to the plant’s root system and determine amount of stratification both horizontally and vertically. Across all treatments, there was stratification vertically, with the top layer having the highest amount of potassium and decreasing with depth. Additionally, there was stratification horizontally from the banding of the fertilizer in the strip till treatments. This suggests strip-till applications may place potassium in a location that is more available for plant uptake in contrast to broadcast applications stratification at the soil surface.

Despite soils meeting the critical potassium threshold levels, the application of 200 pounds of potassium yielded the highest of the K treatments. Methods of application that incorporated potassium applications yielded higher (strip till and chisel till). Plants with Aspire™ fertilizer had higher grain moistures and delayed maturation leading us to believe these plants had better overall plant health and grain fill period. Again, a key takeaway is tillage treatments that incorporated the potassium placed the nutrient for improved access by plant root systems, and that was an important factor for final grain yield. However, because the Aspire product contains both potassium and boron, it is unknown which nutrient or combination of nutrients played a role in the yield increase.

More research will be needed to better understand the changing roles and relationships with potassium in our farming systems. We are seeing indications of yield increases with more precise placement of K. Interesting yield results with K applications on soils meeting the critical threshold levels indicate a need for more research on plant uptake and allocation of K in these situations, particularly as yield increases following K application on soils above the critical K level suggests corn may be limited, given the wide variety of soils and management strategies across the state. Information regarding placement, rate, and timing of potassium applications will be critical information for Indiana producers as they make fertilizer decisions this coming fall.


This research was generously supported by the Indiana Corn Marketing Council and The Mosaic Company.


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