Effects of Adding Potassium to Corn Sidedress N Applications

INSIGHTS

• Certain soil characteristics and dry weather conditions can limit potassium (K) availability.

• Sidedressed N fertilizer in addition to base rates increased yield by 7.7 Bu/A.

• K sidedressing as a standalone or paired with N did not enhance yield.

Introduction

Potassium is a critical element for corn, which requires approximately 0.8 lbs. K₂O to produce every bushel of grain.¹ Its true availability to plants can differ due to soil moisture, soil texture, clay mineralogy (e.g., 2:1 clay soils), or soil structure issues (i.e., compaction). Since it is such a critical nutrient for high-yield corn production, there is curiosity around whether adding K fertilizer to a sidedress nitrogen (N) application is a time- and cost-effective practice that can further elevate yield.

Agronomy in Action Research Trial

A trial was performed at six Agronomy in Action (AIA) research sites in 2024 to quantify the value of sidedressing K as a standalone application and combined with UAN. Specifically, trials attempted to understand if adding K to a planned N sidedress trip could reduce the impact of drought- or compaction-induced K deficiencies on yield. The locations exhibited a range of yield levels, organic matter content, pH levels, and CECs, but had sufficient K per soil testing (Table 1). Two hybrids with varying degrees of overall response to fertility that matched the RM range of the respective sites (G03B19 and G03UO8 brands at Grundy Center, IA, and Janesville, WI; G10B61 and G10U97 brands at Clay Center, KS, Clinton, IL, Malta, IL, and Slater, IA) were sidedressed using Y-drop tubes at the V4 growth stage.

The standalone K treatment consisted of 10.2 lbs/A (4 gal/A) of potassium acetate (0-0-24). The standalone N treatment consisted of 56 lbs/A (16 gal/A) of UAN (32-0-0). The K+N combination treatment used these same individual nutrient rates so as to understand if there were any potential synergistic affects when combined. Due to the lack of compatibility to mix higher rates of K with a normal sidedress rate of N, a lower K rate was used. A 4:1 N/K ratio was the lowest achievable ratio that would allow for proper fertilizer mixing without compatibility issues.

Table 1. Soil test information at six AIA research sites in 2024.

Results

The individual N sidedress application resulted in a significant yield increase of 7.7 Bu/A over the control, indicating all sites were responsive to additional in-season fertility (Graph 1). The trial saw marginal value of the standalone K sidedress treatment over the control (+0.3 Bu/A) or adding K to the UAN treatment (+0.8 Bu/A), and these results coincide with other K sidedress trials in Iowa, Illinois, and Wisconsin.^2,3 The data also did not show any significant differences in the response to K between hybrids with varying degrees of overall fertility responses.

Graph 1. Yield response to sidedress K and N at six Agronomy in Action research sites in 2024. Different letters indicate differences between treatments (P<0.05).

Figure 1. Corn trial after Y-drop application of treatment in 2024.

Summary

The original intent of this trial was to understand if sidedressing K in compacted soils or drought conditions could help mitigate K deficiencies caused by these stresses. Potassium fertilization as a standalone sidedress or in combination with N did not increase yield across six Agronomy in Action research sites in 2024. This is likely because of adequate soil test K and ample precipitation promoting plant K uptake. Situations may exist where K sidedressing may be advantageous, particularly when dry soils limit its uptake. However, it will likely be difficult for surface-applied K to move deep enough into the soil profile for uptake by roots when conditions are dry. In addition, it will be challenging to conveniently apply a sufficient K rate in the same trip as a previously planned N application. The total amount of K that was applied via sidedress in this trial was very low (10.2 lbs/A) due to N compatibility, thus likely limiting responses. These are important constraints to consider before applying K and N sidedress combination.

References

¹ Bender, R.W., J.W. Haegele, M.L. Ruffo, and F.E. Below. 2013. Nutrient uptake, partitioning, and remobilization in modern, transgenic insect-protected maize hybrids. Agronomy Journal 105:161-170.

² Mallarino, A. P., L.B. Thompson, D.L. Maxwell. 2020. Corn and Soybean Response to Sidedressed Liquid Potassium Fertilizer in Southwest Iowa. Iowa State University Research and Demonstration Farms Progress Reports 2019.

³ Miller, R.D. 2017 Sidedressing potassium and nitrogen on corn. The Fluid Journal 25:7-10.