Potassium (K) is considered an essential nutrient for plant growth and is taken up in large quantities during the soybean life cycle. Most Kansas's soils can supply enough K for crop production, but there are still areas (e.g., eastern Kansas) where it is common to observe K deficiencies during the growing season. The purpose of this research is to evaluate diagnostic tools for K in soybean, including soil test, tissue analysis, and cation exchange resins.

Soil testing is one of the best tools to identify responsive soils to K fertilizer. However, more research is needed to find the K extraction method that better correlates to soybean grain yield across soils and regions. Some soil test methods used to estimate K availability (e.g., 1 M NH4OAc) are not always good grain yield indicators. Also, tissue analysis has practical applications in assessing possible K deficiencies and predict future fertilizer needs. However, there are still limitations for this method. In case of cation exchange resins, most of the studies focused on greenhouse conditions and more information is needed in order to make recommendations . The use of ion-exchange resins to measure soil nutrient supply has potential applications for fertilizer management. In order to make recommendations, it is imperative to have a strong understanding of sampling moment, burial length, and critical levels.

The overall objective of this project is to improve potassium management for soybean production in Kansas, increasing yields with improved diagnostic tools and fertilization strategies based on soil types in the main soybean producing regions in Kansas. Specific objectives include:

1. Determine the impact of K deficiencies on soybeans yields for different soil types in Kansas.
2. Evaluate current soil test interpretations for K fertilization in soybean, including the evaluation of new soil test methods and the effect of soil clay types on critical levels for soybean.
3. Assess plant K levels during the growing season and determine possible yield limitations related to the high rate of K uptake in high yielding systems.

Update:
Studies were established at 10 locations during 2019-2021 seasons with focus on K deficient soils, but also including a location with traditionally high K level. Soybean showed visual response (until the end of seed-filling) to K fertilization at low soil testing K locations. Soil samples were collected from each individual plots and detailed analysis completed.
Multiple soil test methods (as well as CEC and clay types) are evaluated for improved K recommendations. Also, a novel approach is evaluating K in the soil solution with the use of an exchange resin method, providing both pre-plant and in-season K availability.

The current data we collected is showing that some soils in Kansas will likely require different recommendations for K management. Therefore, in addition to the field study locations, we initiated a survey of soil samples across all the soybean producing counties in Kansas. This survey will allow us to evaluate region/county specific soils, and provide specific K recommendations for soybean, including the best suited soil test methods and diagnostic tools.

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Three experiments were conducted as part of this project to evaluate soil test methods and tissue analysis as diagnostic tools for potassium (K) in soybean. The first study assessed the relationship between K adsorption by cation exchange resins (CER) and K uptake by soybean in field conditions. The study was conducted at two locations with contrasting soil test K levels and two treatments, including a control (0 lbs K2O/ acre) and a high K rate with 150 lbs K2O/ acre applied pre-plant and incorporated. Cation exchange resins were buried in the field in multiple periods to cover the entire soybean reproductive growth stages. In addition, whole plant samples were collected at R2, R4, and R6 stages to measure plant K uptake. Soil volumetric water content and soil temperature were measured using a TEROS 11 sensor. This study found that CER tends to decrease in inverse proportion to plant K uptake, suggesting a measure of soil K surpluses because of root competition. The fertilized plots were able to maintain higher K supply rates during the peak plant demand. Depending on each location, soil temperature and soil moisture content were highly correlated with CER adsorption in control plots.

The second experiment evaluated tissue nutrient concentration and nutrient ratios as predictors of soybean response to K fertilization. It was conducted at 10 locations throughout eastern Kansas during 2019, 2020 and 2021. Four treatments were selected to evaluate soybean response to K fertilization. Treatments included a control with no K fertilization and rates with 50 lbs K2O/ acre increments until reaching a maximum of 150 lbs K2O/ acre. Aboveground plant samples were collected at V4, R2, R4, and R6 stages to measure plant K and Magnesium (Mg) concentration. K concentration and K/Mg ratio at V4 growth stage were well correlated to K uptake at R6 and grain yield. Considering grain yield, the critical concentration range for K and K/Mg ratio was 1.6 to 1.8 % and 2.3 to 2.4, respectively. The nutrient ratio was slightly better in predicting K uptake.

The third experiment compared different soil test K (STK) methods and evaluated the correlation to soybean yield and K uptake response in low testing soils. Additionally, the study assessed the effect of sampling moment on STK results for NH4OAc and Mehlich-3 tests using dry and field moist samples. It was conducted at 10 locations throughout eastern Kansas during 2019, 2020 and 2021. The treatments were a control with no K fertilization and rates with 50 lbs K2O/ acre increments until reaching a maximum of 150 lbs K2O/ acre. Aboveground plant samples were collected at R6 stage to measure plant K uptake. In general, moist tests were better correlated to K response than dry tests, especially with NH4OAc. Among all evaluated methods, the CaCl2 dry and moist, NH4OAc moist, Resin K, and NaBPh4 tests were the best when correlating to relative yield and K uptake. CaCl2 dry is one the easiest and cheapest tests, also having a consistent correlation coefficient (around 0.70 for both variables). Furthermore, it might be an alternative to the NH4OAc moist test because of the high correlation (r=0.91). Three out of ten locations had STK changes for dry samples regardless of K fertilization between fall and the subsequent spring sampling. However, almost all locations had little or no STK change when considering moist samples. Overall, the NH4OAc moist test was one of the best methods to estimate K availability in low testing soils; however, other non-conventional tests like CaCl2 dry might perform similarly but without the typic disadvantages of moist samples.

The observed soybean K deficiency problems in parts of Kansas is likely impacting yield, and can be corrected through fertilizer applications based on accurate soil test critical values and/or plant tissue data and adequate for soil types. Preliminary work funded by the Soybean Commission more than 10 years ago showed impact on soybean yield and the need for a revised/updated this nutrient diagnostic tool.

Results will be delivered to Kansas producers and crop advisors in various ways. Information will be disseminated through field days, KSU Soybean Production Schools, extension publications, and the KSU nutrient management webpage. Results of field experiments and any revised recommendations that arise will be summarized and distributed to the public via news releases. Brief articles will be prepared for publication periodically in the Kansas State University Agronomy eUpdates. Results will be shared with county/district extension agents in the state who provide information to soybean growers on a regular basis. Finally, this work will be completed in close collaboration with the KSU soil testing laboratory, and results will be shared with the KSU soil testing lab as well as private laboratories who advise Kansas growers each year.