To determine the frequency of soybean yield response to potassium fertilizer across a range of soil test potassium levels and soil types; to assess the effectiveness of different combinations of potassium fertilizer rates and placements for increasing soybean seed yields; to investigate the capacity of Manitoba soils to retain applied potassium in non-exchangeable forms that might not be available to crops.
Research Objectives: 1. Determine the frequency of soybean yield response to potassium (K) fertilizer additions across a range of soil test levels and soil types; 2. Assess the effectiveness of different combinations of potash rates and placements, for increasing soybean seed yield, including a) 30 or 60 lb K2O/acre sidebanded or b) 30, 60 or 120 lb K2O/acre broadcast and incorporated; 3. Investigate the capacity for MB soils to retain added K in non-exchangeable forms, which may not be available for crop uptake (this objective was not completed, due to the lack of K response at our research sites).
Summary of Activities and Results – In 2017 and 2018, 20 on-farm trials and 7 small plot trials were used to investigate the frequency of soybean response to K fertilization across a range of soil test K concentrations (49–451 ppm), and paired microplots within on-farm trials were used to investigate the response relationships at a more detailed scale. In all three types of trials, soybean yield responses were rare or non-existent and there was no significant relationship between soil test K concentrations and soybean yield response to K fertilizer. Small plot trials were established to measure the effectiveness of different K fertilizer rate and placement combinations for improving seed yield. However, there was no significant seed yield response to K fertilization at any small plot site-year; therefore, the effectiveness of the different rates and placements could not be determined.
Relevance to Farmers – Predicting soybean K response is unreliable using the current soil test method and 100 ppm threshold. However, maintaining K fertility in the soil to support K nutrition for all crops in the rotation (e.g., barley, which responded to K fertilization at our small plot sites where soybean did not) is important.