Impacts of soil pH and extractable aluminum on winter canola production in the southern Great Plains

Winter canola (Brassica napus L.) is a potential alternative to winter wheat in Oklahoma rotation systems as a means to overcome the negative impacts of continuous wheat systems. However, its performance may suffer in the acid soils that are common to intensive agriculture systems in the southern Great Plains region. The objective of this study was to quantify the impact of soil pH and Al3+ concentration on winter canola production. Two field trials were conducted in 2016 and 2017 across four different locations in central Oklahoma to evaluate the performance of four canola cultivars in soils with a range of pH from 4.0 to 8.0. Yield, soil pH, and Al3+ concentrations were measured from each plot to study the relationships between these variables. At all but one location, soil pH and Al3+ concentrations were inversely related and when data across these locations were combined, Al3+ concentrations decreased as pH increased to a critical value of 4.8 (r2 = 0.83). In general, it was found that yield decreased linearly as pH decreased below 5.8 (r2 = 0.75) and increased linearly as Al3+ concentrations decreased below 87 mg kg-1 (r2 = 0.73). Above these thresholds, yield was unrelated to pH or Al3+ concentrations. It was also observed that at one location with very low naturally occurring Al3+, high canola yields were achievable even in low soil pH. Therefore, Al3+ concentrations may be a better indicator of the effects of acidic soils on winter canola yields than pH values alone.