Impacts of variable‐rate potassium fertilization on corn and soybean yield and soil‐test variability

Variable‐rate (VR) technology allows for changing nutrient rates within fields. This study compared VR and uniform‐rate (UR) potassium (K) fertilization for corn ( Zea mays L.)–soybean [ Glycine max (L.) Merr.] rotations. Grid soil‐sampling (0.1–0.3 ha cells), global positioning systems (GPS), and yield monitors were used to establish multiyear strip trials on eight Iowa fields (29 site‐years). Treatments were a control and K for the 2‐year rotation applied once before corn or soybean using UR or VR based on soil‐test K (STK). Strip‐mean yields showed increases ( p ≤ 0.10) in 15 site‐years, but the application methods differed only in two. Potassium increased yield in 39 of 60 instances with STK Very Low, Low, or Optimum (<171 mg kg −1 ), but in three of 23 instances with higher STK. The application method did not influence yield response to K consistently in analyses by site but on average across sites and crops, VR increased yield 3.2% more than UR with Very Low STK (<91 mg kg −1 ). Across all site‐years, VR applied 17% less K than UR and increased STK of low‐testing soils (<130 mg kg −1 ) 40% more than UR, with inconsistent effects in high‐testing soils, which represents cost savings and increased K use efficiency. We conclude that the K application methods seldom differed, but on average across fields, reduced the amount of K applied and increased STK of low‐testing areas more than UR. Results contribute to farmers’ understanding about the potential advantages of VR for K management but also its limitations.