Corn and Soybean Responses to Phosphorus and Potassium Application Timings and Methods and Fertilizer Sources

This research was driven by concerns over phosphorus (P) and potassium (K) losses from fall-applied fertilizers and the increasing marketing popularity of liquid-P and K fertilizers over traditional granular-dry forms. We evaluated corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] yield and tissue nutrient concentration responses to fertilizer-P and K application timings, methods, and sources across 9-13 site-years on silt-loam soils in Louisiana from 2020-2024. For timing and method study, five treatments were evaluated: a no fertilizer-P and/or K check, fall broadcast followed by rehipping the seedbed, spring broadcast without rehipping, spring broadcast with rehipping, and spring top-of-bed application. In P- and K-deficient soils, no significant differences in yield or leaf nutrient concentrations were measured among fertilizer-P timings and methods for corn, or fertilizer-K timings and methods for soybean. Spring broadcast-K with rehipping increased corn yield by 7.6% over fall broadcast-K with rehipping. When fertilizer-P and K were applied together in suboptimal-P and K soils, no significant yield or nutrient concentration differences were found across timings and methods for either crop. The fertilizer source study included five treatments: a no-fertilizer check, and half- and full-recommended rates of both dry and liquid fertilizers. Triple superphosphate (0-46-0) and muriate of potash (0-0-60) were used as dry-P and K sources and ammonium polyphosphate (11-37-0) and Nachurs K-fuel (0-0-24) as liquid sources. In P- and K-deficient soils, corn yield did not differ between dry and liquid sources, but full-rate applications produced 5-13%, depending on nutrients, greater corn yield than half-rate. For soybean, no significant yield differences were noted between sources or rates for either P or K. However, when both nutrients were applied together, full-rate treatments produced 9-10% greater soybean yield than half-rate. These yield differences did not consistently correlate with leaf nutrient concentrations for either crop. Overall, results suggest that fertilizer-P can be applied in either fall or spring, but fertilizer-K should be applied in spring, at or before planting, especially in coarse-textured, low cation exchange capacity (CEC) soils. Additionally, liquid-P and K sources offer no significant advantage over traditional granular-dry fertilizers, and the nutrient rates should follow soil-test-based rate recommendations.