Soil Potassium Test Recalibration and Sampling Time for Corn in North Dakota.

Potassium (K) fertilizer recommendations for corn (Zea mays, L.) are commonly guided by yield response calibrations to soil test potassium (STK) levels; however, soil sample drying and date of soil sampling have been shown to affect STK values. Furthermore, yield responses to K fertilizer may be inconsistent on low and high STK soils. Field trials were established in southeastern North Dakota to evaluate soil K testing methodologies and their relationship to corn grain yield and to assess temporal STK variation. Plot treatments included six KCl rates (0 to 167.5 kg K₂O ha^-1) and one fallow control treatment to assess STK changes without plant uptake. Soil samples (0-15 cm depth) were collected biweekly during the growing season and analyzed for NH₄OAc-extractable K on air-dry (DK) and field-moist (MK) subsamples. A cation-exchange resin and a NaBPh₄ extraction method were also evaluated for their ability to extract some plant-available nonexchangeable K in addition to exchangeable K. A mineralogical analysis for K-bearing minerals and clay speciation was employed to help address inconsistences between STK and corn yield responses. Sample drying significantly increased STK, and the DK/MK ratio was related to the MK level but not the DK level. Both DK and MK levels were significantly affected by sampling date at most sites, with highest STK in spring and lowest STK in fall, appearing to follow a sinusoidal pattern. Significant yield responses were not adequately predicted by the current 150 mg kg^-1 DK critical level. Quadratic models of relative yield regressed against STK from the different testing methodologies were evaluated for their ability to predict yield response to K fertilization.