Release and Fixation of Ammonium and Potassium under Long‐Term Fertility Management

Improved nutrient use efficiency in production agriculture is a research priority for both agronomic and environmental reasons. To achieve this goal, a thorough understanding is needed of both short‐term and long‐term nutrient dynamics in soils. The specific object of this study was to investigate the effect of long‐term fertility management on the dynamics of fixed and exchangeable NH 4 and K in a typical Upper Midwestern agricultural soil. Soil samples (0–20 cm) were collected from long‐term (30‐yr) fertility plots near Arlington, WI. Fixed and exchangeable NH 4 and K were determined for plots receiving 0 and 252 kg ha −1 K (from 1962–1982) and 0, 68, 112, 168 kg ha −1 N (from 1962–1993) as urea or NH 4 NO 3 . Levels of exchangeable NH 4 , fixed NH 4 , and fixed K were similar for all treatments. Exchangeable K was significantly higher in plots receiving 252 kg ha −1 K than in those receiving no K. Levels of exchangeable K were also higher in plots receiving no N than in plots receiving N fertilizer. Fixation capacities for both NH 4 and K were significantly reduced by sustained high rates of K fertilization, but not by N fertilization. These results suggest that levels of fixed NH 4 can be neither built up nor depleted through long‐term fertility management. On the other hand, levels of exchangeable K and both NH 4 and K fixation capacities are influenced by long‐term fertility management.