RELEASE AND FIXATION OF AMMONIUM AND POTASSIUM UNDER LONG-TERM FERTILITY MANAGEMENT

Improved nutrient use efficiency in production agriculture is a resear ch priority for both agronomic and environmental reasons. To achieve t his goal, a thorough understanding is needed of both shortterm and lon g-term nutrient dynamics in soils. The specific object of this study w as to investigate the effect of long-term fertility management on the dynamics of fixed and exchangeable NH4 and K in a typical Upper Midwes tern agricultural soil. Soil samples (0-20 cm) were collected from lon g-term (30-yr) fertility plots near Arlington, WI. Fixed and exchangea ble NH4 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 NH4NO3. Levels of exchangeable NH4, fixed NH4, and fixed K we re similar for all treatments. Exchangeable K was significantly higher in plots receiving 252 kg ha(-1) K than in those receiving no K. Leve ls of exchangeable K were also higher in plots receiving no N than in plots receiving N fertilizer. Fixation capacities for both NH4 and K w ere significantly reduced by sustained high rates of K fertilization, but not by N fertilization. These results suggest that levels of fixed NH4 can be neither built up nor depleted through long-term fertility management. On the other hand, levels of exchangeable K and both NH4 a nd K fixation capacities are influenced by long-term fertility managem ent.