To compare the sustainability of alternative cropping systems, a long-term study entitled the Wisconsin Integrated Cropping Systems Trial was initiated in 1990. Maintaining soil test P and K (STP and STK) levels is one aspect of maintaining productivity and thus sustainability.  In this poster we report on the STP and STK levels after 16 years of cropping. Soil test levels were initially quite high (P=80-100 mg/kg; K=200-250 mg/kg), typical of well-managed productive prairie-derived soils.  Using a linear regression model, slopes were between -1.5 to -2.5 mg P/kg of soil/per rotation cycle for the grain rotations.  It was interesting to note that 16 years after initiating an organic grain rotation (c-s-w/rcl), STP was still well above optimum levels.  The slope was nearly zero for the two manure amended alfalfa rotations.  Once stocking rates were modified in the grazing system, (2000), the P-drawdown was also nearly zero. Potassium drawdown was higher, between –2.2 to -6.5 mg K/kg of soil/rotation cycle over the six systems.  In summary, by 2005, STP was at equilibrium, nearing one, or decreasing so slowly that more P will not be required by any of the six systems for at least five years. On the other hand, STK levels have declined in all the systems except continuous corn and are now at the level where standard fertilizer recommendations should be followed.