Alfalfa yield and nutrient uptake as affected by pH and applied K

Approximately 30% of the cultivated cropland in the state of Wisconsin is devoted to alfalfa (Medicago sativa) production. Much of the alfalfa forage produced is used to feed the approximately 1,350,000 dairy cattle found in the state. A key concern of dairy producers is the cation to anion balance in rations for dry and early lactation cows, which is greatly influenced by soil K levels. This study was designed to examine the interactive effects that varying soil pH levels and annual potassium applications would have on forage mineral balance, yield, and quality. Three locations with varying soil textures were used for this 4-yr study. Results indicate that soil pH and applied K can both significantly influence forage yield, quality, and cation levels. At soil pH levels that are near optimum for alfalfa production and yield levels in the 8-10 Mg ha^-1 range, it appears that approximately 224 kg K₂O ha^-1 is required annually to optimize yield and stand quality and to maintain soil test in the optimum category. At all locations, as soil K increased, tissue K increased, and to a much smaller extent, tissue Ca and Mg decreased. Annual K application resulted in a buildup of soil test K and a decrease in soil test Ca and Mg. Increasing soil pH, and consequently soil test Ca, did not consistently result in reduced tissue K levels. However, liming these acid soils was essential to optimize alfalfa yield. Keeping soil test K levels in the optimum range appears to be the best strategy for keeping forage tissue K levels in acceptable ranges for use as dry cow and early lactation dairy feed.