Potassium fertilization recommendations for alfalfa (Medicago sativaL.) vary depending on the area of production, soil levels, and crop management. The objectives of this study were to determine the impact of K fertilization on irrigated alfalfa yield, yield components, and soil exchangeable K (Ke) values in a Mediterranean climate. A field experiment was conducted during a period of 4 yr (2002–2006) in Spain, on a soil with moderate levels of Ke(161 mg K kg –1). The treatments applied were five annual rates of K (0, 100, 200, 300, and 400 kg K ha–1). Total 4-yr dry matter (DM) yields averaged 74.9 Mg ha–1 without differences between K fertilizer rates, although the unfertilized control produced the lowest DM yields. The 4-yr crop uptake of K reached 1738 kg ha–1 with the application of 400 kg K ha–1yr–1 , and 756 kg K ha–1for the 0 K fertilization. The removal was greater than the K applied. The results suggest that large amounts of K should not be applied to alfalfa because the crop uses excess K without increasing yield. Soil Ke concentrations decreased every year for all K rates except for the highest treatmentof 400 kg K ha–1yr–1. Lack of K fertilization did not affect stand density but the shoot weight was the yield component most closely related to K fertilization.