Potassium (K) is a critical macronutrient for alfalfa growth; however, reports of the effects of K fertilization on alfalfa yield, quality, and persistence have been inconsistent. Five K fertilizer rates ranging from 0 to 403 kg K ha–1 were applied to eight alfalfa cultivars during the establishment year (2011) and three subsequent production years (2012–2014) at three locations varying in baseline soil test potassium (STK) levels. Alfalfa yield, nutritive value, and STK were measured annually. During the final year, stem density and belowground traits were quantified. Forage yield, nutritive value, and root traits varied by cultivar, but there were no interactions between cultivar and K rate. Crown rot varied by cultivar but was not affected by K rate. Averaged across cultivars, the effect of K on yield varied by location and stand age. At the site with the lowest baseline STK values, K fertilizer had a positive quadratic effect on yield, which plateaued at a K rate of 296 kg K ha–1. There was no effect of K on yield at the site with the greatest baseline STK. Potassium fertilizer reduced forage nutritive value and increased forage K concentration. Aboveground and belowground biomass K concentration increased with K fertilizer at rates beyond those that maximized yield, indicating luxury consumption of K. However, mineralization of soil K resulted in a net increase in STK after accounting for K removal during harvest. Potassium removal rates that resulted in maintained STK were 167, 227, and 268 kg K ha–1 yr–1 at three locations.