Grass or grass-clover leys are important parts of many crop rotations, providing forage for livestock and exerting positive effects on soil structure and fertility. One of the most important nutrients for optimum ley and especially clover growth is phosphorus (P). Based on eight Swiss and Danish long-term field experiments, we examined the response of grass and grass-clover yields to up to 26 years of varying P fertilization rates, the development of soil test P (STP) values and potential effects of clay content, organic carbon content and pH of the soil, average temperature and precipitation. While different fertilization rates often resulted in significantly different STP values, yields mostly showed significant differences between unfertilized and fertilized treatments, but not between treatments with different fertilizer amounts. Increasing soil clay contents increased relative yields in relation to Olsen-P values.