Phosphorus Fertilization Improves Growth, Yield, Resource-Use Efficiency and Sustainability in Common Vetch ( Vicia sativa L.) Cultivars Under Rainfed Mediterranean Conditions

Common vetch ( Vicia sativa L.) is a well-adapted winter legume in the Mediterranean area, used for both forage and grain production. Phosphorus (P) is a key nutrient influencing plant growth, development, yield, and nutritional quality. This study evaluated how phosphorus availability (0 vs. 60 kg ha −1 P 2 O 5 ) affected the growth, physiological characteristics, yield and environmental resource-use efficiency of two common vetch cultivars, BK-45 and Evinos, over two growing seasons (2020–2021 and 2021–2022). Phosphorus fertilization significantly enhanced vegetative growth, increasing plant height (37.5%) and leaf area index (57%) compared with the control. Improved physiological performance was also observed, as P application increased the chlorophyll content (SPAD) and normalized difference vegetation index (NDVI), particularly during later growth stages. Evinos showed better growth and chlorophyll content around anthesis, whereas BK-45 retained more chlorophyll at maturity. These influences on canopy development and photosynthetic capacity translated into improved yield components, with increases in seeds per pod (40%) and pods per plant (33%), resulting in a higher seed yield (0.127 kg m −2 vs. 0.06 kg m −2 in the control). The dry biomass increased by 50%, with BK-45 showing the strongest response to P fertilization. P fertilization also improved water-use efficiency (WUE) and radiation-use efficiency (RUE), thereby promoting resource use and also the sustainability of the crop. These findings underscore that phosphorus fertilization plays a key role in improvement of common vetch seed yield, forage yield and the sustainability of the cultivars, with the interactions depending on the seasonal variation.