Cultivar response in starch potato (Solanum tuberosum L.) under mineral and organic P fertilisation: a 2-year assessment within a long-term field experiment

Phosphorus (P) is essential for starch potato production, particularly during critical growth stages from tuber initiation through bulking. At the same time, finite mineral P reserves and environmental concerns associated with P losses to water bodies highlight the need for more efficient P management strategies. This study investigated how mineral and organic P fertiliser treatments, cultivar choice, and seasonal conditions influence soil-test P, rhizosphere phosphatase activity, plant P status, and yield formation in starch potato, using a 2-year field study (2020–2021) within a long-term field experiment on P management established in 1998 in northeastern Germany. Two cultivars (cv. Cardoso, cv. Kuba) were grown under six P fertiliser treatments: control (no organic or mineral P input), triple superphosphate (TSP), biomass ash, cattle manure, cattle manure + TSP, and biowaste compost. Yield formation was significantly affected by year and cultivar, whereas fertiliser treatment had only minor and inconsistent effects on yield and plant P uptake. Main-harvest tuber yield decreased from 53.1 t ha −1 in 2020 to 43.5 t ha −1 in 2021. In contrast, soil-test P responded clearly to P management, indicating differences in soil P status without proportional yield gains. Rhizosphere phosphatase activity varied mainly with year and cultivar rather than fertiliser treatment. Cv. Kuba generally achieved higher tuber and starch yields, particularly in 2021, whereas cv. Cardoso showed higher rhizosphere acid phosphatase activity in 2021. These results suggest that contrasting long-term P fertilisation histories clearly modified soil-test P but did not translate into proportional differences in tuber or starch yield during the two potato growing seasons. Yield formation was driven mainly by year and cultivar, with cv. Kuba maintaining higher tuber and starch yield particularly in 2021. Under these site conditions, cultivar choice and soil-test-based P management therefore appeared more relevant for resource-efficient starch potato production than additional P input alone, whilst regular soil testing remains necessary to avoid both soil P depletion and excessive accumulation.