<![CDATA[This research was conducted in Surbakti Village, Simpang Empat District, Karo Regency, North Sumatra Province, Indonesia, at an altitude of approximately 1,250 meters above sea level (masl). The research was carried out from August to November 2025. This research was supervised by Dr. Ir. Muhammad Rizwan, M.P. as the Head Supervisor and Dr. Ir. Diapari Siregar, M.P. as the Assistant supervisor. Potato is a strategic horticultural commodity in North Sumatra, whose productivity has declined due to decreasing soil fertility and increasing pest and disease incidence. This study aimed to determine the effect of volcanic ash and organic matter formulations on soil nutrient status in potato cultivation fields in Surbakti Village.The experiment employed a Randomized Complete Block Design (RCBD). The first factor was the formulation of volcanic ash and organic matter, consisting of: A0 = control; A1 = 5 tons/ha (60% volcanic ash + 40% rice husk); A2 = 5 tons/ha (60% volcanic ash + 40% cattle manure); and A3 = 5 ton/ha (40% volcanic ash + 30% rice husk + 30% cattle manure). The second factor was the application time of the volcanic ash and organic matter, consisting of: B1 = one week before planting; B2 = at planting; and B3 = one week after planting.The results showed that the volcanic ash formulations significantly affected plant height, number of leaves per plant, stem diameter, number of tubers per plant, tuber yield, tuber diameter, and the severity of late blight disease. The application timing significantly affected plant height, number of leaves per plant, number of tubers per plant, tuber yield, and tuber diameter. Furthermore, the interaction between volcanic ash formulation and application timing significantly affected plant height and the number of leaves per plant]]>
