Soil fertility index based on altitude: A comprehensive assessment for the cassava development area in Indonesia

The local variety of Jarak Towo cassava has a distinctive sweet taste and soft texture, suitable for use as raw materials for the creative food industry. The development of quality commodities requires suitable soil and land. This study aims to identify soil fertility index (SFI), determinants of SFI, and provide land management recommendations for developing the Jarak Towo in Jatiyoso Sub-Regency, Indonesia. The research phase consists of (1) determination of land map unit (LMU); (2) field survey; (3) soil sample analysis; (4) determination of the minimum soil fertility indicator (MSFI); (5) determination of soil fertility index (SFI) and (6) data analysis. Land Map Units are based on an altitude of 400, 600, 800, 1000, 1200, and 1400 m asl (meters above sea level), with 4 sampling points at each LMU. The results showed that soil fertility in Jatiyoso Sub-Regency was classified as moderate and high. Altitude greatly affects SFI. Area 1200–1400 m asl have moderate SFI while area 400–1000 m asl have high SFI. An altitude of 1000 m asl has the potential for the development area of Jarak Towo. Determinant factors of soil fertility are total nitrogen, ratio C/N, cation exchange capacity, exchangeable calcium and exchangeable magnesium. Land management recommendations to improve soil fertility and Jarak Towo varieties' development include adding organic matter and applying manure, lime and dolomite before planting.