History and interpretation of early soil and organic matter investigations in Deli, Sumatra, Indonesia

•Early soil studies on the soils of Deli were directed towards tobacco production.•Van Bemmelen formalised a large-scale long-term fallow plantation system in late 19th century.•Continuous cropping in the tropics can decrease soil OC levels up to 80%.•Soil mapping and classification based on parent materials were described by Druif.•Soil OM dynamics and soil provenance concepts have been established since early 20th century. This paper provides a history of the investigation of the soils and organic matter of Deli in Sumatra, Indonesia, for growing tobacco in the early 20th century and an interpretation based on current data, knowledge and understanding. We first review some early chemists and agrogeologists’ investigations on the soils of Deli to increase tobacco production. Van Bemmelen studied the humus of the soil of Deli in 1890 and formalised an 8-year fallow plantation scheme for growing tobacco. While maintaining organic matter had been established, the complexity of soil distribution in the area was more important in determining the quality of tobacco. It took another 40 years for the soil in Deli area to be properly mapped. Jan Henri Druif in the 1930s mapped and classified the soils of Deli based on their parent material and mineralogical composition. We then describe the rise and demise of the tobacco industry from 1930s-current. We examine the implication of the fallow system and soil distribution with the current understanding of soil carbon processes and recent data. The results are interpreted and discussed considering i) the myth of ”poor” tropical soils, ii) nutrient availability after slash and burn, iii) soil organic matter decline after forest conversion and recovery after fallow, and iv) soil mapping and provenance. Based on published studies and observed data coupled with modelling, we attempt to explain early researchers’ observations and deductions. We summarise soil organic carbon dynamic conditions in the tropics after 50 years of forest clearance: under fallow rotation, it is possible to maintain, on average, a constant value of 20% organic carbon (OC) decrease from the original level, while continuous cropping can decrease OC levels up to 30–40%. An extreme condition with continuous cultivation and little organic matter input can result in an OC decline of up to 80%. The historical studies enable to appreciate aspects of soil mapping and organic matter that are repeatedly overlooked in present-day research.