THE INTERACTIVE EFFECT OF OIL PALM AGE AND SLOPE SLOPE ON SOIL PHYSICAL PROPERTIES IN BILAH SUB-WATERSHEDS

Abstract

Oil palm (Elaeis guineensis Jacq.) expansion in tropical watersheds has raised concerns regarding soil physical degradation and its implications for hydrological functioning. This study investigated the effects of plantation age and slope gradient, as well as their interaction, on soil physical properties in the Bilah sub-watershed, North Sumatra, Indonesia. The study was conducted at Negeri Lama Village, District of Bilah Hilir, Labuhanbatu Regency from May to August 2025.  A field survey was conducted across oil palm stands of different ages (0, 5, and 12 years) and slope classes (3–8% and 8–15%) using a factorial design with three replications. Soil samples were collected at depths of 0–30 cm and 30–60 cm to analyze bulk density, soil organic matter, total pore space, water content, and permeability. The results showed that increasing plantation age significantly improved soil physical quality, as indicated by higher soil organic matter, total pore space, water content, and permeability, alongside reduced bulk density. In contrast, steeper slopes were associated with soil degradation, reflected in increased bulk density and reduced porosity, moisture retention, and permeability. The interaction between plantation age and slope gradient revealed spatial variability in soil physical recovery, particularly under moderate slopes where improvements were more pronounced. These findings highlight the importance of considering both biotic (plant age) and topographic (slope) factors in managing soil quality within oil palm-dominated landscapes. The study provides a scientific basis for site-specific soil conservation strategies to enhance long-term productivity while maintaining watershed ecological functions.