Oil palm empty fruit bunch amendments improved the structural and functional properties of a tropical Acrisol

Raw oil palm (Elaeis guineensis Jacq.) empty fruit bunch (EFB) and its biochar derivative (EFB_BC) share some properties but differ in ways that influence soil characteristics. Yet, relatively limited research exists on their effects in low-fertility, structurally poor tropical soils. This study evaluated the impact of EFB applied as organic amendment (EFB_OA) and EFB_BC on soil chemical properties such as pH, organic carbon (SOC), electrical conductivity, cation exchange capacity (CEC), specific surface area (SSA), and physical properties comprising soil water retention (SWR), gas transport, and pore characteristics in a tropical sandy clay loam Acrisol. Treatments included EFB_OA and EFB_BC applied at rates of 20 and 40 t ha⁻¹, along with a unamended control (0 t ha⁻¹). Twenty-one months post-application, intact and disturbed soil samples were analyzed. Both amendments, especially at 40 t ha⁻¹, significantly (p < 0.05) reduced bulk density and increased pH, SOC, CEC, and SSA compared to the control. SOC was highest under the 40 t ha⁻¹ EFB_BC treatment. SWR between matric potentials of −10 and −50 hPa was significantly increased by the EFB_BC at 40 t ha⁻¹. Neither of the amendments significantly affected available water content. At 40 t ha⁻¹, EFB_BC increased air-filled porosity and relative gas diffusivity by 40%–60% and 67%–200%, respectively; EFB_OA increases ranged from 20%–71% and 54%–67% compared to the control. Neither treatment significantly altered soil pore structure complexity. The enhancements in SOC and related soil properties suggest these amendments have the potential to improve water and gas dynamics in weathered tropical soils. Long-term effects, however, require further investigation beyond 21 months.