Effect of Application of Different Combinations of Organic and Chemical Fertilizers on Soil Fauna in Marginal Oil Palm Fields in the Low Country Wet Zone of Sri Lanka

This study at Talgaswella Estate, Elpitiya Plantations PLC, in the low country wet zone of Sri Lanka, examined the impact of various combinations of organic and chemical fertilizers on soil fauna and pH in marginal oil palm fields. Using a randomized complete block design, eleven treatments ranging from absolute control to 100% inorganic and organic fertilizers and their combinations were tested in field plots, replicated three times. The aim was to understand how soil biota, particularly earthworms, responds to different fertilizer regimes and soil pH, highlighting the interaction between fertilizers and soil biodiversity, which influences long-term soil fertility and productivity. The fertilization strategy combined organic fertilizers like poultry litter (PL), empty fruit bunch (EFB), and palm oil mill effluent (POME) with inorganic fertilizers such as ammonium sulphate (AS), muriate of potash (MOP), and Eppawala rock phosphate (ERP) to meet the NPK needs of oil palm. Soil fauna, including earthworms, centipedes, millipedes, and beetles, were assessed over a one-week period every six months from May 2022 to March 2024 using field traps, covering both day and night. Two and a half years of post-treatment, earthworms were collected from two depth ranges using excavation techniques, while other fauna were collected from the topsoil. The study revealed that earthworm populations thrived in treatments with organic inputs, especially T₆ (50% CF + 50% OF [10% PL + 90% POME]), T₇ (75% CF + 25% OF [20% PL + 80% EFB]), and T₈ (75% CF + 25% OF [10% PL + 90% EFB]), supported by stable pH levels. Chemical fertilizers, like T₂, showed lower, though not significant, earthworm populations. Millipede populations were stable, with T₁₁ (100% organic fertilizer) showing significant increases. In conclusion, organic fertilizer treatments, particularly T₁₁, significantly promoted fauna diversity, emphasizing the importance of balanced fertilizer regimes for soil health and biodiversity.