The long-term application of compost and biogas residue fertilizers enhances soil qualities in rambutan orchards

Context Perennial fruit orchards in the Mekong Delta, Vietnam, are increasingly constrained by soil degradation and declining fertility on acid sulfate soils. Excessive reliance on inorganic fertilizers exacerbates soil acidification and nutrient depletion, limiting rambutan (Nephelium lappaceum L.) productivity. Aims This study aims to assess the long-term effects of integrating organic amendments with inorganic fertilization on soil properties and yield in a 17-year-old rambutan orchard on Endo-Proto-Thionic Gleysols. Methods A randomized block design was implemented with three treatments: farmer’s practice (high inorganic inputs), recommended NPK combined with sugarcane compost, and recommended NPK combined with biogas residue. Organic inputs (18 kg·plant−1·year−1) and lime (7.5 kg·plant−1·year−1) were applied. Soil samples (0–20 cm) were collected at 3, 6, and 12 months over three cropping cycles to determine pH, soil organic matter (SOM), labile nitrogen, cation exchange capacity (CEC), exchangeable cations, and bulk density. Key results The results show that organic amendments significantly improved soil quality compared with inorganic fertilization alone (P < 0.05). Soil pH increased from 3.4 to 4.7, SOM exceeded 50 g C·kg−1, and labile nitrogen availability was enhanced, particularly under sugarcane compost. Exchangeable Ca2+, Mg2+, and Zn2+ increased markedly, CEC reached 16.5 cmol·kg−1, and bulk density declined, indicating improved soil structure. These improvements translated into yield gains of 36.9–57.5%, with sugarcane compost achieving the highest productivity (14.87–22.70 t·ha−1). Conclusions Overall, integrating organic amendments with inorganic fertilization and liming effectively restores soil fertility, enhances nutrient availability, and supports sustainable orchard management on acid sulfate soils. Implications By using these strategies, farmers can enhance crop yields, mitigate environmental impact, and extend the lifespan of fruit orchard systems and similar agroecosystems.