Effects of fertilization practices and understory on soil health and oil palm performances in smallholdings: An Indonesian case study

Smallholders play a considerable role in oil palm production in Indonesia. Their specific constraints lead to contrasted plot management, especially for fertilization and understory vegetation. Studies disentangling the agro-environmental impacts of oil palm smallholders' management practices at the plot scale remain scarce. The extent to which smallholders' practices affect both agronomic and environmental performances is still to be investigated with robust in-field studies to provide improvement tracks. This study aimed at investigating the effects of both fertilization and understory vegetation management on soil health and oil palm performances. It explored a gradient of management practices, within homogeneous soil and climate conditions and historical development. The objective was to isolate and decipher the specific effects of some key practices on the overall plantation performances. We surveyed 16 ex-plasma oil palm smallholders' 25-year old plots, gathering detailed information on management and implementing field measurements (48 points). For soil health assessment, we applied Biofunctool® that consists of nine in-site indicators informing on the soil ability to transform carbon, recycle nutrient and maintain its structure. For the oil palm performances, we relied on conventional appraisal parameters, such as yields, leaf area and nutrient leaf analyses. Our results highlighted that fertilization, especially potassium, played a significant role in sustaining oil palm performances. Understory vegetation had a direct and positive effect on soil structure maintenance indicators. Soil structure improvement was correlated with enhanced oil palm performances. Those results highlighted that, besides nutritional aspects, increasing plot biodiversity through understory vegetation maintenance was associated with enhanced soil health hence potential greater ecosystem functions. The findings have implications in terms of better understanding the effects of combined farming practices on the soil-oil palm continuum. It especially highlighted the key role of understory vegetation to improve soil structure maintenance without competing with oil palms in the studied context.