<![CDATA[Producing vigorous oil palm (Elaeis guineensis Jacq.) seedlings in the main nursery is essential for field establishment, yet Ultisols, widely used as nursery media, are acidic and low in fertility and organic matter. Volcanic ash is a mineral amendment whose nutrients are released slowly, whereas cow manure supplies organic matter and nutrients that may accelerate its weathering. This study evaluated the interactive effects of volcanic ash and cow manure on the vegetative growth of oil palm seedlings in Ultisol during the main nursery stage. A factorial experiment in a Completely Randomized Design combined three volcanic ash rates (75, 150, and 225 g polybag⁻¹) with four cow manure rates (375, 750, 1,125, and 1,500 g polybag⁻¹), giving 12 treatment combinations with three replications. Seedling height, stem diameter, number of fronds, and leaf area were recorded at 18 weeks after transplanting and analyzed by two-way ANOVA followed by Duncan's Multiple Range Test (P ≤ 0.05). Significant interactions were found for seedling height and stem diameter. The tallest seedlings (99.67 cm) occurred with 225 g volcanic ash and 1,500 g cow manure, while the thickest stems (48.00 mm) occurred with 225 g volcanic ash combined with 1,125–1,500 g cow manure. Number of fronds (12.28) and leaf area (3,361.44 cm²) responded only to cow manure, peaking at 1,500 g polybag⁻¹. These results indicate that volcanic ash and cow manure act synergistically to promote stem elongation and thickening, whereas leaf development depends on the supply of organic nutrients.]]>
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