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Liming and manuring to ameliorate soil acidity and enhance mineralization in acid tropical soils could promote their agronomic productivity. Synthetic lime and poultry-droppings manure’s effects in sandy-loam Ultisols were evaluated on soybean growth, exploring its relationships with soil pH, soil available P and plant P content. Lime was applied at 0, 2.5 and 5 t ha–1 equivalents and manure at 0, 25 and 50 t ha–1 equivalents to 2.5-kg potted soils. Crop growth was monitored, and soil-plant analysis done during and after six weeks of growth, respectively. Treatment had more pronounced effects on plant height than leafiness of the soybean plants, being generally highest in lime2.5+manure50 where the plants were about three times taller compared to unamended control. Soybean plants grew better due to 2.5 than 0 and 5 t ha–1 lime and to 50 than 0 and 25 t ha–1 manure. Plant height and leaf area depended on soil pH’s influence on soil available P (R2 = 0.69**) and plant P content (R2 = 0.85**), respectively; above-soil biomass depended on soil pH or plant height (R2 = 0.74**). Moderate liming (2.5 t ha–1) with ample manuring (50 t ha–1) could enhance crop early-stage vegetative growth in low-fertility tropical soils, due largely to amelioration of soil acidity to enhance plant uptake of bioavailable P.