Burning biomass for energy generates ash that could be applied as a soil amendment to ameliorate acidity and mitigate nutrient losses associated with biomass harvesting. These soil improvements may also enhance tree growth and foliar nutrition. In this study, we applied low- and high-carbon wood-derived ash at rates of 0 (control), 1000, and 10 000 kg·ha⁻¹ (dry weight equivalents) to soils planted with Picea mariana (Mill.) B.S.P. and Picea glauca (Moench) Voss in a replicated (5) factorial design. We measured soil properties, tree seedling height, and foliar nutrient contents prior to and 4 mo after wood ash addition to determine the immediate effects on soil physical, chemical, and biological properties, and tree seedling performance. We conclude that there were no negative effects of applying either wood ash and that application of ash at 10 000 kg·ha⁻¹, particularly with the low-carbon ash, produced the greatest changes. We anticipate that changes may become more evident over the longer term, especially with respect to tree growth and nutritional responses (e.g., as nutrient uptake demand increases) once the seedlings become more established.