Blackwood (Acacia melanoxylon R. Br.) is a species with poor apical dominance, and in plantations, silvicultural interventions are required to improve stem form. Planting blackwood with a nurse crop and pruning to remove large branches and multiple leaders are common methods used in blackwood plantations. An experiment was established to determine whether applications of nitrogen (N), phosphorus (P) or magnesium (Mg) could also be used to improve stem growth and form. Other aims of the experiment were to compare responses of the blackwood and the Pinus radiata D. Don nurse crop to fertiliser application, and to identify changes in blackwood physiology resulting from fertilising. Application of P had a more marked positive effect on blackwood stem growth than application of N or the combination of N and P. Mg had no effect on stem growth. The response to P may have been related to a substantial increase in root nodulation and a 35% increase in coarse root dry mass. Foliar nutrient analysis suggested that these responses led to an improvement in plant nutrition. In contrast N application reduced root nodulation by 60%. P application improved stem form in the short term by increasing biomass partitioning to stems at the expense of branches. However this response was no longer apparent 24 months after fertilising, suggesting that frequent applications of P may be necessary for long-term improvements in stem form. While light-saturated CO2 assimilation was increased by applications of N, this was not translated into greater biomass production, probably because of shading of many of the blackwood crowns by the nurse crop. The P. radiata nurse crop responded to fertilising differently to the blackwood, with greatest growth increases following application of either the combination of N and P or Mg alone. This highlights the need to understand responses to fertilising of both blackwood and the nurse crop. The most favourable nurse crop:blackwood height ratio was observed following P application, meaning that this treatment is the least likely to result in suppression of the blackwood by the nurse crop.