Vachellia nilotica, a nitrogen-fixing tree, can improve nutrient-poor soils in grasslands but faces challenges from nutrient deficiencies and competition with important grasses such as red oat grass/kangaroo grass. The tree grows better in phosphorus-rich soil and forms nodules for nitrogen fixation, whereas nutrient-poor soil and grass competition reduce growth and nitrogen efficiency. This study highlights how low soil phosphorus content and plant competition with grass species can reshape legume growth and nitrogen source utilisation. Photograph by Naledi Zama.