Magnesium (Mg) deficiency is common in a number of forest regions in the world. It has been linked to a condition in P. radiata called ‘upper mid crown yellowing’ (UMCY) in New Zealand and ‘new type forest decline’ in Europe. Mg concentrations are low in many of New Zealand"s forest soils. With increases in the number of rotations and increased growth rates through tree breeding, Mg deficiency is expected to increase. This study was conducted to determine the fate of calcined magnesite (calmag) fertiliser applied at 150 kg Mg/ha at 2 sites in the Kaingaroa Forest near Rotorua, New Zealand. It also investigated the effectiveness of calmag in increasing the soil solution and soil exchangeable Mg in pumice soils and Mg concentrations in the pine needles, and in reducing the likelihood of UMCY 2 and 3 years after fertiliser application. In both sites and for both years of sampling the application of calmag fertiliser resulted in a significant increase in soil exchangeable and soil solution Mg in the 0–5 cm soil layer. Soil and soil solution pH had also been increased in the top 5 cm soil layer. Two years after application about 90% of the fertiliser applied had dissolved and about 70–80% of the Mg remained in a plant-available form (ammonium acetate exchangeable Mg) in the top 10 cm of soil. Calculations suggest that 3–10% of applied fertiliser had been lost due to leaching. Magnesium fertiliser application also resulted in significant reduction in the exchangeable K: Mg ratio and reduced exchangeable Al in the 0–5 cm soil layer. After 3 years, foliar Mg concentrations increased at all sites in the fertilised trees compared with the control trees, although differences were not yet significant. UMCY severity in the trees was also not significantly affected by the application of Mg fertiliser.