Effect of calcined magnesite on soil and Pinus radiata foliage magnesium in pumice soils of New Zealand

Magnesium (Mg) deficiency is common in a number of forest regions in the wo rld. It has been linked to a condition in P. radiata called 'upper mid crow n 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 incr eases in the number of rotations and increased growth rates through tree br eeding, Mg deficiency is expected to increase. This study was conducted to determine the fate of calcined magnesite (calma g) fertiliser applied at 150 kg Mg/ha at 2 sites in the Kaingaroa Forest ne ar Rotorua, New Zealand. It also investigated the effectiveness of calmag i n increasing the soil solution and soil exchangeable Mg in pumice soils and Mg concentrations in the pine needles, and in reducing the likelihood of U MCY 2 and 3 years after fertiliser application. In both sites and for both years of sampling the application of calmag fert iliser resulted in a significant increase in soil exchangeable and soil sol ution 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 remaine d in a plant-available form (ammonium acetate exchangeable Mg) in the top 1 0 cm of soil. Calculations suggest that 3-10% of applied fertiliser had bee n lost due to leaching. Magnesium fertiliser application also resulted in s ignificant reduction in the exchangeable K: Mg ratio and reduced exchangeab le Al in the 0-5 cm soil layer. After 3 years, foliar Mg concentrations inc reased 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.