MAGNESIUM AND POTASSIUM FERTILISER EFFECTS ON FOLIAR MAGNESIUM AND POTASSIUM CONCENTRATIONS AND UPPER MID-CROWN YELLOWING IN PINUS RADIATA

摘要

Upper mid-crown yellowing (UMCY) is a disorder in Pinus radiata D.Don caused mainly by a high potassium/magnesium (K/Mg) ratio in the soil exchange complex and in the tree needles. To study the effects of a range of soil exchangeable K/Mg ratios on po tassium and magnesium uptake and UMCY in P. radiata, a trial was established in September 1996 on a Pumice Soil in northern Kaingaroa Forest in the central North Island of New Zealand in a second-rotation stand of 20-year-old P. radiata . The trial teste d the effects of magnesium applied as kieserite at 200 kg Mg/ha and potassium applied as potassium sulphate at 200 and 400 kg K/ha on soil-exchangeable and soil-solution magnesium and potassium, on tree needle magnesium and potassium, and on severity of UMCY. The applications of magnesium and the two rates of potassium significantly (p<0.05) increased soil-exchangeable and soil-solution magnesium and potassium concentrations respectively in the top 10 cm soil depth during the first 2 years of the trial (1997 and 1998). Magnesium application significantly (p<0.05) reduced the soil-exchangeable K/Mg ratio from 0.7-1.3 (control treatment) to 0.2-0.3, whereas the low and high rates of potassium application significantly increased this ratio to 0.8-1.7 and 1.3-2.5 respectively in the 2 years. The magnesium fertiliser application significantly (p<0.1) increased tree foliage magnesium concentration in 1999, but had no effect on foliage K/Mg ratios in any of the 4 years of sampling (1997, 1998, 1999, and 2002 ). Potassium fertiliser at the high rate significantly (p<0.1) increased the foliar potassium concentration in 1998 and 2002. Neither magnesium nor potassium fertiliser application had any effect on the change in foliar magnesium, potassium, or K/Mg rati o between 1997 and any of the other years sampled. Individual tree UMCY values ranged from 1 to 6 in a system of increasing severity from 1 to 8. Magnesium fertiliser significantly (p=0.074) reduced UMCY values (assessed in 1997 and 2001) in the 2001 sco ring. Similarly, the UMCY value significantly (p=0.055) decreased from 1997 to 2001 for the magnesium fertiliser treatment compared to the control treatment and high potassium rate treatment Potassium fertiliser application had a significant effect neith er on UMCY values nor on changes in UMCY values between 1997 and 2001. The severity of UMCY was not related to the soil-exchangeable K/Mg ratio in spite of the widely different soil-exchangeable K/Mg ratios (0.3 to 2.0 in the 0-10 cm soil depth) produced by the fertilisers. Nor was it related to the foliar K/Mg ratio. The reasons for this could be that the site was not high risk for UMCY during the trial period, the trees were possibly taking significant amounts of potassium and/or magnesium from deeper layers of soil which were probably not strongly influenced by the fertiliser, the amounts of potassium applied were not excessive, and/or the impact of the fertilisers was relatively short-term.