Site selection for both experiments was based on UMCY information provided by aerial mapping and site visits of Kaingaroa Forest.
UMCY scoring in FR190/5 was carried out in December 1996.
Exchangeable Mg levels in the control plots of both trials (0-5 cm depth) were well above the critical level suggested by Will (1961) but the trees in FR190/5 have displayed minor UMCY symptoms with scores in the range 1.9-2.5 (relating to yellow needle tips sub-apically), suggesting that the critical level is higher or there are other factors, such as nutrient antagonisms (e.g.
High foliar K:Mg ratios have been linked to more severe UMCY (Beets et al.
Therefore, it could be expected that the difference in foliar Mg concentrations between the control and fertilised plots should continue to increase with time, but fertilised plots still may remain at concentrations where slight UMCY could still occur, based on correlations by Beets and Jokela (1994) between UMCY score and foliar Mg concentrations.
High foliar K: Mg ratios have been linked to UMCY (Beets et al.
Effect of Mg fertiliser application on UMCY in FR190/5
6) but this increase had no effect on UMCY scores (Fig.
Due to moderately high foliar Mg concentration in the unfertilised trees, and climatic and genetic factors, UMCY scores were not significantly different between fertilised and unfertilised trees.