LEAF NITROGEN AS A GUIDE FOR FERTILIZING MACADAMIA

Despite the lack of evidence for a critical level of leaf nitrogen in macadamia, fertiliser management has been largely based on tentative s tandards for high yielding trees. Trees on a lower plane of nitrogen n utrition, however, produced higher yields of good quality nuts. This s tudy was therefore carried out to establish the relationship between y ield and nitrogen status of trees. Three rates of nitrogen fertiliser (0.5, 1.5 and 2.5 kg urea/tree.year; 230, 690 and 1150 g nitrogen resp ectively) were applied to macadamia trees in 1 of 5 application strate gies: 1 application in April (floral initiation); 2 applications, one in April and one in June (inflorescence development); 3 applications, April, June and November (rapid nut growth and premature nut drop); 4 applications, April, June, November and January (oil accumulation); an d 12 monthly split applications. Multiple applications were all equal in size. The association between high yields and low nitrogen status w as confirmed. In some, but not all, years, yield was negatively correl ated with leaf nitrogen, accounting for 47 and 59% of the variation in yield of commercially acceptable nuts (>19 mm diameter) in 1991 and 1 993, respectively. It is therefore recommended that the standard for l eaf nitrogen in macadamia be lowered from 1.4-1.5 to 1.3% under Austra lian conditions. These results raise concerns at the current trend for leaf nitrogen to be as high as 1.8%. It would be prudent to cease nit rogen applications on at least a small experimental block until leaf n itrogen declined to 1.3% and then maintain this level for at least 3 y ears and monitor yields.