Leaf nitrogen as a guide for fertilising macadamia

Summary. Despite the lack of evidence for a critical level of leaf nitrogen in macadamia, fertiliser management has been largely based on tentative standards for high yielding trees. Trees on a lower plane of nitrogen nutrition, however, produced higher yields of good quality nuts. This study was therefore carried out to establish the relationship between yield 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 respectively) were applied to macadamia trees in 1 of 5 application strategies: 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); and 12 monthly split applications. Multiple applications were all equal in size. The association between high yields and low nitrogen status was confirmed. In some, but not all, years, yield was negatively correlated with leaf nitrogen, accounting for 47 and 59% of the variation in yield of commercially acceptable nuts (>19 mm diameter) in 1991 and 1993, respectively. It is therefore recommended that the standard for leaf nitrogen in macadamia be lowered from 1.4–1.5 to 1.3% under Australian 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 nitrogen applications on at least a small experimental block until leaf nitrogen declined to 1.3% and then maintain this level for at least 3 years and monitor yields.