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.
Potential of UAV-Based Active Sensing for Monitoring Rice Leaf Nitrogen Status