B.W. Cull’s research while affiliated with University of the Sunshine Coast and other places

Annual soil and monthly leaf nutrient analyses, monthly flushing assessments, meteorological data and yield were recorded for two macadamia cultivars, ‘Keauhou’ (246) and ‘Kakea’ (508), at Beerwah, Maleny and Baffle Creek in south east Queensland over three seasons, and subjected to multiple regression analysis of yield. Apart from cultivar, which had the largest influence on yield, high leaf nitrogen (N) and tree dormancy during rapid nut growth and oil accumulation, leaf boron (B) prior to and during anthesis, and to a lesser extent soil zinc (Zn) were important. A large amount of hardening flush during the nut set and premature nut-drop stages also contributed to high yield, probably through a higher potential photosynthetic capacity of recently mature tissue during the critical nut growth and oil accumulation stages.

For elucidating the role of fertilizers in enhancing nut yields in macadamia (Macadamia integrifolia Maiden and Betche), three soils were sampled for characterisation of their nutrient supply capacity. The soils were a red podzolic, a krasnozem and a sandy red earth. All were well-drained and uniform in texture to a depth of 1.5 m. Soil was taken from the dripline of 10 mature trees each of cultivars ‘Keauhou’ (246) and ‘Kakea’ (cultivar 508). Initial nutrient levels were determined in late 1979 and repeated in late 1980 and 1981 after application of a variety of fertilizers during the previous 12 months. Fertilizer applications raised the levels of phosphorus, potassium, calcium and magnesium; pH remained steady and low; nitrogen and sulphur were maintained except on the krasnozem where nitrogen, and on the red podzolic soils where sulphur, diminished somewhat. None of the other soil constituents analysed occurred in harmful quantities. Nut-in-shell yields varied only slightly with soil type and could not be related to changes in fertilizer applications or soil nutrient levels.

The mean monthly nutrient composition of macadamia leaves was assessed over three seasons on 10 trees each of two cultivars at three orchards in south east Queensland. Leaf nitrogen (N), phosphorus (P), sulphur (S) and boron (B) levels were lower and potassium (K), copper (Cu), iron (Fe), zinc (Zn) and manganese (Mn) were higher than the Hawaiian standards.Tentative standard leaf levels of 1.4–1.5% for N, 0.6–0.7% for K, 0.18% for S and 40–70 mg kg−1 for B are suggested as being more suitable for macadamias under Australian conditions. The other Hawaiian standards are accepted. Additional work is needed to further refine the standard levels, with emphasis on the effect of leaf nutrient composition on yield and quality of kernels.In Australia, leaves should be sampled in spring, preferably at the commencement of the spring flush and after anthesis. At this time, 6–7-month-old leaves which dominate the canopy and which have a relatively stable nutrient composition from the previous summer flush should be sampled from dormant terminals.Small but statistically significant differences were obtained in leaf nutrient levels between cultivars ‘Keauhou’ (246) and ‘Kakea’ (508). Differences are unlikely to be of physiological importance, except perhaps for N and K which were higher in (508), and Ca which was higher in (246).

The nutrient composition of macadamia (Macadamia integrifolia Maiden and Betche) leaves was monitored at monthly intervals to provide information for the development of nutrition management guidelines more suited to local conditions.Leaf nitrogen (N) and potassium (K) followed similar seasonal patterns: levels rose during autumn to high levels in winter, declined in association with the spring flush, flowering and nut-set period, and remained stable over the summer nut-growth/oil-accumulation stage, before declining again in late summer, coinciding with the major summer flush. Leaf phosphorus (P) was similar, but declined earlier during winter. The patterns for calcium (Ca) and boron (B) were different and were determined largely by leaf age. There were no consistent trends in levels of sulphur (S), sodium (Na), magnesium (Mg), copper (Cu), iron (Fe), zinc (Zn), manganese (Mn) or chloride (Cl).Site, involving soil fertility, management and climate, had a dominant effect on levels of N, P and Ca but not on K, which was influenced more by monthly effects. Leaf nutrient levels reflected soil levels and fertilizer applications in some cases, but this was not consistently so.

Monthly flushing patterns on both north and south sides of cultivar ‘Keauhou’ (246) and cultivar ‘Kakea’ (508) macadamia trees at Beerwah, Maleny and Baffle Creek in south east Queensland were recorded from November 1979 to April 1982. Major flushes occurred in late summer and early spring when mean monthly maximum and minimum temperatures were within reported threshold limits. Total flush growth was greater in 508 than in 246, mainly due to larger spring flushes. In autumn and winter, the northern sides of trees produced more growth than the southern sides. The desirability of monitoring flushing patterns to more completely assess the effects of management practices on sustained production is confirmed.