Shell/Nut Ratio
When numerous thick shells and low kernel recovery are affecting crop yields but the trees appear healthy, with plenty of flushing, what are the likely causes?
Typical Signs & Symptoms
Field symptoms of this complex thick shell phenomenon may not be immediately evident in the growing crop apart from vigorous shoot extension and soft, lush green growth.
Comments
The amount and type of fertilizer used, in particular the amount of nitrogen provided, will directly influence growth vigour. The consequences of inappropriate or excessive nitrogen fertilizer use may include unacceptable thickness of shell (at the expense of kernel recovery), premature nut drop and lowering of oil content. Of the nutrients used in macadamia culture, nitrogen requires the highest level of management. Use tissue testing to determine leaf N levels as a basis for fertilizer application. The recommended level is 1.3-1.5%
Treatment, Timing & Application
From the perspective of optimal yield management, it is important that applications are timed to supply N at the peak demand periods, one-third from mid-Sep. to late Nov., with the balance mid-Feb. through mid-Sep. No nitrogen should be applied during the period Dec. through mid-Feb.(nut fill and oil accumulation). The total annual amount of N needed is around 550g per tree, which should be applied in small quantities approximately three monthly, or more frequently if possible. Because N is the nutrient most likely to be lost through leaching or volatilization, excessive use only increases production costs. The mid-year Feb. to September application of N ideally should be applied in the nitrate form as this form of N is more readily available to the crop. Note the importance of building N levels to assist flower formation at this time of the year (see Nut Drop).
Calcium Ammonium Nitrate (CAN) is the recommended form of N for this purpose, and again tissue testing is recommended as the method to determine how much CAN to apply. By selecting a CAN based blend, P & K can also be added (see under Nut Drop and Nut Quality). The Paton Macadamia Blends 212 and 418 have been formulated to meet these requirements.
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