Nut Drop
When macadamias appear to grow well and the trees look healthy but suffer with premature and excessive 'nut drop', what are the likely causes?
Typical Signs & Symptoms
The premature fall of nuts during September-October.
Comments
Premature nut drop is a consequence of poor flower set. Effective nutrient management can positively influence flower set by increasing the availability of nitrogen, phosphorus, potassium, calcium, zinc, and boron within the crop cycle. As flower formation is initiated in late autumn, and the flowers continue to develop through winter and into spring, it is important that all these nutrient inputs are continuously available.
Some nutrients are best applied in a dry granular form, whilst others, typically trace elements such as boron and zinc, are more available to the tree when applied to the foliage. To reduce the costs of application, many of Patons foliar fertilizers can be applied in conjunction with fungicide sprays (provided compatibility is checked before mixing and application). Nitrogen will be available by following the recommendations under Shell/Nut Ratio. Phosphorus and potassium should be applied as part of a dry granular blend in the mid Feb. to July period at the same time as the nitrogen is applied. Phosphorus and potassium should also be applied in liquid form direct on the ground in early spring. A further ground application of potassium may be required depending on leaf tissue levels.
Trace elements such as boron, zinc, and copper have a major effect on plant health and nut retention. Boron is responsible for translocation of nutrients and formation of pollen tubes and should be applied as the flower develops over winter through the application of a foliar spray. Zinc is a stress reliever for plants and will contribute to shoot elongation, longer flowering racemes and, as with boron, will work better if applied as a foliar. Zinc should be applied at flushing on young growth.
Macadamia trees, in their natural state as part of a rainforest ecology, were subject to continual organic matter deposition over their root zone. In a cultivated situation modern harvesting practices can reduce organic matter. The long term organic content should be monitored through reference to soil tests (say over a ten year period) and organic amendments such as humates applied as necessary.
Treatment, Timing & Application
Apply the necessary nutrients strategically, i.e. based on soil and leaf tissue levels, at the appropriate time, with a knowledge of peak nutrient demand periods. Zinc should be applied during periods of flushing growth and following heavy rainfall. Liquid 20 +Zn is the most effective zinc source available (outperforms zinc sulphate sprays by as much as 16 times). The recommended dosage rate is 2 litres per hectare. Boron application is not recommended while flowers are open. Organic K-Humate should be applied at up to 10L/ha. Patons provide Liquid 16+P (Ammonium polyphosphate) and Liquid K & S (Potassium thiosulphate) for spring and autumn use. A dry granular NPK blend formulated in response to a soil test should be applied in the mid-Feb. to July period. Patons have two standard macadamia blends, 212 (12.9-5.3-12.2) and 418 (8.3-2.4-17.2), both with magnesium and a small organic content.
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