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Dr. PJ Raath: Citrus tree nutrition and carbohydrate management in Autumn and Winter:

The new vegetative shoots of citrus trees are responsible for the positions where flowers develop. These new vegetative shoots are strong sinks for carbohydrates derived from both mature leaves and older shoots, stems, or roots.

One could expect that citrus trees should be able to meet all the carbohydrate requirements since it is an evergreen plant that has ample time for photosynthesis. However, it is evident that citrus trees are often “source-limited” and that a lack of availability of photosynthate then restricts vegetative growth, flower development and fruit set.

Fruit is a major carbohydrate sink and can disturb the balance between vegetative shoot development, root growth and flower development. The persistence and health of the previous year’s foliage in citrus, therefore, play a critical role in the provision of photosynthate during the first spring growth. 

Nutrient deficiencies, particularly in autumn, will contribute to a reduction in carbohydrate availability. Producers should therefore ensure that their trees’ nutrient status should be adequate at the onset of flower induction (April – June).

Nutrient elements (especially N and K) that were not present in sufficient quantities during the leave sampling period (Feb – May) should be brought back to adequate levels through a combination of soil and leaf application.

Foliar sprays with low biuret urea (eight weeks prior to budbreak) are essential for leaves to provide sufficient carbohydrates (sucrose) required for budbreak, spring shoot, growth, and flower development.

For more information on fertiliser programs and services, call the Atlantic Fertilisers team on 021 972 1013.

Dr. PJ Raath, (PhDAgric) is the pre-harvest research manager at Citrus Research International (CRI.).