By Tripti Vashisth, Jeff Brecht, Yu Wang and Faisal Shahzad
In the last decade, fruit quality and size has become a major issue. Moreover, fruit peel color is another challenge to overcome for citrus growers who intend to sell for fresh fruit consumption.
Many nutrients are known to improve fruit growth and quality in citrus trees. For example, potassium increases fruit size, peel thickness and acid content. Calcium is involved in cell division and enlargement, maintaining membrane stability and cell integrity that prolongs fruit resistance to decay. Boron acts as a sugar transporter and improves peel color, juice content and yield. In the past few years, many studies have evaluated nutrient management for HLB-affected sweet oranges, but very few to none have investigated potassium and boron to enhance fruit quality.
University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) researchers conducted a field trial on Sugar Belle and Tango mandarins to determine the effects of potassium, calcium and boron on fruit quality. The nutrient treatments were foliar applied from March through October every 45 days in addition to grower standard fertilization.
After two years of evaluation, both foliar potassium and boron treatments resulted in better yield, fruit size, total soluble solids/titratable acidity (TSS/TA) ratio and peel color than in untreated trees. Additionally, potassium treatment was found to be most preferred for sweetness and flavor among all the treatments. Calcium treatment did not result in better yield or quality. However, calcium-treated fruit had better fruit firmness and shelf life as well as less disease incidence than other treatments.
| Parameter | Variety | Control | Potassium | Calcium | Boron | p value |
| Fruit yield (pounds/ tree) | Sugar Belle | 152 b | 216 a | 98 b | 231 a | 0.001 |
| Tango | 103 b | 119 ab | 134 ab | 154 a | 0.09 | |
| TSS/TA | Sugar Belle | 10.3 b | 12.7 ab | 11.7 ab | 13.7 a | 0.01 |
| Tango | 16.5 | 14.8 | 16.7 | 16.9 | NS |
Although this study was done on mandarins, these findings can be extrapolated to sweet oranges. It is well known that a developing fruit requires more potassium and boron as the season progresses. This is also evident by the fact that the leaves from a fruiting branch are often deficient in potassium and boron, especially from August to December. Therefore, considering the role of potassium and boron in fruit development and the field study described, supplementing potassium and boron is a good option, especially during stage 2 and early stage 3 of fruit development (June–October).
Tripti Vashisth and Yu Wang are associate professors, Jeff Brecht is a research foundation professor, and Faisal Shahzad is a postdoctoral associate — all with UF/IFAS.
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