In 2010, SQM began a 3-year trial with Dr. Brian Boman, University of Florida, to test foliar applications of Ultrasol® K Plus on grapefruit. The trial was conducted in a citrus orchard in the Indian River, Florida area, on the 'Ruby Red' grapefruit variety grafted onto bitter orange stock, primarily to produce fresh grapefruits. The trees (40 ha), irrigated with micro-sprinkler systems using surface water, are cultivated on sandy soils with double-row beds and with the usual drainage system. The block suffered two of the most problematic diseases in the citrus industry, citrus canker and citrus greening, but in very low intensity. The sandy soils of Florida are chronically deficient in K, especially after a heavy summer rain, when much of the K applied to the soil is leached. Foliar applications of Ultrasol® K Plus combined with standard pesticide sprays are recommended to solve this problem.
The trial was designed in completely randomized blocks with 4 repetitions each treatment. Applications were made above the row only with forced air sprinklers delivering 125 gallons per acre. In order to adjust the foliar applications of Ultrasol® K Plus (Table 1) with the critical growth stages, the sprays were applied in February / March (dormancy, cell division), April / May (after flowering, cell growth) and in July / August (summer, fruit filling). This time is important to increase the size of the fruit, since around 70% of the final size is related to the number of cells in the fruit, therefore, more cells generally means bigger fruit.
Cell division commonly stops in late April and the change in size throughout the rest of the year comes from cell growth. Consequently, the maximum effect is achieved from applications that allow K to be available during flowering and after flowering, when K can be used during both phases of cell division and rapid cell growth. An additional application of Ultrasol® K Plus is also recommended in combination with the summer spray (usually July) to ensure enough K through the summer growing season.
Table 1. Ultrasol® K Plus applied dose rates.
The harvest was carried out during the first week of February 2011 and the analysis of the data clearly confirms that the foliar applications of Ultrasol® K Plus in the 'Ruby Red' variety resulted in:
- Larger fruit size (Figure 1).
- Greater number of boxes per tree (Figure 2).
- More fruit in the top price class of grapefruit, which generated 39% more gross income per tree for growers at 12.5 lb/acre/application (Figure 3).
Figure 1. Ultrasol® K Plus resulted in larger fruit sizes.
Figure 2. Ultrasol® K Plus resulted in a greater number of boxes per tree.
Figure 3. Ultrasol® K Plus resulted in more fruits in the top grapefruit price class, which generated more gross income for the grower.
