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Foliar Applications with Ultrasol® K Plus Increase Grapefruit Grower’s Gross Income by 39%.

In 2010, SQM started a 3-year trial with Dr Brian Boman, University of Florida, to test foliar Ultrasol® K Plus on grapefruit. The trial was performed in a citrus grove in the Indian River area, Florida, on ‘Ruby Red’ grapefruit on sour orange rootstock, primarily grown to produce fresh grapefruit. The trees (98/acre), irrigated with micro-sprinkler systems using surface water, are grown on sandy soils with double-row beds and typical drainage systems. The block suffers from the two most challenging diseases in the citrus industry i.e. citrus canker and citrus greening, but at very low rates. Florida sandy soils are chronically K deficient, especially after heavy summer rainfall when much of the soil applied K is leached. Foliar applications with Ultrasol® K Plus, combined with the grower’s standard pesticide sprays, are recommended to overcome this challenge.

The trial is designed as a randomized complete block design with 4 replications of each treatment. Applications will be made to bed tops only with an air blast sprayer delivering 125 gallons per acre. In order to match the foliar Ultrasol® K Plus applications (Table 1) with critical growth stages, spraying is applied in February/March (dormant, cell division), April/May (post-bloom, cell enlargement) and in July/August (summer, fruit fill). This timing is important to enhance fruit size, since around 70% of final fruit size is related to the number of cells in the fruit, so more cells usually means larger fruit. Cell division typically stops by late April and size change throughout the rest of the year comes from cell enlargement. Therefore, the maximum effect is achieved from applications that make K available during bloom and post-bloom when K can be used during both cell division and rapid cell enlargement phases. An additional application of Ultrasol® K Plus along with the summer spray (normally in July) is also recommended to ensure sufficient K throughout the summer growing season.

Harvesting took place during the first week of February 2011 and the assessed data have clearly proven that foliar applications of Ultrasol® K Plus on ‘Ruby Red’ resulted in:
• Larger fruit sizes (Figure 1).
• More boxes per tree (Figure 2).
• More fruits in the higher grapefruit price class, which in turn generated 39% more gross income per tree for the grower in case of the12,5 lbs/acre/application (Figure 3).

Table 1. Applied dose rates of Ultrasol™ K Plus.


Applied dose rates of Ultrasol® K Plus (lbs/acre*)







Per application






Total (3 applications)






*1 lb/acre = 1,12kg/ha

Figure 1. Ultrasol® K Plus resulted in larger fruit sizes.

Figure 2. Ultrasol® K Plus resulted in more boxes per tree.

Figure 3. Ultrasol® K Plus resulted in more fruits in the higher grapefruit price class, which in turn generated more gross income for the farmer.

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