Sandy soils with low water and nutrient-holding capacities and high nutrient-leaching characteristics are why large amounts of added potassium (K) fertilizer are required to grow healthy and productive citrus trees to yield high-quality fruit in Florida.
“Potassium is extremely necessary to grow citrus successfully in this region,” said Mongi Zekri, regional citrus agent with the University of Florida Cooperative Extension Service in LaBelle, Fla.
The macronutrient K is as important as the macronutrient nitrogen (N) in citrus production, says Zekri who advises on citrus in Charlotte, Glades, Lee, Hendry, and Collier counties in southwest Florida. The area is home to about 130,000 acres of orange, grapefruit, and tangerine trees.
Zekri says mature citrus trees (five years and older) require about 200 pounds each of applied K2O and N per acre annually. Up to 300 pounds of K2O is required for alternate year-bearing tangerine varieties during “on year” production. Sweet oranges require more K than grapefruit.
K increases fruit size, yield, and juice quality. More than 90 percent of Florida oranges are grown for juice compared with about 50 percent of the grapefruit.
A K deficiency in citrus orchards slows the vegetative growth which causes thinning in the highest tree foliage, according to Thomas Obreza and Kelly Morgan, authors of the book “Nutrition of Florida Citrus Trees.”
Inadequate amounts of K increase fruit creasing, plugging, and drop, plus decrease juice soluble solids, acid, and vitamin C content, the authors report. Symptoms of K deficiency in orange, lemon, and grapefruit include yellow to yellow-bronze chlorotic patterns on the older leaves.
A leaf tissue analysis is the best way to determine the K level in citrus to assess the total nutrient concentration in the leaf tissue.
“We get a lot of rain during the summer which causes potassium to easily leach below the root zone,” Zekri said.
That is one reason why the leaf tissue analysis is so important to keep tabs on the tree’s K level.
K can be foliar applied, soil applied, or applied through the irrigation system (fertigation). The actual method used depends on the grower’s personal preference.
Zekri says dry K should be applied about three times a year. K fertigation can be applied weekly or once a month. Foliar applications are recommended at post-bloom to enhance fruit set, size, and yield.
A variety of potassium products are available to growers. Which one to use depends on the tree type and need, time of year, soil type, salinity level, and other issues.
“If the soil has a high salinity level, I do not recommend the use of potassium chloride (muriate of potash). Potassium chloride has a high salinity index which can increase salinity problems. Potassium chloride should not be applied foliarly because it will burn the leaves and cause leaf drop and fruit drop,” Zekri said. “If salinity is not an issue, potassium chloride is the cheapest potassium source available and can be soil applied.”
Other good K products for foliar use, Zekri says, include potassium nitrate and monopotassium phosphate.
Potassium sulfate (sulfate of potash) is a good choice on calcareous-type soil which contains calcium and magnesium.
“We have a lot of calcareous soil with higher pH levels in the gulf citrus area,” Zekri said. “Sulfate of potash is an effective product to apply when the soil pH is 7.5 or higher.”
When the pH level is below 5 (acidic level), Zekri recommends applying calcitic or dolomitic lime to raise the pH to the optimum 6 to 7 level for citrus production.