What are the nutrients that are most critical to citrus fruit color, weight and size; juice content and color; acid and soluble solids content; and peel thickness — the most important characteristics of a good citrus crop?
It’s about applying the right nutrients at the right time, says Bob Rouse, associate professor and citrus horticulturist at University of Florida IFAS. Citrus trees are most active and develop new growth the first six months of the year, so that would be the time to apply about 2/3 of nutrients applied to a crop.
“Rains always work against you,” Rouse says. “That’s another reason to get 2/3 of fertilizer out in the first half of the year. Florida’s rainy season doesn’t start until June or early July. The tree stores fertilizer nutrients really well before then.”
According to Rouse, there are four nutrients that most affect these key characteristics of citrus: nitrogen, phosphorus, potassium and magnesium.
Of course, the number one element is nitrogen and the amount taken up by trees can positively or negatively affect citrus fruits. There is a balance between under and over application that can make the difference between a successful citrus harvest and a disappointing one.
• Adding nitrogen increases fruit juice content.
• Increases soluble solids (sugars) and increases acid slightly. Growers need to balance the increases from the sugars and the slight increase in the acid, which affects the flavor of the fruit and can lead to an acidic-tasting juice.
• Increases the juice color. Increasing solids or sugars on per box basis is what growers are paid for. “[Increasing nitrogen] will increase the sugar, so that means growers get more money,” Rouse says. “There’s a relationship there between profit and use of nitrogen.”
• Too much nitrogen can make the fruit green, preventing it from coloring up, and also makes an undesirable thicker peel.
• Too much nitrogen will make fruit puffy inside with less juice, as well.
Phosphorus applied during citrus production can work to counteract too much nitrogen, and a healthy balance between them is important.
• If you exceed the recommended ranges of phosphorus, it can negatively affect the acid content, pulling it down.
• Excess phosphorus can also increase the ratio of sugars to acid, since acid is decreasing during fruit maturity.
• Phosphorus has no effect on fruit size or weight, but will make fruit stay green if over applied, as nitrogen also does.
• Phosphorus can also help to keep peel thickness to a minimum.
Potassium does have quite an effect on the external part of the fruit.
• Potassium can increase your fruit size and weight.
• Over application of potassium can cause the fruit to stay green and not color up at the end of the season.
• Potassium can keep the peel thickness from being too thick.
• Potassium may increase acids, but it can also increase sugars.
Magnesium is the last nutrient that has much effect on the fruit directly. Chlorophyll is the driving force for making sugars in fruit. The magnesium is the center of the chlorophyll molecule, so plenty of magnesium means plenty of chlorophyll.
• In addition to increasing sugars, magnesium will also increase the ratio of the amount of sugars to the amount of acid and will result in a better tasting fruit or juice. That’s the main function of magnesium.
• Externally, magnesium can increase size and weight of fruit, because you’re increasing sugar content, which results in a heavier fruit.
• Magnesium can negatively affect the peel thickness, however.
To apply the optimum amount of nitrogen to citrus, refer to the ranges set forth by University of Florida Extension for leaf content for nitrogen, potassium on page 27 of Nutrition Of Florida Citrus Trees.