By Davie Kadyampakeni, Alisheikh Atta and Edilaine Traspadini
Citrus trees require optimal nutrition to be able to maintain high fruit yields, canopy size and good root health. Adequate nutrition supports not only yield and growth, but also tree resilience to disease and environmental stress, which is particularly important in HLB-affected groves.
A balanced mix of macronutrients and micronutrients is needed to ensure the tree’s requirements are met. Macronutrients are categorized into two groups: primary and secondary. Primary macronutrients include nitrogen, phosphorus and potassium. Secondary macronutrients include calcium, magnesium and sulfur. Micronutrients are needed in small amounts but are important for regulating metabolic processes and enabling the efficient use of macronutrients in citrus trees. Micronutrients include zinc, boron, manganese and iron.
The key to nutrient management is to apply nutrients at the correct time, with the right rate, source and placement to ensure adequate availability and uptake by the tree. The nutrients can be applied as controlled-release fertilizer (CRF), conventional granular fertilizer, liquid fertilizer or foliar fertilizer.
WHEN AND WHAT TO APPLY
The University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) recommends applying CRF products two to three times per year to maintain year-round nutrient availability in the root zone. Conventional granular fertilizer can be applied four times per year, starting in February and ending in September or October. Liquid fertigation can be done weekly, bi-weekly or monthly, depending on the grower program, but can be switched to CRF in the summer months to avoid excessive leaching due to rain. Foliar fertilizers are encouraged, especially for micronutrients, to mitigate deficiencies and can be applied three to four times per year.
For now, growers are encouraged to use the rates recommended in the Florida Citrus Production Guide or in the Nutrition of Florida Citrus Trees while research is underway to update some nutrient recommendations, such as nitrogen and phosphorus.
Fertilizer sources that are good for HLB trees are those that contain a bit of every nutrient to allow for spoon-feeding of the weak root systems of HLB trees. Soil-applied fertilizers should be placed in the wetted or irrigated zone to ensure optimal uptake.
SOIL AND LEAF ANALYSIS
Finally, the amount of fertilizer to apply should be determined based on both soil tests and leaf analysis. Interpreting these results together is essential to understanding not only nutrient availability in the soil, but also actual nutrient uptake by the tree. When the soil test indicates high or very high nutrient levels, it does not necessarily mean the nutrients are available in the soil. This can be confirmed through leaf analysis to determine whether the nutrient of interest is low, deficient, optimum, high or very high.
Typically, when the leaf nutrient levels are excessive, it is recommended to omit that nutrient for the next six months of the fertilizer program. When leaf nutrient levels are optimal or high, continue the current fertilizer program. If leaf analysis reveals low or deficient nutrient levels, it means the fertilizer program is sub-optimal and needs immediate improvement.
Although soil and leaf analyses are commonly used to evaluate individual nutrients and determine whether specific application rates should be adjusted, these tools are also essential for assessing overall nutritional balance. Nutrient imbalances, whether due to deficiency or excess, can interfere with the uptake of other nutrients and should be carefully considered when making fertilizer management decisions.
Davie Kadyampakeni is an associate professor, and Alisheikh Atta and Edilaine Traspadini are postdoctoral research associates, all at the UF/IFAS Citrus Research and Education Center in Lake Alfred.