Soil-Applied Nutrition

• Plants uptake nutrients when they are in a water solution.

• Soil-applied nutrients are moved into the root during water uptake and therefore distributed in the tree along with water.

• Mobile and immobile nutrients have an equal and uniform distribution in all parts of the plant.

• Granular fertilizer, controlled-release fertilizer, and fertigation are all soil-applied fertilization methods.

Huanglongbing (HLB) Consideration

• HLB-affected trees have a small root system; therefore, the nutrient uptake capacity of roots is limited.

• The constant availability of nutrients is critical.

Foliar-Applied Nutrition

• Nutrient sprays should be timed at two-thirds-expanded to nearly fully expanded young leaves.

• Mature leaves with thick leaf cuticles limit nutrient uptake.

• Even when foliar sprays are applied at the right time, immobile nutrients can get locked in leaves and cannot move into growing leaves, roots, or fruit; as a result, their distribution within the plant is poor.

Soil pH and Nutrient Interactions

• At high soil pH, most of the micronutrients bind to the soil and become unavailable for plant uptake.

• At extremely low soil pH, most of the macro- and secondary nutrients become unavailable for plant uptake.

• The goal is to have the right soil pH range for nutrient uptake.

• The preferred range for soil pH in Florida citrus production with HLB-affected trees is 5.8–6.5.

Movement of Mobile Nutrients

• Mobile nutrients move to areas of active growth within the plant.

• Nutrients move in all directions.

• These nutrients are transported via xylem and phloem.

• Deficiency symptoms appear first in older leaves, and if the deficiency is not corrected, they show up in new growth.

• Mobile nutrients include Nitrogen (N), Phosphorus (P), Potassium (K), Magnesium (Mg), and Sulfur (S).

• Soil- and foliar-applied nutrients are both adequate.

Movement of Immobile Nutrients

• Immobile nutrients do not redistribute within the plant.

• These nutrients are transported only via xylem.

• Immobile nutrients do not move to areas of active growth.

• Deficiency symptoms show up in the new growth first, and if not corrected, they will show up in older leaves.

• Immobile nutrients include Calcium (Ca), Iron (Fe), Zinc (Zn), Copper (Cu), Manganese (Mn), Boron (B), and Molybdenum (Mo).

• Foliar applications have limited efficacy and distribution in the plant.