Avocados are highly nutritious, and the trees need plenty of fertilizer and nutrients for healthy growth and fruiting. A healthy tree will be better able to resist pests and diseases and will produce more fruit. Use a balanced fertilizer such as 6-6-6 or 10-10-10 containing magnesium. Do not use weed and feed products, because these can kill the tree.
The fertilization needs of avocado trees vary depending on the soil in which the tree is planted. If possible, test the soil before planting the tree. The soil test will make specific recommendations for amending the soil and indicate any micronutrient deficiencies.
Avocado trees prefer a soil pH between 6.2 and 6.5. The actual pH of your garden soil is indicated in the soil test. Adjust the soil pH by adding lime to raise the pH. Substitute dolomite for part of the lime if the soil is magnesium deficient. Mix the lime into the soil before planting the tree. Soil that is too acid or basic will interfere with nutrient uptake and make your fertilizer ineffective.
The University of Hawaii recommends leaf tissue analysis to determine nutritional deficiencies of mature trees. Deficiencies found by leaf analysis can be corrected with fertilizer or nutritional sprays. Take six to eight healthy, full-sized leaves from the tree and rinse them gently with water to remove soil and spray residues. Place them in a labeled plastic bag and refrigerate until transportation to the lab. Contact your County Extension Agent to find a lab near you.
Fertilizing Young Trees
Fertilize avocado trees when planting with a balanced fertilizer mixed into the loose soil removed from the hole. After planting, fertilize young trees every other month with 1/4 to 1/2 lb. of 10-10-10 fertilizer. Broadcast fertilizer over the root zone starting about 1 foot from the trunk and working out 1 to 2 feet beyond the foliage. Increase the amount of fertilizer to 1/2 to 1 lb. of fertilizer during the second year.
As the tree grows, its need for fertilizer grows. Increase the amount of fertilizer every year, adding 1/2 lb. of fertilizer to each application for every year of tree age, up to a total of 5 lbs. per application. Apply fertilizer six times per year for the first three years, then four times a year in subsequent years.
Nutritional sprays provide additional nutrients needed by avocado trees and not found in fertilizer formulas. During the spring and summer, apply three or four applications of a nutritional spray containing copper, zinc, boron and manganese. Use a spray containing iron if indicated as deficient in your soil test.
The University of Florida IFAS Extension recommends continuing the nutritional sprays six times a year for the first four years, then decreasing to four times a year. After the first four to five years, only zinc, manganese and possibly boron applications are needed.
In areas with high soil pH, apply 1/2 to 3/4 oz of an iron chelate drench during the late spring to early summer. Increase the dosage of iron chelates by 1/2 oz. per year during the first four years, and then apply 2 to 4 oz. per year during subsequent years.