Post-Harvest Fertilization of Tree Crops

Post-harvest fertilization of tree crops (the period after harvest and before leaf senescence) can play an important role in the following season crop. In many tree crops, it is one of the best times of the season for uptake of nitrogen and potassium. Trees are still active, and the roots and foliage readily absorb applied nutrients. These nutrients become reserves for the following growing season.  While there may be little difference in tree productivity between fall and spring fertilizer applications, it may be better to fertilize in the fall.

Fall applications are more efficient since trees are still actively taking up nutrients, cold and wet conditions in the spring can delay dormancy break, and spring-applied fertilizers are subject to leaching and runoff. After harvest, trees and vines are recovering from stress, so post-harvest fertilization programs accomplish several things. They replace nutrients removed by the harvest and provides an opportunity to address nutrient deficiency issues before the next growing season. Fall applied fertilizers do not impact tree vigor so what is applied is taken up and goes into tree reserves.

Requirements for macro- and micronutrients varies depending on the crop. Consult a crop advisor on crop needs based on leaf analysis and harvest removal data.

Nutrients to consider and their function

• Boron is important for flowering and fruit set. It also helps develop healthy floral parts, pollen tubules, calcium translocation.
• Zinc is needed for flowering and fruit set. Aids the synthesizing of auxins for cell division for uniform budburst, fruit set, retention, and sizing.
• Magnesium is needed for photosynthesis as it is an important component of chlorophyll.
• Calcium contributes to fruit firmness and cell wall structure. It is an important component of pectin.
• Nitrogen increases yield and fruit size. After budburst, vegetative growth is driven by stored nitrogen in the form of amino acids.
• Phosphorus is needed fall root flush development and for next year’s dormancy break.
• Potassium has a growth regulator effect for water uptake, nutrient uptake and assimilation, and cell reproduction.

Post-Harvest Fertilization of Tree Crops by Species

Almonds – post-harvest fertilization supports the development of next year’s blossoms, growth of the earliest spring leaves and development of immature fruits. Nitrogen can be applied any time after hull split to a few weeks after harvest. The amount needed depends on leaf tissue N from a July sampling.  If sufficient N is present, up to 20% of the N budget can be used not sure what this means. Less N or no N may be needed post-harvest for higher leaf N levels. K can be applied as well although needs may be minimal since the almond hull is the primary sink for potassium. P should only be considered if deficiency symptoms are observed. Fall is a great time for B if hull levels are deficient.  Adequate B is needed for nut set and yield in the following year.

Avocado – since avocados are fertilized several times during the year to match nutrient needs at different growth stages, post-harvest fertilization of nitrogen, phosphorus, and potassium may not be needed. Nitrogen is important for growth and development.  Potassium is needed for growth and fruit development.  Boron and zinc are important for growth and fruit development, respectively. Fertilizers can be applied at any time of the year to support developmental growth needs. There are some things to note.  When leaf concentrations of phosphorus exceed 0.14%, it may have already interfered with zinc uptake in some varieties. Avocado root rot caused by Phytophthora cinnamomi kills root hairs resulting in Zn deficiency symptoms. Hence, Zn fertilization as a corrective measure will not be effective.   

Grapes are a crop where post-harvest fertilization is important. The time required to restore carbohydrate and mineral reserve varies from very little to up to 8 weeks depending on crop load. July or late September applications may result in the highest concentration of N in dormant storage tissue. This nutrient storage fuels the following season’s early growth until roots become active.

Olives are not big feeders, so fertilization is less important than water and good weed control unless deficiency symptoms are observed.  Phosphorus and potassium deficiencies are less common in olives. However, nitrogen deficiencies can appear. Nitrogen is best applied regularly to maintain adequate shoot growth of 8 to 20 inches.

Peaches and other stone fruits should receive sufficient nitrogen to obtain an annual growth rate up to 18 inches. The minimum growth target varies with different fruit. Anywhere from 20-50% of the nitrogen budget for the crop can be applied in the fall before leaf senescence. Fall applications of Zinc can also be applied, but growers should be aware that toxicity can result in a zinc excess.

Pistachio requires phosphorus and potassium that can help trees tolerate infection by Verticillium dahliae. Foliar nitrogen can be used in the fall to supplement and correct deficiency symptoms.

Prunes/Plums, foliar nitrogen can be used to address a nitrogen deficiency, or when conditions for soil nitrogen uptake is low.  Zinc, a micronutrient is usually applied in the fall at the beginning of leaf drop. Note that earlier zinc applications may accelerate leaf drop.

Walnuts, common practice are two to three applications of fertilizer during the year with the last application concluding in August.