Nutrient Needs and Fertilization of Apple Trees

To have vigorous, healthy, and productive apple trees with high-quality fruits, the grower needs to pay careful attention to creating a suitable fertilization program to cover the nutrient needs of his/her trees in the different developmental/growth stages and moments.

Essential Nutrients for Apple Trees: Nutrient Levels and Function

Apple trees, like most fruit trees, require macronutrients (nitrogen, phosphorus, and potassium) as well as micronutrients (such as calcium, magnesium, boron, and zinc). These elements play specific roles in the tree's development and fruit production.

In modern and commercial apple orchards, farmers are advised to apply a plant tissue (leaf) analysis once a year (in mid-end summer). To do that, the grower should collect at least 100 leaves from our apple trees and send them to a laboratory for a detailed nutrient analysis. If we detect that the average leaf has some nutrient deficiencies (some important chemical elements are below acceptable standards), then we can intervene and correct the nutrient problems by adding the proper element, for instance, a calcium amendment, through soil or foliar application.

The normal levels of the most important nutrient are listed below:

N: 2% – 2,4%

Nitrogen is perhaps the most important nutrient for apple trees as it supports vigorous vegetative growth, including leaves and shoots. However, excessive nitrogen can lead to overly lush foliage and poor fruit quality, which makes it crucial to apply the correct amount. According to Cornell Cooperative Extension, nitrogen requirements vary based on tree age, growth, and soil conditions. Too little nitrogen results in weak growth and smaller fruit, while too much leads to overly large, poorly colored fruit with storage problems.

P: 0,1% – 0,3%

Phosphorus is vital for root development and flower and fruit formation. While apple trees do not have a high demand for phosphorus compared to nitrogen, it is essential for ensuring healthy root systems, which are the foundation of a strong tree. Phosphorus deficiency, though uncommon, can cause poor fruit set and weak root growth.

K: 1,1% and above

Potassium supports fruit size, color, and overall yield. It also plays a role in water regulation and disease resistance. Apples are considered potassium-intensive, meaning that a deficiency in potassium will lead to poor fruit quality. Symptoms of potassium deficiency include leaf chlorosis, reduced fruit size, and premature fruit drop.

Ca: 1% and above

Calcium is crucial for cell wall strength, directly impacting fruit firmness and shelf life. A calcium deficiency can lead to disorders such as bitter pit, where fruit develops small brown spots beneath the skin. To prevent this, apple growers often apply calcium sprays or supplements during the growing season.

Mg: 0,25% and above

Magnesium supports photosynthesis by being a central component of chlorophyll. Deficiency symptoms include yellowing between the veins of older leaves, often in the late summer, which can reduce fruit size and tree vigor.

Other important nutrients are boron and zinc. Boron plays a role in pollen tube growth and fruit set, making it important for fruit development. A deficiency of this nutrient can result in poor fruit set and corky, misshapen fruit. On the other hand, zinc is involved in enzyme function and is necessary for leaf growth and flowering.

Soil testing must also be performed before establishing the orchard and once every three years later on, since it provides a baseline of existing nutrient levels and pH, allowing growers to make informed decisions about fertilizer applications.

Apple trees grow best in slightly acidic soil with a pH range of 6.0 to 7.0. If the soil is too acidic or alkaline, nutrient uptake can be impaired even if nutrients are present in sufficient quantities. Liming acidic soils can raise pH levels, while sulfur or other acidifying agents can lower pH if necessary.

When and How to Fertilize Apple Trees

The timing of fertilizer applications is critical for optimizing apple tree growth and fruit production. Different nutrients are required at specific stages of growth, such as during early spring for vegetative growth or after fruit set for fruit development.

As a rule of thumb, mature fruit-bearing trees need more fertilization than young trees that have not entered their fruition period. The average mature apple tree needs 0,6 lbs. (270g) of N per year, while the average young non-bearing fruit needs only 0,15 lbs. (68g) of N per year. These are the rates for standard height trees, so the needs of dwarf trees are normally decreased by 40%.

A common fertilization scheme used by many apple growers is adding once a year 1- 4 lbs. (0,5-2 kg) N-P-K 12-12-12 or 11-15-15 per young tree and 6,5 to 11 lbs. (3-5 kg) N-P-K 12-12-12 or 11-15-15 per mature tree. Another commonly used fertilization scheme in an average commercial apple orchard with 600 mature trees per hectare is adding 240 lbs. (110 kg) N and 440 lbs. (200 kg) K20 per hectare, whereas Phosphorus, Magnesium, and Calcium fertilizers are added according to leaf analysis.

Spring Application: The first (soil) fertilizer application of the year is typically applied in early spring, just as buds begin to swell. Slow-release nitrogen fertilizers, such as ammonium sulfate, can provide a steady supply of nitrogen throughout the growing season and are important for supporting new vegetative growth.
Post-Bloom Fertilization: After the trees have bloomed and fruit set has occurred, a second application of fertilizer is sometimes recommended. Potassium and phosphorus are particularly important during this time to support fruit development and overall tree health. Phosphorus fertilizers should not come in direct contact with the tree roots to avoid toxicity problems. Finally, excessive nitrogen fertilization should be avoided post-bloom since it can lead to shoot growth at the expense of fruit quality.
Fall Fertilization: Some growers apply a light amount of fertilizer in the fall after harvest to support root growth and prepare the tree for the following year. However, this should be done carefully, as applying too much nitrogen in the fall can cause unwanted late-season growth that may not harden off before winter.

However, these are just common patterns that should not be followed without doing your own research. Every field is different and has different needs. Checking the soil nutrients and pH is vital before applying any fertilization method. Moreover, if a full coverage irrigation system is combined with the presence of Alfalfa or other legumes (as a cover crop) in a dense planting system, the need for fertilizing apple trees is remarkably reduced because the cover crop normally produces organic matter that is decomposed, releasing important nutrients to the soil.