The goal of fertilization is to replace the fertilization units which are removed during harvest and pruning, as well as to cover the needs of the crop for sprouting, flowering, and fruit setting. The annual requirements of a tree in nutrients depend on its age, variety type, early ripeness, intended yields, and the soil and climate conditions of each area.

From winter dormancy to the onset of bud swelling (Dec. – Mar.)

During the winter months, apple trees – as all deciduous trees – are dormant. During this time, the flower buds accumulate the hours of cold required and they become fully grown. The trees have no nutritional requirements at that time and, due to the low temperatures, the roots do not absorb water and nutrients from the soil until March, when bud flushing begins.

From bud swelling to flowering (Mar. – Apr.)

As the temperatures rise, the buds open and the first leaves flush at the base of the fruiting bodies, with the first flowers following shortly afterwards. At the same time, the root system of the tree is activated and starts absorbing nutrients and water.

During this time, the rate of nutrient uptake by the roots and the photosynthetic capacity of new leaves cannot sufficiently cover the increased nutritional requirements of flowering and fruit setting. In order for the trees to nourish the buds, the flowers, and the newly formed fruits, they mobilize the substances they had stored in their roots, trunk, and branches during the previous growing season.

At this developmental stage, the key nutrients are:

Nitrogen (Ν), which contributes to the tree breaking dormancy, supplies the buds and flowers with proteins, and boosts fertilization and fruit setting.

Phosphorus (Ρ), which contributes to the growth of the root system and the renewal of the root hairs, and provides the tree with the necessary energy for flowering and the structural elements for cell division in the new fruits.

Potassium (Κ), which participates in all metabolic reactions and regulates the water balance of the trees, and through it, the redistribution of the stored substances in the developing bodies.

When it comes to trace elements, Boron (Β) increases pollen germination and has a direct effect on fruit setting, while Zinc (Ζn) and Iron (Fe) play an important part in covering the needs of sprouting.

From flowering to full growth (Apr. – Jun.)

Flowering is followed by fruit setting and fruit growth, which is initially the result of cell division and then of cell elongation. The tree also forms its annual produce by June. The growing cycle usually concludes in June and the differentiation of the flower buds which will flower next spring begins.

At this stage, the trees exhibit their highest requirements in nutrients and water. Their requirements in Nitrogen (N) are greater now than at any other point of the annual cycle. Nitrogen supply, however, needs to be moderate so as to avoid excessive vegetative growth which antagonizes fruiting in the current year and undermines the formation of flower buds for the following year.

Phosphorus (Ρ), Boron (B) , and Calcium (Ca) are required in large quantities in order to cover fruit growth and the increased sprouting activity, while the requirements in Potassium (Κ) are high and progressively increase as the fruits grow.

From fast growth to ripeness (Jul. – Sep.)

uring the summer, the fruits increase in size and weight, while the shoots stop elongating and start accumulating dry matter and lignifying.

It is a period of lower requirements in Nitrogen (as it negatively affects the quality and storability of the fruits) and increased requirements in Potassium, which migrates from the leaves to the fruits and takes part in the production of storage substances for the following year.

Α limited Nitrogen supply, combined with an adequate amount of Potassium, increase the size of the fruits and improve their qualitative characteristics, and should therefore constitute a standard practice at this stage. Phosphorus is highly absorbable in August and it increases the weight and binding power of the fruits, while Calcium supply contributes to their storability after the harvest.

Post-harvest (Oct. – Nov.)

It is during this period when many nutrients, in the form of carbohydrates and amino-acids, migrate from the leaves to the trunk and the roots of the trees. These substances are vital to the normal course of sprouting and flowering the following spring.

Supplying the trees with Nitrogen, particularly through foliar application, increases the storage substances and contributes to robust growth and healthy fruiting in the following year.