Influence of Fertigation Regimes on Nitrogen Concentration in Apple (Malus × domestica Borkh.) Leaves at Different Age Stages

Nitrogen (N) deficiency in apples (Malus × domestica Borkh.) leads to characteristic physiological symptoms, including leaf and fruit discoloration. Fertigation, i.e., the application of dissolved fertilizers, can significantly improve the growth and fruit quality of apples while optimizing nutrient uptake through a more precise and better timed application than conventional fertilization. This study therefore investigates how different fertilization treatments affect the N concentration of different age categories of apple leaves. Apples of the variety ‘Braeburn’ were grown hydroponically on the low-vigorous rootstock M9. Four fertilizer treatments were used: (1) Hoagland solution (HS); (2) HS nitrogen excluded; (3) HS iron excluded; and (4) HS magnesium excluded. Through vegetation, leaf samples were taken from three shoot positions representing different leaf ages (young, semi-young and old) and then chemically analyzed. The lowest N concentrations across all leaf ages and sampling moments were found in the treatment with N excluded (1.69–2.07% N), while the highest values occurred in the treatments where iron (2.00–2.49% N) or magnesium (1.98–2.37% N) were excluded. The seasonal changes in N concentration reflect interactions between the leaf age and the sampling moment. These results show that the N concentration of apple leaves strongly depends on the type of fertilization.