Assessing nutrient uptake by field-grown orange trees

Mineral nutrients applied to crops are a cost to growers and could represent a cause of air and water contamination, especially when supply exceeds nutrient needs. Assessing the amount of nutrients that trees need to absorb in order to successfully complete a vegetative and reproductive growth is therefore a fundamental step for developing rational fertilization strategies in orchards. In this paper, the estimation of cumulative yearly nutrient needs and the dynamics of nutrient uptake along the vegetative season in "Tarocco" orange [Citrus sinensis (L) Osbeck] orchards in Southern Italy were studied. In a first experiment, allometric relationships between branch circumferences and wood or leaf biomass were obtained. These relationships were used to quantify the growth patterns of the above ground organs. Woody organs accounted for the majority of the above ground biomass (AGB). Although the leaves accounted in total for less than 21% of AGB biomass, they included more than 38% N, 31% P, 44% K, 32% Ca and 33% Mg. In a case-study represented by a mature citrus orchard, most AGB annual increase was accounted by the fruits and the shoots. Calcium was the nutrient absorbed with the highest rate, followed by N. K, Mg and P. Most Ca and Mg were recovered in the abscised leaves, while most P and K were recovered in fruits. Nitrogen was distributed more equally among fruits, pruning wood and abscised leaves. Seasonal uptake data indicate that N, P and Ca are absorbed from April to November, while the net uptake of K and Mg is almost complete by the summer. From December to February the amount of nutrients present in the tree canopy either remained stable or decreased, therefore indicating no net nutrient uptake during this period, and, as in the case of N and K, internal nutrient translocation to perennial organs. (C) 2012 Elsevier B.V. All rights reserved.