Methodologies and Concepts in the Study of Nutrient Uptake Requirements and Partitioning in Fruit Trees
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Relatively few published papers report the annual nutrient uptake by fruit trees, a fact that is surprising considering that it represents the backbone of proper fertilization in sustainable agriculture. Nutrient uptake by fruit trees can be quantitatively assessed by following the uptake of isotopically-labeled nutrients or by mass balances, the former being accurate but not always suitable for field studies. By the mass balances approach, that often involves tree excavations and is less accurate than isotopes, the amounts of nutrients taken up under field conditions can be approximated to the amount of nutrients contained in the new biomass produced by the tree in one year. The differential allocation of nutrients within the tree influences the extent of nutrients withdrawn from the system (by fruit harvesting) and of nutrient recycle (by internal recycling and through litter decomposition). Nutrient uptake along the season differs according to the tree species and the type of nutrient considered but, in general, trees tend to absorb nutrients faster in spring. Net uptake of nutrients like nitrogen and phosphorus lasts longer than that of potassium, which is remobilized in significant amounts from leaves to fruits in the last part of the season. Data on the amount and the dynamics of nutrient uptake are already available for many fruit trees. There is still a need to integrate this knowledge into user-friendly site-specific systems (predictive models), to be used by field-advisors and growers.