Most nutrients are stored and retained as reserves in tree buds, bark and roots. These are then remobilized after winter to fuel leaf growth, flower bloom, fruit formation and growth. In addition, a significant proportion of nutrients taken-up by the tree are recycled via the soil as a result of annual pruning, fruit thinning and leaf fall.
A significant proportion of nitrogen is used early season to provide good vegetative leaf growth, bloom and fruit set. Excessive, later applied nitrogen can adversely affect fruit quality. As a result, it is common practice to apply nitrogen at two main stages of growth. Firstly in the autumn after harvest, to boost reserves in the tree, which can then be re-mobilized to support flowering, fruit set and early leaf development in the early spring. The second, in the spring at flowering, to ensure further fruit and leaf development in late spring and summer.
Peak potassium uptake occurs later than that of nitrogen, being a key driver for yield but also having a major influence on fruit sweetness. Relatively low levels of phosphorus are required. Phosphorus is particularly important during periods of new growth of plant tissue – e.g. when new roots and flowers are formed and during cell division in the developing fruitlets.
Magnesium is important in ensuring good growth, but at relatively low levels. It has a critical role to play in leaf growth and good flowering with minimal fruit drop.
While much lower concentrations of micronutrients are needed to satisfy good growth, boron, copper, manganese and zinc are key elements within pome fruit.
Boron and Zinc have a critical role to play in bud development, flowering and fruit set, and are the most widely applied micronutrients.