Nutrients can be absorbed by plants either through roots or leaves. For nutrients such as nitrogen and potassium, foliar application alone can not be sufficient without soil application.
Fertilisers for postharvest application
(continued from last issue)
Some foliar fertilisers for postharvest application have certain amounts of nitrogen in their formulation.
Considering that the nitrogen is usually withheld from a tree to reduce vigour or to develop colour in fruit, leaf nitrogen application after harvest can be enough to supply the fruit buds for spring growth and activate metabolism of micronutrients applied.
Considering that boron is a phloem-mobile nutrient, postharvest foliar boron will be efficiently transported from the leaves into storage tissues for the next year's growth.
However, soil-applied boron remains mostly in the roots while very little will be translocated to the above-ground parts of the tree at full bloom.
Bud building for next season
The main purpose of postharvest foliar application of fertilisers is bud building for the coming season.
For that purpose, boron, zinc and other micronutrients play important role.
Besides boron, zinc is beneficial for bud development and fruit set, due to its role in auxin production (plant growth hormone).
Several researchers have suggested a synergistic effect of application containing both boron and zinc. According to some trials (Keshavarz et al., 2011), zinc and boron applied either independently or in combination showed that all boron and zinc applications, and combinations, had a significant effect on reproductive and vegetative growth.
Pollen germination, fruit set, vegetative growth and fruit weight were all highest when B and Zn were applied simultaneously at 174 and 1050 mg/L concentrations respectively.
See this article in Tree Fruit April 2017
