Nitrogen and its Effect on the Balance of Vegetative and Reproductive Growth
With the big avocado crop out there, the question came up about whether a grower should add extra nitrogen to encourage more canopy growth to protect the fruit from sunburn or whether that would cause the fruit to drop. There is in our understanding the idea that there is a competition for resources and that in that competition one must best another. A commonly held belief is that if you apply nitrogen at the wrong time it will push resources to vegetative growth at the expense of fruit. This is somewhat true for annual plants that get most of their nutrients from outside sources (soil, air, fertilizer, water), but trees have a huge buffer in their storage organs (roots, stems, leaves, etc.). Most growth in trees occurs from this storage source and most importantly from photosynthesis and the sun. The more sun captured the more energy for flowering and fruit production.
So it is this competition for photosynthates that becomes the most limiting factor. When there is not enough to go around, the tree sheds fruit. If you see fruit dropping off a tree after applying a slug of fertilizer, it's a salt effect. Too much salt and it causes a water competition and the tree is stressed. It's not the nitrogen, but too much salt. With fertigation this is not so likely to happen as when dry fertilizers were applied and someone got too aggressive with the application
In fact a dose of nitrogen fertilizer is a good idea at this time when there are lots of clusters of fruit. This can encourage a flush of leaves that will protect the fruit from sunburn and damage that would cause fruit to drop. Also at this time, persea mite has been building its population and is starting to cause leaf drop. A bit of nitrogen to encourage leaf replacement is a good approach to dealing with this mite damage.
For further reading about the competition between vegetative and reproductive growth as affected by nitrogen (or little affected in fruit trees by nitrogen), D.O. Huett wrote a wonderful review of past research on this topic:
http://www.publish.csiro.au/?act=view_file&file_id=AR9960047.pdf
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